Building Terms Glossary

Also: C16 timber, C16 joists, regularised timber, carcassing timber, structural softwood

Standard-grade structural softwood timber graded to C16 strength class. Available in common sizes (47x100mm, 47x150mm, 47x200mm). Used for floor joists, roof rafters and general structural framing. Adequate for most domestic spans when correctly sized.

Also: C24 timber, C24 joists, C24 carcassing

Higher-grade structural softwood timber graded to C24 strength class. Stronger and stiffer than C16, allowing longer spans or smaller section sizes. Sometimes specified by structural engineers for specific load-bearing applications.

Also: cabinet bolts, carcass connectors, inter-screw connectors, cabinet clamp bolts

Specialist bolts for joining adjacent kitchen unit carcasses together for rigidity.

Also: CAT scanner, CAT and Genny, utility locator, underground services detector, cable detection equipment

Specialist equipment used to detect buried underground services (electricity cables, gas pipes, water mains, telecoms conduits) before excavation. Distinct from wall-mounted cable detectors. HSE confirms a legal duty to use cable detection equipment before excavation. LSBUD only covers public utility networks — private property cables require physical CAT scanning by a qualified utility locator.

Also: capping, mini-trunking, oval mini-trunking, plastic capping, metal capping

Flat plastic or metal channel fixed over cables run across walls before plastering. Distinct from conduit — capping is NOT mechanical protection under BS 7671 Reg 522.6.204, but protects cables from trowel damage during plastering.

Also: thermal derating, derating factor, current capacity derating, cable in insulation derating

The reduction of a cable's safe current-carrying capacity when installation conditions limit heat dissipation, primarily when cable is buried in thermal insulation.

Also: pipe and cable detector, stud and cable detector, wall scanner, internal cable detector

A handheld scanner for detecting electrical cables, water pipes, and metal objects hidden behind walls and floors. Essential before drilling into any wall to avoid hitting live cables or pipes. Some models also detect timber studs.

Also: BS 7671 reference method, reference method, reference method C, reference method 100, reference method 101, reference method 102, reference method 103, method C, clipped direct, method 103

The BS 7671 Appendix 4 reference method classification system (Methods A, C, 100-103) that determines which current-carrying capacity figure applies to a given cable in a given installation scenario. The same 2.5mm² cable carries 27A clipped direct (Method C) but only 13.5A fully surrounded by insulation (Method 103).

Also: cantilever beam, cantilevered structure

A structural arrangement where a beam extends beyond its support point, commonly used in extensions to remove corner posts and create wide openings for bifold doors. Requires specialist structural engineer calculations.

Also: joiner

Works with timber for roof structures, stud walls, door frames, and fitted joinery

The IEC 61010 installation category system for electrical test equipment. CAT III 600V minimum is required for testing fixed building wiring and sockets.

Also: Cat 6 cable, ethernet cable, structured data cabling, Cat6a cable

Data cabling installed at first fix alongside electrical cables. Retrofitting after walls are closed costs 3-4x more. At least 2 drops per zone recommended, kept 100mm from mains cables to prevent interference.

Also: fire stop, cavity fire stop

A fire-stopping element required by Approved Document B at party wall junctions, eaves, and wall plate level, with a maximum uninterrupted cavity run of 20m. The BCO checks cavity barriers at the structural stage.

Also: cavity boards, cavity batten, cavity cleaning board, mortar catching board

A strip of timber, foam, or polystyrene dragged up through the cavity as brickwork courses are laid, catching mortar droppings that would otherwise land on the DPC or wall ties and bridge the cavity. Removed and cleaned at intervals during bricklaying.

Also: insulated reveal closer, window closer, door reveal closer

A prefabricated insulated frame that closes the cavity at window and door reveals, preventing thermal bridging and moisture penetration. Must be installed during wall construction (first fix) rather than retrofitted. Building control inspectors check their presence at every opening.

Also: insulated cavity closer, cavity closers, window reveal closer

Prefabricated insulated frames installed at window and door openings to close the cavity and prevent thermal bridging. Must comply with Parts B (fire), C (moisture), and L (thermal) of Building Regulations, with minimum 0.45 m²K/W thermal resistance under Part L. First-fix installation during wall construction is required by NHBC/LABC standards.

Also: cavity tray DPC, cavity flashing

A damp proof course installed within a cavity wall to redirect moisture that has entered the cavity back to the outside via weep holes. Required where a roof meets a wall, above openings, and at any point where moisture could bridge the cavity.

Also: preformed cavity tray, cavity tray DPC, lead-in cavity tray, cavity tray unit

Preformed angles and straights made from pitch-polymer or CPT, installed at abutments where the extension roof meets the existing house wall to direct cavity moisture back outside through weep vents.

A wall made of two skins (leaves) with an air gap or insulation between them

Also: partial fill cavity insulation, cavity batts, DriTherm, Knauf DriTherm 32, Knauf DriTherm 37

Semi-rigid mineral wool or glass fibre batts designed to partially fill the cavity in a masonry cavity wall. Held in place against the inner leaf with retaining clips. Must leave a minimum 50mm clear cavity to prevent moisture bridging.

Also: drain camera survey, drainage CCTV, drain survey, CCTV survey

A survey of existing drainage using a camera fed through the pipes to record their condition, depth, gradient, and connection points. Costs £150-350 and takes half a day. Reveals pipe depth and invert level (needed to calculate new drainage gradients), confirms which system each pipe connects to, and identifies existing defects such as root ingress or collapsed sections before they affect the new connection.

Construction (Design and Management) health and safety regulations

Also: HardieBacker, tile backer board, cement backer board, Aquapanel, CBU, cement backer unit, NoMorePly

Rigid cement-based board used as a tile backing in wet areas such as showers and around baths. Water-resistant but not waterproof - the board itself will not degrade when wet, unlike plasterboard. Joints and fixings must be sealed. Essential behind tiling in shower enclosures; requires tanking membrane for fully wet areas.

Also: ceramic tiles, ceramic floor, glazed ceramic tiles

A lower-cost alternative to porcelain for floor tiling. More porous and less hard-wearing than porcelain. Same adhesive and preparation requirements as porcelain tiles.

Also: Certass Ltd, Certass trade association

UK competent person scheme operator for window and door installations, providing self-certification for Building Regulations compliance. A direct alternative to FENSA - Certass-registered installers can self-certify replacement windows and external doors without a separate building control application, and issue the certificate insurers and future buyers' solicitors ask for.

Also: regularisation certificate, regularisation application, retrospective building control

The retrospective route for obtaining building regulations approval for work done without prior notification. Available for work carried out after November 1985.

Also: snap line, chalk reel

A reel of string coated in coloured chalk powder. Stretch it between two points and snap it to mark a perfectly straight line on floors, walls, or timber. Used for setting out layouts, marking cut lines, and aligning brickwork.

Also: chalk refill, chalk line powder, marking chalk

Coloured powder used to refill chalk line reels for marking straight lines on surfaces

Also: ACO drain, linear drain, threshold drain, slot drain, ACO AS350, drainage channel

A surface water drainage channel installed flush with paving at the junction between a patio and the house wall. Required when a flush-threshold patio is needed next to bifold or sliding doors to intercept surface water before it reaches the wall, while maintaining DPC clearance.

Also: debit card chargeback, card chargeback, chargeback claim

A process where your bank reverses a debit card payment if goods or services weren’t provided. Unlike Section 75, there’s no minimum spend, but time limits are strict — typically 120 days from the payment.

Also: CEng, chartered status, MIStructE, MICE

A professionally qualified engineer who has demonstrated competence through examination and supervised experience. The letters CEng and MIStructE or MICE confirm chartered status.

Also: checkatrade.com

Platform providing cost guides and tradesperson vetting for UK homeowners.

Also: resin anchors, chemical fixings, resin cartridge, epoxy anchors

Two-part resin adhesive in a cartridge, injected into drilled holes to bond threaded studs or rebar into masonry or concrete. Stronger than mechanical anchors and essential in hollow or weak masonry where expansion anchors cannot grip. Used with a specialist applicator gun.

Also: chemical DPC, injection damp proofing, remedial DPC, damp injection

A remedial method for installing a new DPC in an existing wall with a failed or missing DPC, by drilling holes and injecting waterproofing fluid. Governed by BS 6576. Costs £50-150 per linear metre.

Also: paint stripper, paint remover, Paint Panther, Peel-away, Nitromors

Solvent or caustic gel applied to painted surfaces to break down the paint film without heat, making it the safest method for lead paint removal. Preferred over heat guns when lead paint is confirmed or strongly suspected. Paint Panther and Peel-away are the most commonly recommended UK brands. No heat means no lead vaporisation risk and no power source needed.

Also: chimney stack

A projecting section of wall that houses a fireplace flue, often shared between semi-detached or terraced houses as part of the party wall

Also: P5 chipboard, flooring grade chipboard, moisture resistant chipboard

T&G moisture-resistant chipboard for domestic floor decking. Cheaper than plywood with a smoother surface than OSB. P5 grade is load-bearing and moisture-resistant. Standard sheet 2400x600mm (T&G) or 2440x1220mm (square edge).

Also: CSM, chopped strand matting, 450g CSM, 600g CSM, fibreglass matting, GRP reinforcement mat

Also: Chartered Institute of Architectural Technologists, architecturaltechnology.com

The professional body for architectural technologists in the UK. CIAT members specialise in the technical design and detailing of buildings.

Also: circuit list, consumer unit schedule, circuit identification schedule

Record showing what each protective device in the consumer unit protects. Required under BS 7671:2018+A2:2022. Must be updated if circuits are moved during second fix work.

A handheld power saw with a rotating blade for making straight cuts in timber, sheet materials, and decking. Essential for cutting plywood, OSB, and structural timber to length on site.

Also: saw guide rail, circular saw track, saw track, guide rail, straight edge guide

A clamped aluminium track that attaches to a circular saw base, enabling accurate straight cuts in sheet materials and worktops. Essential for chip-free laminate worktop cutting.

Also: clearance zone, working space, NHBC 1m circulation

NHBC 9.4.3 mandates minimum 1 metre clearance in front of all work surfaces, cupboards, and appliances.

Also: CISRS, Construction Industry Scaffolders Record Scheme, scaffolder competency card

The industry competency certification for scaffolding operatives in the UK (Construction Industry Scaffolders Record Scheme). HSE guidance requires that only competent persons erect, alter, or dismantle scaffold. Homeowners should ask to see CISRS cards before allowing a scaffolding contractor to begin work.

Also: citizensadvice.org.uk, Citizens Advice Bureau, CAB

A free, independent advice service that can help with consumer disputes, contract problems, and your rights when dealing with builders or tradespeople.

Also: external cladding

External wall covering material (timber, composite, render, etc.). Certain cladding changes are specifically excluded from permitted development in conservation areas.

Also: carpenter's hammer

The standard carpenter's hammer with a flat face for driving nails and a curved claw for pulling them out. Used throughout a build for fixing timber, hanging doors, and general nailing. Choose a weight that feels comfortable for extended use.

Also: ground heave, soil heave, heave

The upward swelling of clay soil when moisture returns after tree removal, requiring compressible material on trench sides and suspended ground floors.

Also: clay tiles, plain clay tiles, handmade tiles, interlocking clay tiles, pantiles, Dreadnought tiles, Keymer tiles, Marley Acme tiles

Traditional clay tiles with excellent longevity and appearance. Often required in conservation areas or to match older properties. Lighter than concrete tiles but more expensive. Available in plain, interlocking and handmade profiles.

Also: lump hammer, mash hammer

A short-handled, heavy hammer (typically 1-1.5kg) for striking bolster chisels, driving stakes, and light demolition. The essential companion to a bolster chisel for cutting bricks and blocks.

Also: carriage bolts, cup head bolts

Large bolts with a smooth, domed head and square shank that grips into timber to prevent spinning. Used with a nut and washer for heavy-duty timber-to-timber or timber-to-steel connections. Available in M8, M10 and M12 diameters.

Also: 20mm stone, gravel, 10mm aggregate, coarse gravel, crushed stone, concreting gravel

The third component of a site-mixed concrete batch alongside cement and sharp sand. Used in standard concrete mixes (1:2:4, 1:2:3, 1:3:6). Purchased separately from sharp sand when batching on site. Distinct from all-in ballast, which is a pre-blended combined aggregate. Homeowners using a concrete mixer must understand the difference between sharp sand and coarse aggregate and how to proportion them.

Also: flat cold chisel

Hardened steel chisel for cutting or shaping metal and removing mortar

Also: cold deck roof, ventilated roof

Roof construction where insulation sits below the deck, leaving a cold ventilated void above. Requires ventilation gap between insulation and covering.

Also: frost protection, cold weather brickwork, cold weather mortar

Building work during winter needs special precautions — concrete can’t be poured in frost, and brickwork must be protected. Expect slower progress and discuss winter working arrangements with your builder before the project starts.

Also: combination boiler, combi

The most common UK boiler type, heating water on demand without a storage tank. Flow rates of 9-16 L/min, halved at two simultaneous outlets.

Also: combination drill, combi driver

A versatile cordless drill that combines drilling, hammer drilling, and screwdriving. Good for wood, light masonry, and general fixings. The first power tool most people buy, but not powerful enough for heavy masonry work.

Also: combined sewer, combined system, combined drain

A drainage system where foul water and surface water share a single pipe to the treatment works. Common in pre-1960s UK properties. Building regulations require separate systems where practicable, but extensions on properties with combined sewers may still need to connect to them if no separate sewers exist in the road.

Also: approved but uninvoiced, committed spend

Costs that have been formally approved (work instructed, materials ordered) but not yet invoiced or paid. One of three budget visibility categories alongside actual costs and remaining budget. Tracking committed costs closes the gap between what has been paid and what has been formally approved.

Also: CIL

A charge that councils may levy on new development to fund infrastructure. Self-build homeowners are exempt but must claim proactively.

Also: find-and-update.company-information.service.gov.uk, Companies House register

The official government register for verifying whether a builder is a registered company, checking directors, and reviewing financial history.

Also: approved inspector scheme, self-certification scheme, NICEIC registered, NAPIT registered, Part P competent person

Government-approved schemes (NICEIC, NAPIT, ELECSA) that allow registered electricians to self-certify Part P notifiable electrical work without a separate building control application.

Also: completion notice, Regulation 16 notice, 5-day completion notice, building control completion notice

Formal notice a homeowner must give to building control within 5 days of practically completing the work, under Building Regulations 2010 Regulation 16. Triggers the 8-week statutory clock for the local authority to issue the completion certificate.

Also: completion notification, building control completion notice, regulation 16 notification

Form that must be submitted to building control before a final inspection can be requested. Introduced by the Building Regulations etc. (Amendment) (England) Regulations 2023 (October 2023). A procedural gate that many homeowners miss, causing unnecessary delays at the final stage of the project.

Also: principal designer compliance declaration, principal contractor compliance declaration, Regulation 18 declaration

Declaration required under the Building (Registered Building Control Approvers etc.) (England) Regulations 2024 (Regulation 18). The principal designer and principal contractor must each submit a compliance declaration to building control before a Final Certificate can be issued. For domestic extensions where the homeowner acts as principal designer, they must submit this themselves.

Also: IsoBloco, CompriBand, expanding window tape, window sealing tape, compressible foam sealing tape

Specialist perimeter sealing tape for window and door frames that expands to fill gaps, providing superior thermal, acoustic, and weathertight performance compared to foam alone.

Also: compression elbows, compression tees, compression couplers

Brass fittings that join copper pipes using a compression olive and nut tightened with spanners. Available as elbows, tees, straight couplers and reducers. Reliable, demountable joints but slower to install than push-fit. Preferred where high temperatures or pressures are involved.

Also: drainage search, water and drainage search, CON29DW, drainage and water enquiry

Concrete products, reinforcement, formwork and associated materials used in foundations, floor slabs and structural elements.

Also: dense blocks, solid blocks 100mm, 4 inch blocks

Standard 100mm dense concrete blocks used for load-bearing walls and below-ground work. Heavier than aerated alternatives but stronger in compression.

Also: aerated blocks, Thermalite blocks, lightweight blocks 100mm, aircrete blocks

Lightweight aerated concrete blocks (e.g. Thermalite, Celcon) used for inner leaf of cavity walls. Better thermal performance than dense blocks and easier to cut and handle.

Also: 6 inch blocks, 140mm blocks

Wider concrete blocks used where additional structural strength or wall thickness is required, common in foundation walls and below DPC level.

Also: cover to reinforcement, minimum cover, nominal cover

The minimum distance between reinforcement steel and the concrete surface. Protects steel from corrosion. Values vary: 75mm (ground contact without blinding), 40mm (over blinding), 25mm (internal). The most common building control rejection point for reinforced concrete.

Also: spacers, cover blocks, rebar chairs, concrete cover blocks

Small plastic or concrete blocks placed under reinforcement mesh and rebar to maintain the correct cover distance from the formwork. Essential for building control compliance as insufficient cover leads to premature corrosion of the steel.

Also: mix ratio, cement ratio, concrete proportions, mix design

The proportions of cement:sand:aggregate (e.g. 1:2:4, 1:3:6) that determine the strength class of site-mixed concrete. Different ratios apply to different applications: foundations, slabs, blinding. Homeowners encounter mix ratios in structural engineer specifications, building control documents, and builders merchants.

Also: belle mixer, cement mixer, Belle Minimix 150, drum mixer, Altrad Belle mixer

A rotating drum mixer for combining cement, sand, aggregate, and water on site. Used for mixing mortar for bricklaying and small batches of concrete. For foundation pours, ready-mix delivery is more practical. Usually hired.

Also: line pump, boom pump, concrete line pump

Required when a ready-mix lorry cannot reach the trench by chute. Line pump £250-500, boom pump £500-1,200 per half day.

Also: concrete tiles, Marley tiles, Redland tiles, Sandtoft tiles

Machine-made concrete tiles available in a wide range of profiles and colours. Heavier than clay tiles but cheaper and very durable. Common for matching existing roofs on 1960s-1990s properties.

Also: condensing combi boiler, high-efficiency boiler, gas boiler, new boiler, replacement boiler, Worcester Bosch, Vaillant, Baxi, Ideal Logic

The gas appliance installed at second fix plumbing — the primary cost driver and the item that triggers the Gas Safe notification requirement. All new gas boilers installed in the UK must be condensing (high-efficiency) by regulation.

Also: oval conduit, round conduit, PVC conduit, cable conduit

PVC tubing used to protect cables routed through or across walls. Oval conduit is used for cables chased into masonry walls; round conduit for surface-mounted runs. Cables in conduit are protected from accidental damage from nails and screws.

Also: CVI, verbal instruction confirmation

A specific document type under JCT contracts that bridges verbal site instructions into the formal variation process. Under JCT, verbal instructions have no immediate effect until confirmed in writing via a CVI within 14 days.

Also: consequential damages, consequential loss claim

Indirect costs you suffer because of a builder’s breach of contract, such as rental costs while you’re unable to live in your home. These are only recoverable if your contract specifically allows for them.

An area of special architectural or historic interest designated by the local authority. Permitted development rights are restricted: side extensions and certain cladding changes are excluded. Rear extensions may still be possible under PD depending on specific council restrictions.

A glazed extension typically at the rear of a property. A previous conservatory added by a former owner counts against your permitted development allowance, as PD limits are measured against the original house.

Also: build programme, programme of works, project schedule, build schedule

A time-based document that schedules every phase and task in a build project with planned start dates, end dates, durations, and dependencies. Distinct from a schedule of works (which defines scope, not time).

Also: site hours, working hours on site, construction hours restriction, permitted construction hours

Council restrictions on construction activity, typically 7:30am-6:00pm weekdays, enforced by Environmental Health

Also: CCR 2013, Consumer Contracts (Information, Cancellation and Additional Charges) Regulations 2013

Gives you a 14-day cooling-off period to cancel contracts agreed at your home or online, which can apply to some building contracts.

Also: CCA 1974, Consumer Credit Act, s75 CCA

Protects you when using credit to pay for building work. If you pay by credit card or finance, the lender can be jointly liable if things go wrong.

Also: CIDRA 2012, CIDRA, Consumer Insurance Act 2012

The primary UK statute governing a consumer's duty of disclosure to their insurer. Section 2 creates the duty to take reasonable care not to make a misrepresentation; Section 4 defines qualifying misrepresentations as deliberate/reckless or careless with different remedies. Governs what happens if a homeowner fails to notify their insurer of a completed extension. Applies to all policies taken out or renewed after 6 April 2013.

Also: CRA 2015, Consumer Rights Act

Your main consumer protection law. It requires building work to be done with reasonable care and skill, and materials to be of satisfactory quality.

Also: fuse box, fuse board, distribution board, CU

The main electrical distribution board that houses the RCD(s), MCBs and RCBOs protecting individual circuits. Modern consumer units must be metal-clad (amendment 3 of BS 7671). A 12-way or 16-way unit is typical for an extension; the electrician will specify the size needed based on the number of circuits.

Also: CU placement, consumer unit location, fuse board location

The first-fix decision about where the consumer unit is physically located, constrained by the DNO meter tail length rule. All circuit cable routes radiate from this fixed point.

Also: home contents insurance, contents cover

Insurance covering moveable possessions within the home. Commonly overlooked when updating insurance after an extension, but a kitchen extension typically adds significant contents value: new kitchen appliances, bifold doors, tiles, and new furniture.

A budget reserve (typically 10-20%) for unexpected costs during construction

Also: CA, contract admin

The person named in a building contract to manage it on behalf of the employer (homeowner). Issues payment certificates, manages variations, and certifies practical completion. Under JCT Minor Works, this is typically the architect but can be a quantity surveyor or project manager.

Also: site insurance, all-risks insurance, contractors all risk insurance, CAR insurance

Insurance covering works in progress against damage, theft, or destruction during the build. Either the builder or the homeowner must hold this — it is not covered by standard home insurance.

Also: builder going bust, builder insolvency, construction company insolvency

When your builder goes bust mid-project, leaving you with an unfinished build. Having a proper contract, staged payments, and retention money held back will limit your financial exposure if this happens.

Also: Asbestos Regulations 2012, CAR 2012

Requires licensed specialist removal of asbestos-containing materials and prohibits asbestos from the general waste stream

Also: CLAW Regulations, CLAW 2002, Control of Lead at Work, CLAW

Also: Noise Regulations 2005, Noise at Work Regulations 2005

UK statutory instrument establishing legal noise exposure thresholds: lower action value 80 dB(A), upper action value 85 dB(A), exposure limit 87 dB(A). Hearing protection is mandatory at 85 dB(A).

Also: COSHH 2002, COSHH, COSHH Regulations

Primary UK legislation requiring assessment and control of exposure to hazardous substances including construction dust (silica, wood dust, cement dust). Establishes the legal basis for dust mask selection via the hierarchy of controls and workplace exposure limits.

Also: Vibration at Work Regulations 2005, HAV Regulations 2005, Vibration Regulations 2005, SI 2005/1093

Statutory instrument SI 2005/1093 setting legally binding Exposure Action Value (EAV) of 2.5 m/s² A(8) and Exposure Limit Value (ELV) of 5.0 m/s² A(8) for hand-arm vibration from power tools. Angle grinders produce 5.3–5.5 m/s² and SDS breakers are among the highest-risk tools. Applies to employers but the physical health risk applies equally to homeowners acting as their own contractor.

Also: build-over indemnity insurance, BOA indemnity policy, sewer indemnity insurance

Also: cooker switch, cooker outlet plate, CCU, cooker control unit

Wall-mounted switched fused unit serving as the hardwired connection point for a cooker or hob, fed by 6mm T&E cable from the consumer unit.

Also: extractor hood, range hood, kitchen extractor, extractor fan

The appliance category for kitchen extraction with multiple sub-types (chimney, canopy, island, downdraft, integrated), each requiring different duct routing approaches and affecting wall unit layout.

Also: fret saw

Hand tool with a thin blade for cutting curved profiles. Essential for scribing internal corners on skirting board and architrave — the correct technique instead of mitring internal corners.

Also: 15mm copper, half inch copper pipe

15mm diameter copper tube, the standard size for domestic hot and cold water supply runs to individual taps, basins and appliances. Sold in 3m lengths. Joined using compression fittings, solder ring fittings or push-fit connectors.

Also: 22mm copper, three quarter inch copper pipe

22mm diameter copper tube used for main supply runs, central heating flow and return pipes, and higher-flow applications. Sold in 3m lengths.

Also: 28mm copper, one inch copper pipe

28mm diameter copper tube for main incoming supply pipes, boiler connections and high-flow central heating pipework. Sold in 3m lengths.

Also: copper tube temper, R220, R250, R290, soft copper, half-hard copper, hard copper

The R220/R250/R290 grade system (defined in BS EN 1057) determines whether copper pipe can be bent safely. R290 hard-tempered pipe cannot be bent. R220 (soft/annealed, sold in coils) and R250 (half-hard, sold in 3m lengths) are both suitable for bending.

Also: battery platform, cordless platform, battery compatibility, 18V battery system, battery interoperability, one battery system

The purchasing principle that all cordless tools from one brand share batteries. A homeowner who owns a DeWalt 18V drill should buy DeWalt body-only for subsequent tools, saving £50–100 per tool on battery costs.

A specialist drill for cutting large-diameter circular holes through walls, floors, and concrete. Used for waste pipes, ventilation ducts, and extractor fan outlets. Often hired rather than bought for one-off use.

Also: line blocks, line boxes, bricklayers corner blocks

Plastic or rubber blocks that grip corners of brickwork to hold a builder's line

Also: corner leads, building corners first

Bricklaying technique of building up corners first to establish level and plumb references before filling in between

Also: corner-post removal, removing corner post, corner post elimination

Removing the vertical structural post at the corner of an extension to create a wider opening, typically for full-width bifold doors. Requires a cantilever beam design and multiple rounds of structural engineer calculations.

Also: kitchen cornice, kitchen pelmet, light pelmet, wall unit trim

Decorative trim fitted above and below wall units to cap and conceal gaps and lighting.

Also: CIGA 2020, CIGA

Introduced new insolvency procedures including a moratorium that may temporarily prevent you from taking legal action against a struggling builder’s company.

Also: budget overrun, cost overspend

The core risk concept for homeowners budgeting an extension — when actual costs exceed estimates due to structural surprises, specification creep, or undercosted specialist trades.

Also: cost per m2, £/m2, build cost rate

The primary benchmark metric used to sanity-check extension quotes against regional averages.

Also: cost and percentage contract, time and materials contract, T&M contract, open-book contract

A contract where you pay the builder’s actual costs for labour and materials plus an agreed percentage or fee on top. You get transparency on spending but less certainty on the final bill.

Also: planning register, public register

The publicly accessible record of all planning applications submitted to a local authority, including plans, documents, and decision notices.

Also: counter battens, roofing counter battens, rafter-direction battens, drainage battens, 47x25 counter battens

Also: injunction, court injunction

A court order that can stop building work. Under the Party Wall Act, a neighbour can seek an injunction if work starts without proper notice being served.

Also: crack gauge, crack width gauge, tell-tale, crack tell-tale

A calibrated device affixed across a crack to track whether it is stable or progressing over time. Provides a permanent dated record of crack movement. Used to monitor cracks at the old/new junction on extensions during the defects liability period.

Also: ferrule crimper, bootlace crimper, ratchet crimper

A tool for compressing crimp connectors onto the ends of electrical cables, creating a secure mechanical and electrical connection. Used for joining cables, attaching ring terminals, and making connections in consumer units.

Also: critical path method, CPM, critical path analysis

The sequence of tasks in a project that determines the minimum possible project duration. Any delay on the critical path delays the whole project. Tasks not on the critical path have float — they can slip without affecting completion.

The land around the original house. Extensions must not cover more than 50% of the curtilage area under permitted development rules.

Also: slitting gauge, knife gauge

A marking gauge that uses a small blade instead of a pin, severing wood fibres rather than tearing them. The correct tool for marking across the grain where a pin gauge would produce a ragged line.

Specialist tools and accessories for cutting tiles, ceramics, and making holes in hard materials. Used primarily during second fix and finishing stages.