Building Terms Glossary

Also: thermal transmittance

A measure of heat loss through a building element; lower is better

Also: developed surface area U-value, rooflight Ud-value, projected area U-value

Developed surface area U-value used for flat rooflights (as opposed to pitched roof windows which use standard U-values). AD L 2021 Table 4.2 sets different limits: 1.4 W/m²K for pitched roof windows assessed as windows, 2.2 W/m²K for flat rooflights using Ud-value methodology.

Also: screed commissioning protocol, underfloor heating temperature ramp-up, UFH force-drying protocol, UFH screed break-in

Step-by-step temperature ramp-up process required before tiling over UFH screed. 25C for 3 days, 45C for 4 days, max design temp for 7 days per BS 1264 Part 4.

Also: underfloor heating pipe, UFH barrier pipe, PEX-a pipe, EVOH barrier pipe, 16mm UFH pipe

16mm PEX-a or EVOH barrier pipe supplied in coils of 100-200m, laid in continuous loops for wet underfloor heating systems and pressure-tested before screed.

Also: underfloor heating clip rail, UFH staple rail, pipe fixing rail, UFH pipe staple, underfloor heating pipe clips

Snap-in plastic clip rails that hold UFH pipe at the correct spacing (100-300mm) directly on top of insulation before screed is poured. The combined build-up height of pipe plus rail is typically 20mm, which feeds directly into floor level calculations.

Also: UFH pipe pressure test, underfloor heating pressure test, pressure test before screed

Pressure test of UFH pipes at 5-6 bar before and after screed is poured. An NHBC Standards hold point. Skipping it means leaks are only discovered after screed is set, requiring full break-out to repair.

Also: ukplanningmaps.com

Alternative OS-licensed map provider for planning applications with 2026 pricing confirmed.

Also: unbonded floor screed, floating screed, screed on DPM

Screed laid over DPM or insulation without bonding to the slab. Minimum 75mm thickness per BS 8204-1. Standard for modern extensions with Part L compliance.

Also: LED strip light, under-cabinet lighting strip, worktop LED strip, cabinet lighting, LED tape light

A low-voltage flexible LED strip mounted to the underside of kitchen wall units, providing task lighting over the worktop. Requires a 12V or 24V driver concealed above the cabinet run, wired from a switched FCU or dedicated circuit provisioned at first fix.

Also: under-unit lighting, under-cupboard lighting, worktop lighting, LED strip lighting kitchen

LED lighting mounted under wall units to illuminate the worktop surface. Requires a cable run behind the wall unit position at first fix before units are erected.

Also: roof window underfelt collar, Velux underfelt collar, membrane collar, roof window membrane kit, window underlayer collar

Manufacturer-specific junction membrane that bridges between the breathable roof membrane and the roof window frame. Must be the correct type for the window product (Velux and Fakro collars are not interchangeable). Breathable membrane must be dressed beneath the collar, not on top, turned up 75mm against a fixing batten per DuPont Tyvek technical manual.

Also: UFH manifold, heating manifold

Central distribution point for wet underfloor heating loops, typically wall-mounted 400-600mm above finished floor level and connected to the heat source before screed is poured.

Also: drainage bends, drainage junctions, 110mm fittings, underground fittings, push-fit drainage fittings

The bends, junctions, reducers, couplers, and adaptors that connect lengths of 110mm uPVC underground drainage pipe. Includes 45-degree and 90-degree bends, Y-junctions, T-junctions, couplers, reducers, access pipes, and rodding eyes. Push-fit ring-seal connections are standard for domestic drainage. Every change of direction and every junction requires a fitting; fittings accumulate quickly on a complex drainage layout.

Also: under-insurance, average clause

The condition of being insured for less than the true rebuild cost of a property. If a property is insured for less than its rebuild cost and suffers a loss, the insurer may apply the average principle and pay only proportionately. 76–93% of UK properties are currently underinsured. A completed extension that is not reported to the insurer guarantees underinsurance.

Also: spur, socket spur, ring main spur, ring circuit spur

A branch cable tapped directly from a ring circuit to feed an additional socket, without a fuse at the branch point. An unfused spur can only feed one single or double socket outlet and is limited by the 2.5mm² cable's current rating. Distinct from a fused spur (fused connection unit), which limits current to 13A at the branch point.

Also: trade creditor, ordinary creditor, general creditor

A creditor with no security or collateral backing their claim — which is almost certainly your position if your builder goes bust. Unsecured creditors are paid last and often receive only pennies in the pound.

Also: drinking water tap, mains drinking tap, kitchen drinking tap, hard water tap

A tap connected to the supply pipe before any water softener, legally required under Schedule 2.10.26 of the Water Fittings Regulations to provide compliant drinking water. In hard water areas where softened water exceeds 200mg/l sodium, this tap is mandatory. The DWI advises against softened water for infant feeds.

Also: pressurised cylinder, mains-pressure cylinder, unvented cylinder, megaflo, hot water cylinder (unvented)

A hot water cylinder that stores mains-pressure hot water without a cold water header tank. Installation is legally restricted to competent persons under Part G of Building Regulations, with specific discharge pipework and safety valve requirements.

Also: fascia, fascia board, uPVC fascia board, replacement fascia, capping fascia

The board fixed to the ends of the roof rafters along the eaves, carrying the guttering and closing off the roof edge. On a new extension it is usually white or anthracite uPVC, either capping over a timber backing or full-replacement board. Standard lengths are 5m in 9mm or 16mm thicknesses and depths from 150mm to 300mm. Matched to the existing house fascia in profile and colour.

Also: built-up area, town, suburb

A built-up area including towns and suburbs. In planning, urban properties typically use 1:1250 scale location plans. Most householder planning applications are in urban areas.

Also: USB-A socket, USB-C socket, USB charging socket, USB double socket, socket with USB

Sockets with integrated USB-A or USB-C charging ports. Available in standard finishes but cost more than plain sockets. Must be specified before the electrician orders materials.

Also: meter relocation

Relocates gas and electricity meters, often required when extending

Also: Stanley knife, retractable knife, box cutter, craft knife

A retractable-blade knife for cutting plasterboard, insulation boards, roofing membrane, carpet, and plastic sheeting. The most-used cutting tool on any extension site. Trapezoidal blades are standard; hooked blades are safer for membrane and carpet.

Also: PAS 128 surveyor, underground utility detection surveyor, qualified utility locator

A specialist who carries out a PAS 128 utility survey (QL-B geophysical detection up to QL-A physical verification) when a dig is high-risk or where private services are suspected beyond a routine on-site scan. Distinct from the groundworker, who does the routine on-site CAT scan. LSBUD's own guidance directs readers to engage a qualified utility locator for private cables that records will not show.

Also: service trench, meter trench, gas trench

The excavation required to reroute underground gas and electricity supply pipes from the street main to the new meter position.