Quick Answers to Common UK Building Questions

How many bags of plaster for a room?

~4 bags multi-finish (2mm skim) for a standard 3.5m x 4m room

How many plasterboard sheets for a room?

~13 sheets (2400x1200mm) for a standard room

How much render for a wall?

~3 bags sand & cement for a 3m x 2.4m wall at 15mm

How thick should render be?

Two-coat sand and cement is 14-20mm, NHBC minimum 15mm

How much dot and dab adhesive?

~1.5 bags (25kg) for a 3m x 2.4m wall

How much coving for a room?

~14m of coving plus adhesive for a 4m x 3m room

How much bonding coat?

~2 bags (25kg) for a 3m x 2.4m wall at 8mm

How much monocouche render?

~15–20kg/m² for typical 15mm application

How much lime plaster?

~12–15kg/m² for a two-coat system

How long does plaster take to dry?

Skim 2–3 days, bonding 1–2 days, rendering 3–7 days

How much paint for walls?

~5 litres for a 4m x 3m room (2 coats)

How much wallpaper for a room?

~8 standard rolls (10m x 0.53m) for a 4m x 3m room

How much primer?

~2.5L for a typical room at 10–12m²/L

How much masonry paint?

~15L for a typical 3-bed semi (2 coats)

How much ceiling paint?

~2L for a 4m x 3m ceiling (2 coats)

How much wood stain?

~10–15m²/L coverage for exterior wood stain

How much lining paper?

~6–8 rolls of 1000-grade for a 4m × 3m room

How much intumescent paint?

0.8–1.5 kg/m² for 30-min fire protection

How much floor paint?

~3L for a 15m² garage floor (2 coats)

What paint coverage should I expect?

Emulsion 12–14m²/L, gloss 16–18m²/L

How many wall tiles for a bathroom?

~100–120 tiles (300x600mm) plus 10% wastage

How much tile adhesive?

3–5 kg/m² depending on tile size

How much grout?

0.3–0.8 kg/m² for standard wall tiles

How much wet room tanking?

2–3 coats at 1–1.5 kg/m² per coat

How much tile backer board?

Measure wet area in m². Standard board 0.72m². Add 10%

How much shower tray fall?

Minimum 1:80 fall (12.5mm per metre)

What size shower enclosure?

Min 760×760mm. Standard 900×900mm or 1200×800mm

How many fence panels?

10m run = 6 panels (1.83m standard)

How many fence posts?

One more post than panels (6 panels = 7 posts)

How much concrete for fence posts?

~2 bags postcrete (20kg) per post for a standard hole, 1-3 by hole size

How many gravel boards?

Same number as panels (6 panels = 6 boards)

How many picket fence pales?

~80 pales for 10m (75mm pale + 50mm gap)

How many post and rail lengths?

Posts at 2.4–3m centres, 2–3 rails per bay

What is the maximum fence height?

2m without planning (1m fronting highway)

How many bricks for a wall?

~792 bricks for a 5m x 2.4m single-skin wall

How much concrete for a shed base?

0.72m³ for a 3m x 2.4m x 100mm slab

How many blocks for a wall?

~132 blocks for a 5m x 2.4m wall

How much concrete for a foundation?

0.135m³ per metre run (600×225mm trench)

How much mortar for a wall?

~0.03m³ per m² of single-skin brickwork

How many wall ties do I need?

2.5 per m² for standard cavity wall (900×450mm grid)

How much repointing do I need?

Wall area in m². Rate: 3–5 m²/day per bricklayer

How deep to rake out mortar before repointing?

Twice the joint width, 20mm minimum for a 10mm brick joint

How much concrete for a slab?

1.2m³ for a 4m × 3m slab at 100mm depth

What concrete mix ratio do I need?

C20: 1:2:4 (cement:sand:aggregate) or 1:5 ballast

How much reinforcement mesh?

Slab area ÷ sheet area (4.8×2.4m) + 200mm overlaps

How much DPC do I need?

Perimeter length. Rolls typically 30m. 100mm overlaps

How many bricks per m²?

60 per m² (stretcher bond, standard UK brick)

How many blocks per m²?

10 per m² (standard 440×215mm block)

How much gravel for a driveway?

~1.28 tonnes for a 5m x 3m driveway at 50mm depth

How many paving slabs for a patio?

~35 slabs (600x600mm) for a 4m x 3m patio

How much topsoil for a garden?

~10.5 tonnes for a 10m x 5m garden at 150mm depth

How much drainage gravel do I need?

~1.5 tonnes of pea gravel for a 10m drain run

How much decking do I need?

~18 boards + 7 joists for a 4m x 3m deck

How much turf?

~53 rolls for a 50m² lawn + 5% waste

How much artificial grass?

~26.4m² for a 6m x 4m garden (+ 10% waste)

How much bark mulch?

0.75m³ (~0.6 tonnes) for 10m² at 75mm depth

How much aggregate do I need?

~0.8 tonnes for 10m² at 50mm depth (MOT Type 1)

How much does a tarmac driveway cost?

£40–70/m² professionally laid

How much weed membrane do I need?

Ground area + 150mm overlap at joins

How much pond liner do I need?

(L + 2×depth + 0.6m) × (W + 2×depth + 0.6m)

How much block paving do I need?

~50 blocks per m² (200×100mm). 20m² = ~1,000 blocks

How many roof tiles per square metre?

10-60 tiles/m² depending on type

How much felt and batten for a roof?

4 rolls of felt + 145m battens for a 50m² roof

How many roof tiles for a roof?

~500-600 interlocking tiles for a 50m² roof

How many ridge tiles?

~18 ridge tiles for an 8m ridge (450mm each)

How much lead flashing?

Measure total linear metres of abutments and valleys

How many gutter brackets?

One every 900mm max (7 for a 6m run)

How much fascia and soffit?

20–25m total for a typical 3-bed semi

How much flat roof felt / membrane?

Area + 150mm overlaps + 75mm edge upstands

What roof pitch do I need?

30° common UK domestic. Min 15° interlocking tiles

How much roof insulation?

270mm mineral wool (loft) or 120–150mm PIR (rafters)

What roof ventilation is needed?

Cold roof: 10mm eaves + 5mm ridge. Warm: none with VCL

How many roof trusses?

~18 trusses for 10m building at 600mm centres

How much dry ridge?

Ridge length in metres. Kits in 6m or 12m packs

How many tongue and groove boards do I need?

~20 boards (119mm cover) for a 3m x 2.4m wall

How many studs for a stud wall?

9 studs for a 3m wall at 400mm centres

How many spindles?

Gap must be <100mm (Building Regs). ~1 per 100–120mm

How many door linings?

One set per door: 2 legs + 1 head (32×115mm)

How much timber cladding?

Wall area + 10–15% for wastage and overlap

How much skirting board?

~12.5m for a 4m x 3m room after deducting doors

How many balustrade spindles?

Max 100mm gap (Building Regs). ~110–120mm centres

How much picture rail?

~12–13m for a 4m × 3m room (perimeter minus breaks)

How much dado rail?

~12.5m for a 4m × 3m room. Fitted at ~900mm height

How many packs of laminate flooring?

~8 packs (2.0m²) for a 4m x 3.5m room

How much screed for a floor?

0.78m³ for a 4m x 3m floor at 65mm depth

How much floor levelling compound?

~4 bags (90–120 kg) for a 20m² room at 3mm

How many floor tiles?

~37 tiles (600x600mm) for a 12m² room

How much vinyl flooring?

~13.2m² for a 4m x 3m room (+ 10% waste)

How many skirting boards?

~5 lengths (3m) for a standard room

How much carpet?

~14m² for a 4m x 3.5m room, 4m roll width

How much underlay?

Same as floor area + 50mm overlap. Standard roll 15m²

How much floor adhesive?

Vinyl 0.3–0.5 kg/m², engineered wood 1–1.5 kg/m²

How much self-levelling compound?

1.5–1.8 kg/m² per mm. 20m² at 5mm ≈ 6 bags

How much land drain?

Trench length + 10% for junctions, 80–100mm pipe

How much gravel for a soakaway?

1–3m³ clean stone for a typical domestic soakaway

How many inspection chambers?

Every direction change, junction, and 45m on straight runs

What size manhole do I need?

450mm dia. up to 600mm deep. 1200mm for >3m depth

What drainage gradient do I need?

110mm foul: 1:40. Surface water: 1:100. Part H

What size rainwater downpipe?

68mm round for up to 50m² roof area. 110mm for larger

How many radiators per room?

1 for small rooms, 2 for large living areas

How much water does a radiator hold?

About 7 litres for a 600 x 1200mm double panel radiator

How much underfloor heating pipe?

~100–120m for a 20m² room at 200mm centres

How much pipe insulation?

~20–30m for a typical 3-bed house

What size boiler do I need?

24–27kW combi for 1 bathroom, 28–34kW for 2

What size hot water cylinder?

120L for 1–2 people, 180L for 3–4, 250L+ for 5+

How much inhibitor?

1 litre per 100L system volume. Check product data

How much loft insulation do I need?

~6 rolls of mineral wool for an 8m x 6m loft

What U-value do I need?

Part L 2021 new-build: walls 0.26, roof 0.16 (pitched) / 0.18 (flat), floor 0.18

What changes in the 2026 Part L update?

From 24 Mar 2027: low-carbon heating and solar PV for new homes; extension U-values unchanged

What insulation thickness do I need?

Walls 90–100mm PIR, roofs 120–150mm, floors 70–100mm

How much cavity wall insulation?

~80–100m² for a typical 3-bed semi

How much floor insulation?

Ground floor area, 70–100mm PIR boards

How much pipe lagging?

20–30m for a typical 3-bed house. 1m cut lengths

What EPC rating do I need?

A (92–100) to G (1–20). UK average: band D

How much draught proofing?

Perimeter of doors (~5–6m each) and windows

How much loft boarding?

~12 boards (2400x600mm) for a 15m² area

What size dormer do I need?

Min. 2.3m head height, 1.8–3.6m wide

What does a loft conversion cost?

Velux £15–40K, dormer £30–60K, mansard £45–80K

What does a dormer cost?

Small £20–30K, medium £30–45K, large £45–60K

How much loft ventilation?

10mm continuous gap at eaves. Cross-ventilation for cold roofs

How much does damp proofing cost?

DPC injection £40–90/m. Whole house £800–5,200. Survey first

How much tanking membrane?

Wall + floor area + 10% overlaps, ~£15–25/m²

How much timber treatment?

4–5m²/L spray coverage, 4–5L for 20m²

How many plasterboard sheets for a wall?

3 boards (2400x1200mm) for a 3m x 2.4m wall

How many metal studs?

6 studs + top/bottom track for a 3m wall

How much jointing compound?

~0.5 kg/m² of plasterboard, ~15 kg for 30m²

What does a rear extension cost?

£1,500–2,200/m² single storey typical (2026 UK)

What size extension under permitted development?

Detached: 8m, semi/terrace: 6m (single storey)

What does window replacement cost?

uPVC casement £350–550 fitted, timber sash £1,200–3,500

What does secondary glazing cost?

Supplied and fitted £250–700 per window

How much sealant for windows?

~3m of sealant per window (310ml cartridge = 8–10m)

How many trickle vents do I need?

8,000mm² per habitable room, 4,000mm² per wet room (Part F 2021)

What window U-value do I need?

Part L: 1.2 W/m²K new build, 1.4 replacement

How many bricks on a pallet?

~500 facing bricks or ~300 engineering bricks

How big is a skip?

Mini 2yd³ (20 bags) to Maxi 16yd³

What does planning permission cost?

Householder £548 (England, from April 2026)

How much construction waste will I generate?

Typical extension: 5–10m³ waste

What bathroom suite spacing do I need?

Minimum 700mm clear in front of WC and beside bath

What does bathroom tiling cost?

Full bathroom in ceramic £900–2,050

What is the minimum en-suite size?

Minimum ~1.5m × 2.2m. Comfortable: 1.8m × 2.7m

What are standard worktop dimensions?

600mm deep, 900mm high, stocked in 3.0m, 3.6m and 4.1m lengths

What does a basement conversion cost?

£1,500–3,000/m². Typical 30m²: £45,000–90,000

What does basement waterproofing cost?

Cavity drain £80–150/m². Tanking slurry £30–60/m²

What does demolition cost?

House £80–115/m². Garage £1,000–3,000. Chimney breast from £1,500

How much demolition waste?

Strip-out: ~0.5–1m³/room. Full demo: 0.5–1 t/m²

What does an asbestos survey cost?

Management £200–750 by house size. R&D £300–700

What does a strip-out cost?

Kitchen £500–1,500. Bathroom £300–800

What does scaffold hire cost?

Full house wrap £1,500–2,600 incl. erection and hire term

How many scaffold ties do I need?

One per 16m² (4m × 4m grid). TG20 compliance

What size extract fan do I need?

Bathroom 15 l/s; kitchen 30 l/s adjacent to hob (ducted cooker hood over hob) or 60 l/s elsewhere (no hood / recirculating), 13 l/s continuous

What air change rate do I need?

Domestic rooms: 0.5–1.5 ACH

What size MVHR unit do I need?

Typical 3-bed house: 60–80 l/s unit

What duct size do I need?

100mm bathrooms (15 l/s), 150mm kitchens (30 l/s)

What kitchen extraction rate?

30 l/s adjacent to hob (ducted cooker hood over hob) or 60 l/s elsewhere in kitchen (no hood / recirculating), 13 l/s continuous (ADF 2021 Table 1.1)

What cooling load do I need?

~100W/m² office. 50–80W/m² domestic

How many solar panels for my roof?

10–12 panels (3.5–4kW) for typical 3-bed house

What does a heat pump cost to install?

Air source £8,000–14,000. BUS grant £7,500

What size solar battery do I need?

5–10kWh for a typical home

What size steel beam do I need?

152×89 UB to 203×133 UB for 2–4m domestic spans

What size steel column do I need?

Typically 100×100 UC or 152×152 UC domestic

What does structural steelwork cost?

RSJ installed £450–1,500 by span

How do you size a steel beam for an extension?

The calculation method: span, load, bending, deflection

What is the embodied carbon of building materials?

Concrete ~100–150 kgCO₂e/m³. Steel ~1.5–2.5 kgCO₂e/kg

How much rainwater can I harvest?

50,000–80,000 litres/year for a typical UK house

What does a green roof cost?

Sedum extensive £60–100/m², intensive £120–250/m²

What day rate should I charge?

Labourer £120–160, skilled £180–280, specialist £250–400

What profit margin should I target?

Materials 15–25%, labour 30–50%. Net profit 10–20%

How much does van mileage cost?

HMRC 45p/mile (first 10,000), 25p after. Actual 35–55p

Do I need a COSHH assessment?

Yes, for any work with hazardous substances (COSHH 2002)

Do I need a noise level assessment?

Required if daily exposure likely above 80 dB(A)

Do I need CDM notification?

>30 working days with >20 workers, or >500 person days

What wheelchair ramp gradient?

Max 1:12 for short ramps. Preferred 1:20. Part M

What accessible door width?

775mm min effective clear width (new build)

What corridor width for wheelchair?

1,200mm minimum. 1,800mm for passing point

How many smoke detectors?

Min 1 per floor level. BS 5839-6 Grade D for new builds

What fire door rating?

E 20 inside a house. FD30S for garage and flat entrance doors. BS 476 / EN 1634

How many emergency exits?

1 for ≤60 persons, 2 for 60–600. Max travel 18–45m

How thick is plasterboard?

9.5mm ceilings, 12.5mm standard walls, 15mm fire/acoustic board

How thick can you apply multi-finish plaster?

2mm skim finish coat (up to ~3mm built up in two passes)

How many air bricks do I need?

Subfloor ventilation free area (Approved Document C + BS 5250)

How many composite slates do I need?

From cover width and gauge, plus a waste allowance

How much does it cost to board a loft?

Indicative UK per-m² supply and fit (varies by access and spec)

How many fence slats per metre?

Feather-edge board face width minus the overlap

How much do patio doors cost?

Indicative supply and fit by type (sliding, French, bifold)

What size lintel do I need?

Set by span, load and wall — from engineer or manufacturer tables

How is cable size calculated?

The BS 7671 factors — designed and signed off by an electrician

How do you calculate electrical maximum demand?

Connected load after diversity, per IET On-Site Guide Table A2

What is the correct render mix ratio?

Scratch coat 4:1 to 5:1, float coat 5:1 to 6:1 sand:cement, each coat weaker

How deep should drainage pipes be buried?

600mm min under gardens, 900mm under drives, for standard plastic pipe

What cable size for an electric shower?

10mm² on a 45A breaker for 9.5kW; design and sign-off by an electrician

What are standard fascia and soffit board sizes?

Fascia 150-250mm deep, 16-18mm thick, 5m lengths; soffit cut to width

How high should a dado rail be?

900-1,000mm on a 2.4m ceiling (wall height ÷ 5 × 2 convention)

What are accessible lift car dimensions?

1100 × 1400mm minimum car with 900mm clear door, per Part M

What joist spacing for decking?

400mm centres for standard boards; raised decks need structural design

How much does artex removal cost?

Skim over £15-£30/m²; asbestos-positive removal £400 to several thousand by method

How much does a percolation test cost?

Roughly £300-£800 for a BRE 365 test, more with a full site assessment

How much does repointing cost per m²?

Sand:cement £30-£65/m², lime £60-£110/m²; scaffold and chimneys add more

How much does rendering cost per m²?

Sand:cement £30-£80/m², monocouche £50-£95/m² supply and fit

How much does a sewer connection cost?

Application fee plus excavation; road crossings run £7,000-£35,000+

How much does mastic asphalt cost?

Roofing £60-£90/m² installed (two-coat 20mm); flooring £40-£70+/m²

What intumescent paint thickness for a fire rating?

DFT depends on rating and section factor; always per the certified assessment