How Much Does a Home Extension Cost in London in 2025?

Extension cost per square metre (m²) — the standard metric: the standard way to express extension costs in the UK construction industry is cost per gross internal floor area (GIFA) in pounds per square metre (£/m²). This metric allows comparison between extensions of different sizes and types — but it must be used carefully: the cost per m² covers the construction cost (structure, external skin, roof, internal fit-out to a defined specification level, M&E, decoration) but does NOT include: professional fees (architect, structural engineer, planning consultant — typically 10-18% of construction cost); VAT (20% on most construction works — or 5% for certain residential renovation works); planning application fees (£258 for Householder Application); Building Regulations fees (£400-£1,200 for a Householder Application); kitchen or bathroom fit-out costs (kitchen design, units, and appliances are typically a separate budget in any extension cost estimate); external works (landscaping, new patio, fencing, gate, paths); Party Wall Act surveyor costs (£800-£2,000 per party wall per building owner's surveyor); preliminaries and site setup costs (skip hire, site toilet, insurance, project management overhead — typically built into the contractor's price). The cost per m² also varies with extension size: very small extensions (under 10m²) typically cost more per m² than larger extensions (25m²+) because the fixed setup and overhead costs (scaffolding, skip, site setup) are spread over a smaller floor area. Cost per square metre for London home extensions in 2025: these ranges reflect the London construction market in 2025 for a typical residential extension in Greater London (Zone 2-4 — inner and outer London suburbs). Basic (entry-level) specification: budget kitchen units and bathroom fittings; standard roof windows (VELUX single-glazing in a pitched roof extension); standard uPVC windows and doors (acceptable in non-conservation areas); standard decorating; standard M&E spec (boiler extension, standard radiators, no UFH, standard lighting); no structural complexity (standard strip foundations, standard brickwork construction, standard flat or mono-pitch roof). Basic single-storey rear extension: £1,500-£2,000 per m² (construction only — see exclusions above). Mid-range specification: composite stone worktop; standard-spec kitchen (e.g., Howdens or equivalent); timber bi-fold or sliding doors (uPVC or aluminium standard frame); standard roof lantern; standard M&E with underfloor heating to extension floor; timber skirting and architrave; plastered walls and painted; LED downlighters. Mid-range single-storey rear extension: £2,000-£2,800 per m². High-end (premium) specification: Crittall-style steel glazing (bi-fold or sliding doors and fixed glazing panels); fully glazed structural ridge or apex roof lantern; high-specification kitchen (Siematic, Bulthaup, or bespoke kitchen — typically a separate budget item); porcelain tile or natural stone flooring (Mandarin Stone, Artisans of Devizes); LED indirect lighting design (cove lighting, perimeter strip, pendant over island); ASHP; oak or engineered hardwood flooring; bespoke joinery. Premium single-storey rear extension: £2,800-£4,500+ per m². These ranges are construction cost only — add professional fees (10-18%), VAT (20%), and the specific additional costs noted above.

Single-storey rear extension costs London 2025: the single-storey rear extension — adding floor area to the ground floor at the rear of a London terrace or semi-detached house — is the most common extension type in London. Typical configuration: 3m-6m deep, full-width of house (3.0-5.5m), flat or mono-pitch or pitched roof, opens to a new kitchen-diner or living space. Cost estimate (construction only, ex-VAT, ex-professional fees): small (12-18m²): £18,000-£50,400 (at £1,500-£2,800/m²). Medium (20-25m²): £30,000-£70,000 (at £1,500-£2,800/m²). Large (25-35m²): £37,500-£98,000 (at £1,500-£2,800/m²). Add: architect and SE fees £5,000-£12,000; kitchen £6,000-£50,000+ (depending on specification and whether client-supplied); VAT at 20%; external works £2,000-£10,000. Total all-in budget for a medium single-storey rear extension with mid-range kitchen in London 2025: approximately £70,000-£130,000 (all-in, including professional fees, VAT, and kitchen). Double-storey rear extension costs London 2025: a double-storey extension adds floor area on both the ground floor and the first floor — providing a ground-floor kitchen-diner and a first-floor additional bedroom or bathroom above. Double-storey extension cost per m² (total GIA — combined ground and first floor): Mid-range double-storey rear extension: £2,200-£3,200 per m² (combined GIA). Cost estimate (medium 25m² per floor = 50m² total GIA, mid-range): £110,000-£160,000 construction cost (ex-VAT, ex-professional fees). Side return extension costs London 2025: the side return extension — infilling the narrow side return of a London terrace with an extension that widens the ground floor kitchen or living space — is one of the most popular extensions for Victorian terraces. Typical side return extension: 3m-8m long × 1.2-2.5m wide (the width of the side return); typically single-storey; often combined with a rear extension to create an L-shaped extended ground floor footprint. Side return extension cost (construction only, ex-VAT): £2,000-£3,000 per m² (typically more expensive per m² than a rear extension of the same area, because of the narrower access and more complex party wall conditions). A 20m² side return extension (8m × 2.5m): £40,000-£60,000 construction cost (ex-VAT, ex-professional fees). Wrap-around extension (rear + side return combined): a wrap-around extension combines the rear extension and the side return extension into a single L-shaped addition — significantly expanding the ground floor footprint of a London terrace. A full wrap-around typically adds 30-50m² of new floor area. Cost: £2,200-£3,500 per m² (construction only, ex-VAT) — the combined procurement and site costs are shared across a larger area, typically reducing cost per m² compared with separate rear + side return additions. Estimated construction cost for a 35-40m² wrap-around: £77,000-£140,000 (ex-VAT, ex-professional fees). Loft conversion costs London 2025: see london-loft-conversion-dormer-guide for full details. Approximate construction costs: Velux (no dormer — rooflights only): £18,000-£35,000. Flat-roof rear dormer (single bedroom): £30,000-£55,000. L-shaped dormer (3-bed terrace — dormer + hip-to-gable): £45,000-£80,000. Hip-to-gable (semi-detached, large loft space): £45,000-£70,000. Mansard (conservation area — full-width, max head height): £55,000-£100,000. All loft costs above: construction only, ex-VAT, ex-professional fees. Side extension (detached or semi-detached house): a conventional side extension on a detached or semi-detached house (extending the house into the driveway or side garden — NOT a terrace side return). Single-storey, mid-range: £2,000-£3,000 per m²; double-storey, mid-range: £2,200-£3,200 per m² combined GIA.

Key factors that drive extension cost variation in London: specification level: the single biggest driver of cost variation in a London extension — moving from basic (entry-level UPVC glazing, budget kitchen, basic M&E) to premium (Crittall steel glazing, premium kitchen, ASHP, UFH, premium flooring) can double or triple the cost per m². Foundation type: standard strip foundations (900mm depth in London Clay with no significant tree influence) are the baseline; deep strip foundations (1.5-2.5m in tree-influenced London Clay) or mini-pile foundations (close tree proximity) add significant cost to the groundworks. Structural complexity: a straightforward brickwork extension with a flat or mono-pitch felt roof and a standard 3m wide structural opening into the house is the baseline; increasing the opening width (wider steel beam span); adding a glazed ridge/apex lantern; including roof lights; or adding basement level works all increase cost significantly. Glazing specification: UPVC aluminium bi-fold doors are the baseline; Schuco or Reynaers aluminium bi-fold (higher quality, slimmer sight lines) adds 20-50% over UPVC; bespoke Crittall steel glazing adds 100-300% over UPVC bi-fold doors. Access: limited or no vehicular access to the rear of the property (very common in mid-terrace London properties) increases cost by 5-15% compared with a property with rear vehicular access — all materials must be carried through the house and spoil carried out through the house. London weighting: construction costs in inner London (Zone 1-2) are typically 15-25% higher than in outer London (Zone 4-6) — due to higher labour rates, congestion charges, and higher preliminary costs (parking, loading bay permits). What to watch for in contractor quotes for a London extension: like-for-like comparison: ensure all quotes cover the same scope — some contractors quote construction only (excluding M&E, flooring, and decoration); others quote a complete package including kitchen. Exclusions: always ask for the exclusions list — does the quote include: structural engineer's calculations? Building Regulations fees? Scaffolding? Skip hire? Party Wall costs? Kitchen? Appliances? External works? Provisional sums and PC sums: a provisional sum is an allowance for works whose scope or cost cannot be confirmed at the time of quoting (e.g., a provisional sum for heating system modification); a Prime Cost (PC) sum is an allowance for a specific client-supplied item (e.g., kitchen units at a PC sum of £8,000 — the contractor will manage installation; the client sources the units to that budget). Understand what provisional sums and PC sums are included in any quote and ensure you have a realistic budget for them. Contingency: budget 10-15% of the construction cost as a contingency for unforeseen works (hidden drainage, structural discoveries during groundworks, specification upgrades during the project). A well-prepared contractor will flag likely contingency areas in the quote.