Building Terms Glossary

Also: thermal resistance, resistance value

Thermal resistance of an insulation product measured in m²K/W. Higher R-value means better insulation. Calculated as thickness (m) divided by lambda value.

Also: radial final circuit

A wiring topology where a single cable runs from the consumer unit to outlets in sequence, terminating at the last point. All dedicated appliance circuits are radial.

Also: panel radiator, convector radiator, column radiator, designer radiator, double panel radiator, K2 radiator

The primary heat emitter fitted at second fix plumbing. Each radiator is hung, connected to flow and return pipes, fitted with a TRV and lockshield valve, and bled. Designer/column radiators hold larger water volumes than standard convectors and can cause heating system capacity issues.

Also: system balancing, lockshield valve adjustment, hydronic balancing, heating system balancing

Adjusting the lockshield valves on each radiator so hot water flows evenly around the heating system. The most common commissioning defect in new extensions is incorrect balancing — some radiators cold, others very hot.

Also: radon mitigation sump, radon gas sump

A sealed void beneath the floor slab connected to a pipe that allows radon gas to be extracted. Required in high-radon areas of England (parts of Devon, Cornwall, Northamptonshire, Derbyshire) per Approved Document C. Building control inspects for the sump during the oversite visit in affected areas.

Also: reinforced raft, concrete raft

A foundation type that distributes load across the entire footprint, used for variable or poor ground conditions.

Also: freestanding range, range oven, dual-fuel range

A large freestanding appliance (700mm–2,200mm wide) combining hob and oven in one unit. Typically requires both a dedicated cooker circuit and/or gas connection (dual-fuel). Sizing and circuit requirements differ significantly from separate hob and oven combinations.

Also: RCBO, combined MCB and RCD

Combined MCB and RCD in a single device, providing both overcurrent and earth-fault protection for an individual circuit. More expensive than separate MCBs and RCDs but prevent a fault on one circuit from tripping everything else. Increasingly specified in modern installations for better discrimination.

Also: Residual Current Device, residual current device, 30mA RCD

A safety device that disconnects a circuit within 40ms if current leaks to earth. BS 7671 Reg 411.3.3 mandates 30mA RCD protection for all socket outlets up to 20A.

Also: C20 concrete, gen 3 concrete, C20/25

Standard strength ready-mix concrete suitable for domestic foundations, pathways and non-structural applications. Delivered by lorry and pumped or wheelbarrowed into place. Minimum order usually 1m³.

Also: C25 concrete, RC25, C25/30

Medium-strength ready-mix concrete commonly specified for domestic extension strip foundations. The most commonly used strength grade for residential foundation work.

Also: C30 concrete, RC30/37, C30/37

Higher-strength ready-mix concrete used where the structural engineer specifies greater load-bearing capacity, such as reinforced foundations, pad foundations for steels, or ground floor slabs subject to heavy loads.

Also: reasonable skill and care, s.49 CRA, standard of a competent tradesperson

The implied legal standard for all domestic building services under Section 49 of the Consumer Rights Act 2015. A builder must perform work to the standard of a reasonably competent tradesperson in their field. Any contract clause attempting to exclude this standard is legally void.

Also: 12mm reinforcement bar, steel reinforcement 12mm, T12 bar, H12 bar

12mm diameter steel reinforcement bar used in foundations and reinforced concrete elements. Cut and tied on site to the structural engineer's specification.

Also: 16mm reinforcement bar, steel reinforcement 16mm, T16 bar, H16 bar

16mm diameter steel reinforcement bar for heavier-duty reinforced concrete work. Used in pad foundations, ring beams and where the structural engineer requires greater tensile strength.

Also: rebuilding cost, reinstatement cost, reinstatement value, rebuild value

The cost of demolishing and reconstructing a property from scratch if completely destroyed — distinct from market value. Homeowners must insure for rebuild cost, not market value. Adding an extension increases rebuild cost by roughly £1,800–£2,500 per m² (BCIS/RICS data, 2025). The BCIS online calculator is the authoritative free tool for estimating this.

Also: recip saw, sabre saw, Sawzall

A powerful saw with a back-and-forth blade action for demolition, rough cuts, and cutting in awkward spaces. Cuts through timber, metal pipes, nails, and plasterboard. The go-to saw for stripping out old kitchens and bathrooms.

Also: recirculation extractor, non-vented cooker hood, filterless extraction

A cooker hood that filters air through carbon filters and returns it to the kitchen rather than ducting it outside. Does not comply with Building Regulations Part F for new kitchen extensions — building control will reject it as sole ventilation.

Also: reclaimed roof tiles, salvage tiles, second-hand roof tiles, reclaimed clay tiles

Also: application boundary, site boundary line, red edge boundary

The red boundary line drawn on a location plan to mark the extent of the planning application site. A mandatory element — missing or incorrect red line boundaries are a top reason for application invalidation.

Also: RBI, Class 2 RBI, Class 1 RBI

An individual-level professional title created by the Building Safety Act 2022 (implemented April 2024). Distinct from the organisational Registered Building Control Approver (RBCA). Class 1 can only work under supervision; Class 2 can work unsupervised on domestic extensions. It is a criminal offence to carry out restricted building control activities without RBI registration.

Also: Fire Safety Order, RRO, FSO

The primary fire safety legislation for non-domestic premises in England and Wales. Buildings covered by this order must use the Full Plans building control route, not Building Notice.

Applying a cement or lime-based coating to external walls

Also: right to repeat performance, right to redo, s.55 CRA remedy

The primary remedy under the Consumer Rights Act 2015 (s.55) when a service is not performed with reasonable care and skill. The builder must redo the substandard work at their own cost. Homeowners must offer this before pursuing a price reduction.

Also: requestaplan.co.uk

Planning Portal's recommended provider for site plans and block plans for planning applications.

Also: domestic occupier, HGCRA residential occupier exclusion, section 106 HGCRA exclusion

A person who occupies, or intends to occupy, a property as their primary residence. Under Section 106 of the HGCRA 1996, contracts with residential occupiers are excluded from the statutory right to adjudication.

Also: clear cavity, residual air gap

The clear air gap that must be maintained between partial-fill insulation batts and the outer masonry leaf in a cavity wall. NHBC requires a minimum 50mm residual cavity in most locations (rising to 75mm in Very Severe exposure zones) to prevent moisture bridging. Determines batt thickness, wall tie length, and cavity width specification.

Also: RCS, silica dust, crystalline silica

Microscopic dust from cutting concrete, brick, and stone. Group 1 carcinogen with WEL of 0.1 mg/m³. FFP3 mask required.

Also: RPE, respirator, half-mask respirator, P3 respirator, face-fit test, tight-fitting mask

Equipment worn to protect the wearer from inhaling hazardous airborne particles and vapours. For paint stripping work, HSE (HSG 53) requires a P3 half-mask respirator when lead paint is likely. All tight-fitting masks require face-fit testing to be legally effective under CLAW 2002. FFP2/FFP3 disposables are adequate for general paint dust without lead. Distinct from FFP disposable masks — extraction at source is the primary control measure; RPE is secondary.

Also: revised application, resubmit planning application

The alternative to appealing: submitting a revised planning application that addresses the reasons for refusal.

Also: payment retention

Holding back a percentage of payment until work is complete and snagging resolved

Also: architrave setback, door reveal, lining reveal

The 5-10mm strip of door lining left visible between the lining edge and the architrave. Created by setting the architrave back from the lining face. A consistent reveal is the mark of careful second-fix carpentry.

Also: Royal Institute of British Architects, RIBA Find an Architect, architecture.com

The professional body for architects in the UK. They maintain a directory of chartered architects and can help resolve disputes with RIBA-registered practices.

Also: RIBA stages, RIBA Plan of Work

A standard set of project phases (0–7) defined by the Royal Institute of British Architects, from initial brief through to handover. Architects often use these stages to structure their fees and your project timeline.

Also: Royal Institution of Chartered Surveyors, rics.org, RICS Find a Surveyor

Professional body regulating chartered surveyors in the UK, including party wall surveyors; provides a Find a Surveyor directory

Also: RICS DRS, rics.org/drs

The dispute resolution arm of RICS. Operates mediation and adjudication schemes for property and building maintenance disputes. The mediation scheme charges a £240 administration fee plus mediator fee.

Also: apex batten, ridge timber, ridge fix batten, batten support bracket

Also: ridge board, ridge plate

The central horizontal beam running along the apex of a roof where two slopes meet. Can be timber or steel depending on the span and loads.

Also: repointing ridge tiles, rebed and repoint, ridge mortar repair, ridge rebed

Also: ridge caps, half-round ridge tiles, angular ridge tiles, universal ridge tile, hogback ridge tile, angle ridge tile, security ridge tile

Shaped tiles that cap the apex of a pitched roof. Must match the roof tile material and colour. Traditionally bedded on mortar but increasingly installed with dry ridge systems for a maintenance-free finish.

Also: 150mm round duct, 150mm smooth-bore duct, rigid extraction duct

The standard-diameter rigid duct pipe used for cooker hood extraction runs. 150mm is the industry-standard diameter for performance; substituting smaller diameter (e.g. 120mm) degrades airflow significantly.

Also: ring main, ring final circuit

A wiring topology where a cable starts and ends at the consumer unit, allowing multiple sockets to share a single 32A MCB.

Also: ring main, ring circuit

A 32A electrical circuit wired in a ring from the consumer unit back to the consumer unit. The recommended minimum circuit type for all kitchen socket outlets. Separate dedicated radial circuits are required for the hob, oven, and ideally the fridge-freezer.

Also: rising moisture, capillary rise

Moisture from the ground travelling upward through masonry by capillary action, which the DPC is designed to prevent. Often misdiagnosed when the actual cause is condensation, penetrating damp, or a bridged DPC.

Also: flat roof fall, minimum roof fall, design fall, 1:40 fall, 1:80 fall

Installs roof structure, coverings, flashings, and weatherproofing

Materials for pitched and flat roof coverings including tiles, membranes, battens, flashings and ridge systems.

Also: tile battens, roof battens, 25x50 battens, 25x38 battens

Treated softwood battens (typically 25x50mm) fixed horizontally across the rafters at centres matching the tile gauge. Tiles hook over the battens. Must be graded and treated timber to current standards.

Also: roofingsuperstore.co.uk

Also: Roofing Superstore, JJ Roofing Supplies, Midland Lead, Calder Lead

Specialist suppliers of tiles, slates, felt, battens, lead, and roofing accessories

Also: rotating laser level, site laser level

A laser level that spins the beam to project a full 360-degree plane, used for large-scale external levelling such as foundation setting-out and drainage gradient checking. Typically hired rather than purchased for domestic extension projects.

A power tool with a high-speed rotating bit for cutting grooves, rebates, and decorative profiles in timber. Used for creating hinge recesses, decorative edges on worktops, and housing joints. Requires practice to use safely and accurately.

Also: router cutter, straight cutter, roundover bit, rebate cutter, bearing-guided cutter, worktop cutter, profile cutter, TCT router bit

The cutting element of a router — the router is useless without an appropriate bit. Types include straight, roundover, rebate, chamfer, and ogee. Shank sizes (1/4", 1/2", 6mm, 8mm) must match the router's collet. TCT construction preferred over HSS for durability. Trend is the dominant UK supplier. A 1/2" shank is required for worktop cutting — 1/4" is not recommended for this work.

Also: collet, router chuck, collet size, 1/4 inch collet, 1/2 inch collet, 8mm collet, 6mm collet

The clamping mechanism in a router that holds the bit shank. Collet size (1/4", 1/2", 6mm, 8mm) determines which bits can be used. A major UK-specific beginner confusion: 6mm is NOT the same as 1/4" (6.35mm) and 8mm is NOT the same as 1/2" (12.7mm) — they are not interchangeable despite appearing similar, and forcing a mismatch causes dangerous runout. Must be understood before buying router bits.

Also: rolled steel joist, Universal Beam, UB, steel beam

A steel beam used to support load-bearing walls when creating openings

Also: rubber4roofs.co.uk

Also: rural location, countryside

A location outside built-up areas. In planning, rural properties may need 1:2500 scale location plans (rather than 1:1250) to show enough surrounding context, and permitted development rights may differ in designated rural areas.