House Extension Costs in London in 2025: What to Budget for Every Extension Type

Rear extension costs, side extension costs, and the main cost drivers for London house extensions in 2025: COST STRUCTURE FOR A LONDON HOME EXTENSION — HOW TO THINK ABOUT IT: a London home extension cost is best understood as a combination of: (1) STRUCTURE COST (the building shell — foundations, walls, roof, structural beams, windows and doors, weatherproofing and insulation): this is the cost of the building work to get to a WEATHERTIGHT SHELL; (2) FIT-OUT COST (the internal fit-out — first fix services, plastering, flooring, finishes, decoration, and any kitchen or bathroom fit-out if the extension houses these): this is the cost of making the shell into a finished habitable space; most EXTENSION QUOTES from contractors cover primarily the STRUCTURE COST; the FIT-OUT COST is often estimated separately or excluded; this is an important distinction because a STRUCTURE-ONLY EXTENSION QUOTE (weathertight shell) may appear affordable, but the FIT-OUT COST to make it into a finished kitchen or living space may add an additional 40-100% to the structure cost; REAR EXTENSION COSTS IN LONDON 2025: SINGLE-STOREY REAR EXTENSION (SHELL AND FIT-OUT — NO KITCHEN, NO BATHROOM): SMALL (10-15m²): £75,000-£110,000 all-in (structure + first fix services + plastering + basic flooring + decoration + structural opening to existing house + structural engineer); MEDIUM (20-30m²): £90,000-£140,000 all-in; LARGER (30-40m²): £120,000-£180,000 all-in; ALL-IN INCLUDING KITCHEN FIT-OUT (TIER 2 KITCHEN, PORCELAIN FLOOR WITH UFH — 20-30m²): £145,000-£250,000; ALL-IN INCLUDING PREMIUM KITCHEN FIT-OUT (TIER 3 KITCHEN — 20-30m²): £200,000-£350,000+; WHAT IS INCLUDED IN AN ALL-IN EXTENSION PRICE: DEMOLITION AND SITE CLEARANCE: demolition of the existing rear outrigger or structure; removal of debris; skip hire and disposal; FOUNDATIONS: strip foundations or reinforced concrete pad foundations (depending on London Clay depth and structural engineer's specification — London Clay foundations are often deeper than in other parts of the UK because the clay is susceptible to SWELLING AND SHRINKAGE with moisture, which can cause heave — foundations in London Clay are typically 1.0m-1.5m deep for a single-storey extension, and 1.5m-2.5m where trees are present in London Clay); SUBSTRUCTURE: DPC, ground-bearing concrete slab (with insulation — minimum 75mm PIR for Part L 2021 compliance); EXTERNAL WALLS: facing brick, render, or timber cladding (material affects cost — see below); ROOF: flat roof (warm roof construction — PIR insulation, GRP or EPDM membrane, flashings); or pitched roof (felt, battens, slates or tiles to match existing); STRUCTURAL OPENINGS: structural steel beam in the rear wall of the existing house, temporary propping, making good; GLAZED REAR ELEVATION: bi-fold doors or sliding doors; WINDOWS AND ROOFLIGHTS: new windows and rooflights as designed; FIRST FIX PLUMBING AND ELECTRICAL: new plumbing supply and waste connections; new electrical circuits from the consumer unit to the extension; INSULATION: cavity wall insulation (Part L); roof insulation (Part L); floor insulation (Part L); DAMP PROOF COURSE AND TANKING: DPC to all new external walls; PLASTERING: plaster or plasterboard to all new internal walls and ceilings; FLOORING (FIRST FIX): structural floor screed or substrate preparation; BUILDING REGULATIONS AND STRUCTURAL ENGINEER: structural engineer's design fees; Building Regulations fees; WHAT IS OFTEN NOT INCLUDED IN AN EXTENSION QUOTE: KITCHEN FIT-OUT (the kitchen itself is usually priced as a separate package — kitchen designer, kitchen supply, kitchen installation); BATHROOM OR WET ROOM FIT-OUT (where the extension houses a bathroom, this is often a separate package — sanitaryware supply, tiling, waterproofing); FLOOR FINISHES (the extension quote typically includes the structural screed or substrate; the floor covering — engineered hardwood, porcelain tile — is often a separate supply-and-fit cost); DECORATION (the main contractor's quote typically includes a mist coat; final decoration is often left to the client or is a separate cost); PARTY WALL SURVEYOR FEES: party wall notices and surveyor fees are typically NOT included in the contractor's quote — these are a client cost (approximately £2,500-£12,000 depending on the number of adjoining owners); PLANNING APPLICATION FEES AND ARCHITECT'S FEES: typically separate (planning fee: £258 (2025); architect: 10-15% of construction cost); STRUCTURAL ENGINEER'S FEES: occasionally included in the contractor's quote; more often a separate client cost (approximately £1,500-£3,500); MATERIALS THAT SIGNIFICANTLY AFFECT EXTENSION COST IN LONDON: FACING BRICK (MATCHING LONDON STOCK BRICK OR YELLOW STOCK BRICK): if the extension walls are to be built in FACING BRICK matching the existing Victorian London Stock brick (a characteristic yellow-orange brick — London Stock or a similar modern equivalent), the bricklayer cost and the brick cost are significantly higher than for a BLOCKWORK AND RENDER or BLOCKWORK AND CLADDING construction; matching-quality hand-made brick: supply approximately £800-£1,500 per 1,000 bricks; a 20-30m² extension might use 5,000-10,000 facing bricks; brick cost alone: approximately £4,000-£15,000; TIMBER CLADDING: cedar, larch, or Kebony cladding is increasingly popular on London rear extensions as an alternative to brick; supply and fix approximately £60-£120/m² (timber cladding); GLAZING SPECIFICATION: the cost of the glazed rear elevation (bi-fold vs sliding vs frameless glass) varies significantly — see KITCHEN EXTENSION GUIDE for details; SIDE EXTENSION COSTS IN LONDON 2025: a SIDE EXTENSION uses the space between the SIDE ELEVATION of the house and the BOUNDARY; in London semi-detached houses, the side passage between the house and the boundary is typically 900mm-2.0m wide; a SIDE RETURN EXTENSION uses this side passage space (often combined with a rear extension as part of a SIDE AND REAR WRAP-AROUND EXTENSION): SIDE RETURN EXTENSION (INFILL OF SIDE PASSAGE — 10-15m²): £60,000-£95,000 all-in (structure only, no kitchen fit-out); SIDE AND REAR WRAP-AROUND EXTENSION (25-40m², combined side and rear): £110,000-£180,000 all-in (structure + first fix, no kitchen fit-out); FULL-WIDTH SIDE EXTENSION (TERRACED HOUSE EXTENDING TO THE SIDE BOUNDARY — FULL STOREY, 15-25m² per floor): typically requires planning permission; structure cost: approximately £90,000-£160,000 for a single-storey full-width side extension (20-25m²).

Double-storey extension costs, wrap-around extension costs, professional fees, foundation costs, and what makes a London extension quote comprehensive in 2025: DOUBLE-STOREY REAR EXTENSION COSTS IN LONDON 2025: a DOUBLE-STOREY REAR EXTENSION adds accommodation on both the GROUND FLOOR and the FIRST FLOOR above; the first floor addition is typically a new bedroom and/or bathroom above the ground floor extension; DOUBLE-STOREY REAR EXTENSION (15-20m² per floor — total 30-40m²): STRUCTURE ONLY (shell — no kitchen, no bathroom): £130,000-£200,000; STRUCTURE + GROUND FLOOR KITCHEN FIT-OUT (Tier 2) + FIRST FLOOR BATHROOM: £180,000-£280,000; DOUBLE-STOREY REAR EXTENSION (20-25m² per floor — total 40-50m²): STRUCTURE ONLY: £160,000-£250,000; ALL-IN (structure + kitchen + bathroom): £220,000-£350,000+; WHY DOUBLE-STOREY IS MORE EXPENSIVE THAN SINGLE-STOREY × 2: a double-storey extension does not simply cost twice the single-storey equivalent because: the FOUNDATIONS must carry twice the load (deeper, wider, or more heavily reinforced); the STRUCTURAL FRAME must resist lateral forces from the two-storey height; SCAFFOLDING is required at greater height (higher cost and longer hire period than single-storey); PARTY WALL WORKS are more complex (raising the party wall above the existing first floor level requires a more complex party wall agreement and potentially more substantial structural works to the party wall); the STAIRCASE from the existing first floor to the new first floor extension bedroom requires coordination with the existing first floor layout; FIRE SAFETY: if the extension creates additional habitable accommodation on the first floor of a house that already has first-floor bedrooms, the fire safety assessment must be updated; WRAP-AROUND EXTENSION COSTS IN LONDON 2025: a WRAP-AROUND EXTENSION combines a FULL SIDE EXTENSION and a REAR EXTENSION in an L or C shape around the original rear corner of the house; the result is a significantly larger ground floor footprint; SINGLE-STOREY WRAP-AROUND (SIDE + REAR — 30-50m²): STRUCTURE ONLY: £140,000-£220,000; ALL-IN WITH KITCHEN FIT-OUT (Tier 2): £200,000-£320,000; DOUBLE-STOREY WRAP-AROUND (SIDE + REAR — 50-80m² total): STRUCTURE ONLY: £200,000-£350,000; ALL-IN: £280,000-£500,000+; COST PER M² FOR LONDON EXTENSIONS IN 2025 — USEFUL BENCHMARKS: structure-only cost per m² (shell — walls, roof, floor slab, glazing, first fix): approximately £3,500-£6,000/m² for a standard quality single-storey London rear extension; a MID-RANGE FIT-OUT ADDS approximately £1,000-£2,500/m² to the structure cost (fit-out including plastering, flooring, decoration — excluding kitchen and bathroom); a KITCHEN FIT-OUT (Tier 2 — separate from the structure) adds approximately £1,500-£2,500/m² of kitchen area (supply + install + all M&E connections); FOUNDATION COSTS IN LONDON CLAY — A SIGNIFICANT LONDON-SPECIFIC COST DRIVER: LONDON CLAY FOUNDATION DEPTHS: London Clay covers approximately 60-70% of Greater London's sub-soil; London Clay is highly susceptible to SHRINKAGE when dry (particularly in periods of drought or where trees extract moisture from the clay) and HEAVE when re-wetted; BHMA (British Homes and Mortgage Advice) and NHBC guidance: in areas of London Clay, foundations must be at least 1.0m deep for a single-storey extension WHERE NO TREES ARE WITHIN INFLUENCE DISTANCE (approximately 1-1.5× tree height from the foundation for most species); where TREES ARE WITHIN INFLUENCE DISTANCE OF THE FOUNDATION: foundations must be taken to a depth recommended by a SOILS ENGINEER or STRUCTURAL ENGINEER — typically 1.5m-3.0m or more depending on the species and size of the tree and the proximity; MINI-PILES OR DRIVEN PILES: where the clay is very deep, trees are very close, or the soil profile is complex, the structural engineer may specify MINI-PILES (concrete or steel) rather than strip or pad foundations; mini-pile cost adds approximately £5,000-£20,000+ to the extension cost (depending on the number and length of piles); CONCRETE STRIP FOUNDATION COST: the foundation cost for a single-storey London rear extension (20-30m²) in standard London Clay conditions (no trees, standard 1.0-1.2m depth): approximately £8,000-£18,000 (excavation, temporary shoring where required, concrete pour, rebar, inspection); TREE ROOT CONSIDERATIONS: where trees with PRESERVATION ORDERS (TPOs) are nearby, the extension must be designed so that no tree roots are severed or materially damaged by the excavation; a TREE SURVEY and ARBORICULTURAL IMPACT ASSESSMENT may be required by the LPA as part of the planning application; PROFESSIONAL FEES FOR LONDON HOUSE EXTENSIONS (2025): ARCHITECT OR ARCHITECTURAL TECHNICIAN: FEASIBILITY AND PLANNING DRAWINGS ONLY (no tender drawings, no construction drawings): approximately 5-8% of construction cost; or fixed fee approximately £2,500-£6,000 for a standard single-storey rear extension; FULL ARCHITECTURAL SERVICES (PLANNING THROUGH CONSTRUCTION DRAWINGS AND CONTRACT ADMINISTRATION): approximately 12-18% of construction cost; STRUCTURAL ENGINEER: standard single-storey extension (design certificate and drawing): approximately £1,500-£3,500; complex (double-storey, basement, unusual structural conditions): approximately £3,000-£8,000; BUILDING REGULATIONS APPLICATION (FULL PLANS): Local Authority Building Control (LABC) fee: approximately £400-£1,000 for a single-storey extension; Approved Inspector fee (private Building Control): approximately £500-£1,200; PLANNING APPLICATION FEE (HOUSEHOLDER APPLICATION): £258 (2025); PRIOR APPROVAL APPLICATION (NEIGHBOUR CONSULTATION SCHEME): approximately £206 (2025); LAWFUL DEVELOPMENT CERTIFICATE (LDC): approximately £258 (2025); PARTY WALL SURVEYOR: agreed surveyor (both owners share one surveyor): approximately £900-£2,000 per owner (2 owners = £1,800-£4,000 total); individual surveyors (each adjoining owner appoints their own): approximately £900-£2,500 per surveyor × 2-4 surveyors = £1,800-£10,000 total; TOTAL PROFESSIONAL FEES FOR A STANDARD LONDON REAR EXTENSION (PLANNING PERMITTED DEVELOPMENT — NO PLANNING REQUIRED; STANDARD STRUCTURAL SPEC): approximately £5,000-£12,000; FOR AN EXTENSION REQUIRING FULL PLANNING AND COMPLEX STRUCTURAL SPEC: approximately £10,000-£25,000+; GETTING A RELIABLE EXTENSION QUOTE IN LONDON — WHAT TO ASK: to get a COMPARABLE AND RELIABLE QUOTE from London contractors, the client should: obtain FULL ARCHITECTURAL DRAWINGS AND A STRUCTURAL ENGINEER'S DESIGN before requesting quotes; include a DETAILED SCHEDULE OF WORKS or SPECIFICATION (produced by the architect or project manager) that all contractors price against the same document; request that the QUOTE STATES WHAT IS INCLUDED AND EXCLUDED CLEARLY; ask for the FOUNDATION DEPTH assumption (if the contractor quotes for 900mm foundations but London Clay or tree proximity requires 1.5m, the difference in cost could be £5,000-£15,000); ask for the quote to be FIXED PRICE or COST-PLUS (a fixed price quote protects the client against cost overruns from the contractor, but requires complete drawings — cost-plus is appropriate where the scope is incomplete); confirm whether PARTY WALL, PLANNING, AND PROFESSIONAL FEES ARE INCLUDED OR EXCLUDED.