Building Terms Glossary

Also: 45 degree light rule, 45-degree light test

An informal but widely-applied council assessment method for prior approval applications, drawing a 45-degree line from a neighbour's nearest window to assess overshadowing and overbearing impact.

Also: DPM, polythene DPM, damp membrane, visqueen, 1200 gauge polythene sheet, 300 micron DPM

Heavy-duty polythene sheet (1200 gauge / 300 micron) laid under and within concrete floor slabs to prevent moisture rising from the ground. Joints must be overlapped by at least 150mm and taped. Building control will check this is present.

Also: datum level, datum point, reference line, datum mark, site datum, benchmark peg, datum peg

A fixed reference height from which all measurements are taken during a construction task. In groundwork, a benchmark peg is driven into undisturbed ground and all site levels are referenced from it. In kitchen fitting, the 860mm datum is projected from the highest floor point to set all base unit heights.

Also: daylight assessment, sunlight study, BRE daylight report

A specialist technical report homeowners can commission to demonstrate that a proposed extension does not unacceptably reduce daylight or sunlight to neighbouring properties.

Also: pipe reamer, pipe deburring tool, internal pipe reamer

A tool for removing the internal burr left on cut pipe ends. Critical for compression fittings (burr prevents the olive from seating squarely, causing leaks) and soldered joints. Some adjustable wheel cutters include a built-in reamer cone, but fixed-size pipe slices do not — so a standalone reamer is a separate purchase.

Also: officer's report, planning officer report, delegated report

The planning officer's written assessment document that addresses neighbour objections and explains the reasoning behind the planning decision. Published as part of the council's determination.

Hand tools for painting, wallpapering, and surface preparation. The tools you need to apply paint, hang paper, mask off adjacent surfaces, and strip old finishes. Used extensively during the decoration phase at the end of a build.

Also: painter and decorator

Paints, wallpapers, and finishes interior and exterior surfaces

Also: caulk, acrylic caulk, painters caulk, flexible filler

Flexible acrylic filler applied from a cartridge to fill gaps between skirting boards and walls, around architraves and along ceiling lines before painting. Paintable (unlike silicone) and gives a clean, professional finish. Not waterproof; use silicone in wet areas instead.

Also: uncoupling membrane, Schluter Ditra, Ditra 25, tile decoupling mat

Polymer mat installed between screed and tiles to absorb differential movement from UFH thermal cycling. Prevents tile cracking and debonding.

Also: dedicated feed, dedicated radial, single-appliance circuit

A circuit from the consumer unit serving only one appliance, required for high-load kitchen appliances including induction hobs, electric ovens, dishwashers, and fridges.

Also: automatic approval, 42-day silence rule

If the council fails to issue a decision on a prior approval application within 42 days, and no written extension of time has been agreed, the development is automatically deemed approved.

Also: deemed commencement notification, Regulation 16(3C)

From 1 October 2023, horizontal extensions have a statutory 'deemed commencement' trigger: when the sub-surface structure (foundations and ground floor) is complete, the applicant must notify the building control body within 5 days. If this notification is not given and the approval lapses after 3 years, building control approval expires.

Also: DPA 1972

Makes builders and professionals liable if a new dwelling or conversion is not fit for habitation. Claims can be brought up to 15 years after completion.

Also: defects period, rectification period, DLP, maintenance period

A period after your build is finished, typically 6–12 months, during which the builder must come back and fix any defects at no extra cost. Don’t release the final retention payment until this period ends.

Also: desiccant dehumidifier, compressor dehumidifier, industrial dehumidifier

Electric appliance that removes moisture from the air, used to speed up plaster drying in new extensions. Compressor models work well in warm conditions; desiccant models are more effective in cold, unheated spaces. Can be purchased or hired (£44-100/week).

Also: GRP delamination, resin delamination, fibreglass delamination, laminate separation

Also: depth adjustment, drive depth, nail depth setting, countersink setting

The adjustable setting on a nail gun that controls how far the nail is driven into the material. Over-driven nails crush timber fibres and reduce structural holding power — a defect called out in NHBC standards for first fix work. Under-driven nails protrude above the surface and prevent plasterboard from sitting flush. Setting depth on scrap timber before each new material is standard professional practice.

Also: DAS

A document that may be required with a householder planning application, explaining the design principles and how the development will be accessed.

Also: design-and-build contract, turnkey contract

An alternative project route where a single contractor handles both design and construction, potentially reducing costs through better alignment.

Also: designated mix, GEN mix, RC mix, FND mix

BS 8500 system of pre-defined concrete specifications (GEN, RC, PAV, FND families) with guaranteed cement content and water-cement ratio. Different from C-class strength grades which only specify compressive strength. The source of widespread confusion among homeowners and suppliers.

A freestanding house that does not share any walls with neighbours. Under the Larger Home Extension scheme, single-storey rear extensions can extend up to 8 metres (compared to 6m for semi-detached and terraced).

Also: differential movement, foundation movement

The movement that occurs when a new extension settles at a different rate to the existing house, because the new foundations are fresher and the concrete is still curing. The primary cause of cracks at the old/new junction in the first 1–2 years after construction. Normal within limits; requires monitoring.

Also: electronic spirit level, angle finder level

An electronic spirit level that uses sensors to display angle in degrees on an LCD screen. Accurate to 0.1mm/m vs 0.5mm/m for quality bubble levels. Audio beep alerts allow hands-free use.

Also: DIN mounting rail, 35mm DIN rail, top-hat rail

The standard 35mm mounting rail inside consumer units on which MCBs, RCBOs, and SPDs are clipped.

Also: DNO, NGED, UK Power Networks, Northern Powergrid, SSEN, SP Energy Networks

The electricity DNO owns and operates the cable from the street to the meter. Moving an electricity meter that requires cable rerouting requires a separate DNO application and payment.

Also: electrical diversity, load diversity calculation

The engineering principle allowing multiple socket circuits to share a consumer unit without assuming all draw maximum load simultaneously. Used to assess whether an existing consumer unit can accommodate extension circuits.

Also: door frame kit, door lining set, door casing, door liner, lining kit

Pre-cut timber frame kit that lines the door opening in a stud or masonry wall. Includes head and two jambs. Available in different widths to suit wall thickness. Must be installed plumb and square or the door will not hang properly.

Thin timber strip (typically 12x32mm) nailed to the inside of a door lining that the door closes against. Positioned after the door is hung to ensure correct alignment.

Also: flush threshold, low threshold, raised threshold, level threshold, rebated threshold

The threshold is the most failure-prone element of bifold/sliding door installations. Flush thresholds offer accessibility but higher flood risk; raised thresholds provide more robust drainage. Remedial work costs £2,000-£5,000.

Also: dry lining adhesive method, dab and dot

Method of fixing plasterboard or insulated plasterboard to masonry walls using adhesive dabs. Requires perimeter adhesive band plus supplementary mechanical fixings for fire safety.

Also: DCV, double non-return valve, backflow prevention device, non-return valve

A backflow prevention device legally required on outside tap supply pipes under Schedule 2.6.15.20 of the Water Fittings Regulations. Must be installed inside the thermal envelope of the building — not in the tap body. Also used on washing machine and dishwasher supply connections.

Also: IGU, insulated glazed unit, sealed unit, double glazing, 24mm unit, 28mm unit

Standard glazing specification for UK extensions: two panes of glass (typically 4mm) separated by a gas-filled cavity (typically 20mm argon), sealed as a single unit conforming to BS EN 1279.

Also: double lap, double-lap tiling, two-layer overlap

Also: downdraft hood, pop-up extractor, hob extractor, integrated hob extractor, BORA extractor

An extraction system that pulls cooking vapour downward rather than capturing it from above. Requires floor-level ducting installed before screed is poured — a critical sequencing constraint often missed until too late.

Also: double-pole isolator, 20A DP isolator, 20 amp double-pole isolator, double-pole switch

A 20A double-pole isolating switch used to isolate high-load continuous-duty appliances such as immersion heaters and Sunamp units from their dedicated circuit. Safer than a 13A fused connection unit for sustained loads.

Also: damp proof course, damp-proof course

A barrier in walls to prevent rising damp, typically at ground level

Also: DPC bridge, bridging the DPC, DPC bypass

The most common DPC failure mode, where patios, paths, raised flower beds, or render taken below DPC level create a moisture path that bypasses the DPC entirely.

Also: 150mm DPC clearance, DPC to patio clearance, patio DPC gap, DPC height above paving

The requirement to maintain a minimum 150mm clearance between the damp-proof course and any adjacent external ground or paving level. The most common post-extension defect. Failure creates DPC bridging, allowing moisture to bypass the DPC and cause damp. Must be verified before any paving is laid.

Also: DPM tape, polythene jointing tape, membrane lap tape, vapour barrier tape

Self-adhesive tape used to seal the overlapping joints in DPM sheets so the moisture barrier is continuous. Joints must be lapped by at least 150mm and taped; untaped joints are a building control failure point.

Also: air test kit, drain pressure test kit, 110mm drain bung kit

Equipment for conducting a pre-inspection drain air test: 110mm drain plug/bung, flexible hose, hand pump, and pressure gauge. Community forums recommend homeowners buy one (~£40 from Screwfix) and pre-test drainage before the BCO visit to catch and fix leaks before failing in front of the inspector.

Also: underground drainage, foul and surface water drainage, below-ground drainage, rainwater drainage

Below-ground products that route foul water to the existing sewer connection and surface water to a soakaway: inspection chambers, soakaway crates, geotextile membrane, push-fit fittings, and threshold channel drains. Building control inspects every joint before the trench backfills.

Also: drainage test sign-off, CCTV drainage survey certificate

Record of the air or water pressure test carried out on newly laid below-ground drainage to Building Regulations Part H and BS EN 1610, issued by the installing groundworker or drainage contractor and witnessed by Building Control at the drains-before-cover inspection. For connections to the public sewer a CCTV survey is often substituted. The certificate or test record is part of the completion handover pack.

Also: pipe gradient, pipe fall, drain fall, fall, 1:40 gradient, Maguire's rule

The slope of a drainage pipe that allows waste to flow by gravity. For 110mm domestic drainage serving a kitchen extension (no WC), the required gradient is 1:40 — a drop of 25mm per metre. Steeper causes water to outrun solids; shallower causes solids to settle. Both lead to recurring blockages. Maguire's rule states: pipe diameter divided by 2.5 equals the gradient denominator (100 / 2.5 = 40, so 1:40).

Power drills for drilling holes, driving fixings, and breaking through masonry. The most essential power tool category for any construction project.

Also: dry ridge kit, mechanical ridge fixing, roll-out dry ridge system, dry fix ridge, RidgeFast

Mechanical fixing system for ridge tiles that eliminates the need for mortar bedding. Includes a roll of ventilated membrane, clips and screws. Now required by Building Regulations on new builds and extensions for better ventilation and durability.

Also: dry verge caps, dry verge units, verge caps, dry verge kit, Klober Uni-Line, Danelaw dry verge, Con6+

Also: dry firing, empty magazine firing, running out of nails damage

Damage caused by firing a nail gun without nails in the magazine, which can harm the driver blade, piston, and internal seals. Modern nailers have a lockout mechanism (typically activating when 3–7 nails remain) to prevent accidental dry-fire. Homeowners hiring a nail gun need to understand this lockout, what it means when the tool stops firing, and why they should not force the tool or override the safety.

Also: plasterboard screws, black phosphate screws, collated screws

Black phosphate-coated self-tapping screws designed specifically for fixing plasterboard to timber or metal studs. Bugle head sits flush without tearing the paper face. Available in 25mm (9.5mm board), 38mm (12.5mm board to timber), 42mm (15mm board to timber). Use 32mm for metal studs.

Also: external vent cowl, wall vent grille, non-return valve vent, louvred vent cowl

The external termination fitting for a ducted extraction run. Non-return valve type prevents back-draught and rain ingress when the extractor is off.

Also: duct lagging, duct pipe insulation, ventilation duct wrap

Insulation applied to ducting that passes through unheated ceiling voids or loft spaces. Prevents condensation forming inside the duct due to temperature differential.

Also: extraction ducting, cooker hood ducting, extractor ducting

Extraction ducting for cooker hoods and mechanical ventilation. 150mm rigid duct is the industry standard; flexible duct is for short transitions only because its ribbed interior cuts airflow by roughly 25m³/h per metre. External runs need a non-return cowl to stop back-draught, and runs through unheated voids need insulation wrap to prevent internal condensation.

Also: Class M extractor, power tool vacuum, workshop dust extractor, M-class dust extractor

A Class M-rated extraction unit that connects to power tool dust ports (router, mitre saw, sander) to capture fine dust at source. Legally required under COSHH when machining MDF indoors. The primary dust control measure; a dust mask is secondary.

Also: FFP2 mask, P2 respirator

A disposable face mask that filters at least 94% of airborne particles. Suitable for general dust from cutting timber, plasterboard, and light masonry work. Not sufficient for silica dust from cutting concrete or engineered stone.

Also: FFP3 mask, P3 respirator, FFP3 respirator

A higher-grade disposable face mask that filters at least 99% of airborne particles. Required for cutting concrete, brick, stone, and any material that produces silica dust. Also recommended for stripping old paint that may contain lead.

Also: drop cloth, decorators dust sheet, canvas dust sheet, polythene dust sheet

Used to protect floors, kitchen units, and finished surfaces during painting. Essential in a new extension where kitchen installation may be complete before final painting passes, protecting finished floors and appliances from paint splatter.