Home Extension Costs in London in 2025: Single Storey, Double Storey, Side Return, and Wrap-Around

Realistic costs for single-storey rear extensions, double-storey extensions, and side return extensions in London in 2025 — full breakdowns by trade and element, cost per m², and what is included: IMPORTANT PREFATORY NOTE ON EXTENSION COST ACCURACY: the figures in this guide are stated in Q3 2025 LONDON MARKET PRICES; they reflect: current material prices (steel, timber, concrete, brickwork, render, roofing, glazing — all significantly higher in London 2025 than 2021 national averages); LONDON LABOUR RATES (Inner London: Islington, Hackney, Camden, Lambeth, Southwark — labour approximately 25-40% above national average; Inner-Outer London: Ealing, Wandsworth, Lewisham — approximately 15-25% above; Outer London: Bromley, Sutton, Havering, Enfield — approximately 5-15% above); TYPICAL PROJECT SCOPE includes: all structural works (foundations, structural steel, walls, roof); external envelope complete (external doors and windows); internal first fix (plumbing, electrics, insulation, screeding); internal second fix (boarding, plastering, decoration); excludes: KITCHEN FIT-OUT (separate allowance), FLOOR FINISHES (separate allowance), FURNITURE AND SOFT FURNISHINGS; ALL PROFESSIONAL AND DESIGN FEES (architect, structural engineer, party wall surveyor, Building Control) are QUOTED SEPARATELY; SINGLE-STOREY REAR EXTENSION — LONDON 2025 COSTS: WHAT IS A SINGLE-STOREY REAR EXTENSION: a ground floor extension to the rear of the property, extending the footprint of the house into the garden; typical dimensions: 3.0m-6.0m depth × 3.5m-6.0m width; most commonly extending the kitchen or creating a new open-plan kitchen-dining-living space; INTERNAL FLOOR AREA RANGE: typically 10m²-36m² (3×3 = 9m²; 6×6 = 36m²); COST RANGE FOR A TYPICAL LONDON SINGLE-STOREY REAR EXTENSION (4M DEEP × 4M WIDE = 16M² EXTENSION): BUDGET TO MID-RANGE (STANDARD MATERIAL SPECIFICATION — CAVITY BRICK/BLOCK WALLS, FLAT WARM ROOF WITH SOLID POLYCARBONATE ROOFLIGHT OR SINGLE VELUX, BIFOLD OR SLIDING ALUMINIUM DOORS, STANDARD SCREED FLOOR): approximately £55,000-£80,000 for the extension structure (excluding professional fees, kitchen, and floor finishes); MID-RANGE TO HIGH SPECIFICATION (BRICK-CLAD CAVITY WALLS, STANDING SEAM ZINC OR POWDER-COATED ALUMINIUM ROOF CLADDING, LARGE ROOF LANTERN OR MULTI-PANE FLAT ROOF GLAZING, HIGH-END BIFOLD OR SLIDE-AND-FOLD DOOR SYSTEM): approximately £75,000-£120,000 for the extension structure; COST PER M² OF EXTENSION FLOOR AREA (LONDON 2025 — SINGLE-STOREY): budget: approximately £2,500-£3,500/m²; mid-range: approximately £3,500-£5,000/m²; high specification: approximately £5,000-£7,500/m²; WHY COST PER M² IS AN IMPRECISE GUIDE: the FIXED COSTS of a small extension (mobilisation, scaffold, foundations, drainage connections, structural opening and steel beam, Planning fees) are approximately the same whether the extension is 12m² or 24m² — so the COST PER M² OF A SMALL EXTENSION IS MUCH HIGHER than a larger one (the fixed costs are spread over fewer m²); conversely, a 36m² extension has much lower cost per m² because the fixed costs are spread over a larger floor area; the quoted cost per m² is most reliable as a benchmark for extensions of approximately 15-25m²; FULL COST BREAKDOWN (SINGLE-STOREY REAR EXTENSION — TYPICAL 4M×4M = 16M² — LONDON 2025): PROFESSIONAL FEES (DESIGN + PLANNING + PARTY WALL + BC — SEPARATE FROM BUILD CONTRACT): architect drawings (planning and BC): £3,000-£6,000; structural engineer: £800-£2,500; party wall surveyor (2 neighbours — both sides typical Victorian terrace): £1,500-£4,000 if dispute; Building Control full plans application: £400-£1,000; TOTAL PROFESSIONAL FEES: approximately £6,000-£13,000; BUILD CONTRACT (CONTRACTOR COSTS — INCLUDED IN MAIN CONTRACT PRICE): ENABLING AND DEMOLITION (DEMOLISHING EXISTING REAR WALL, EXISTING REAR DRAIN RELOCATION, CUTTING OUT OPENINGS): £2,000-£6,000; EXCAVATION AND FOUNDATIONS (STRIP OR PAD FOUNDATIONS — TYPICALLY 1.2-1.5M DEEP IN LONDON CLAY — WITH BORED PILING IF NEAR TREES): £5,000-£15,000; DRAINAGE (NEW GROUND FLOOR DRAINAGE RUNS TO EXISTING STACK OR NEW GULLY; SURFACE WATER DRAINAGE — SOAKAWAY OR CONNECTION TO DRAIN): £2,000-£6,000; STRUCTURAL STEEL (RSJ BEAM FOR REAR WALL OPENING — TYPICALLY 152-305MM UB; PADSTONES; PROPPING DURING INSTALLATION): £2,500-£6,000; EXTERNAL WALLS (CAVITY BRICK/BLOCK CONSTRUCTION TO WALL PLATE HEIGHT — LABOUR AND MATERIALS): £6,000-£15,000 (depends on wall height and wall length); FLAT ROOF STRUCTURE AND WATERPROOFING (INSULATED FLAT ROOF ON TIMBER JOISTS — GRP FIBREGLASS OR EPDM WATERPROOFING — ROOF LANTERN OR VELUX ROOFLIGHT INCLUDED): £8,000-£18,000; EXTERNAL DOORS AND WINDOWS (BIFOLD OR ALUMINIUM SLIDING DOORS TO REAR — 3-5 PANELS; GLAZED SECTION IN SIDE WALL OR FIXED WINDOW): £5,000-£15,000 (aluminium mid-range specification); INTERNAL INSULATION (WALL INSULATION TO CURRENT PART L STANDARDS; FLOOR INSULATION UNDER SCREED): £2,000-£5,000; CONCRETE FLOOR SLAB WITH DPM (GROUND-BEARING SLAB ON HARDCORE WITH DPM AND POWER-FLOAT FINISH): £2,000-£5,000; FIRST FIX PLUMBING (DRAINAGE AND WATER SUPPLY TO NEW UTILITY OR KITCHEN): £1,500-£4,000; FIRST FIX ELECTRICS (PART P CABLES, CONSUMER UNIT UPGRADE WHERE REQUIRED, DISTRIBUTION TO NEW ZONE): £1,500-£3,500; INTERNAL PLASTERBOARDING, SKIMMING, AND DECORATION: £4,000-£9,000; SECOND FIX ELECTRICS + PLUMBING: £1,000-£2,500; CONTINGENCY (10% — STRONGLY RECOMMENDED FOR LONDON VICTORIAN TERRACE EXTENSIONS): £5,000-£10,000; TOTAL BUILD CONTRACT (INCLUDING 10% CONTINGENCY — SINGLE-STOREY REAR EXTENSION 4M×4M — STANDARD-TO-MID SPECIFICATION): approximately £55,000-£90,000; DOUBLE-STOREY EXTENSION — LONDON 2025 COSTS: WHAT IS A DOUBLE-STOREY EXTENSION: an extension that adds two floors of new space (ground floor and first floor) to the rear or side of the property; almost always requires FULL PLANNING PERMISSION (PD rights for a two-storey rear extension are very limited — the extension must be within 3m of the rear wall and at least 7m from the rear boundary, which most London Victorian terraces cannot meet); DOUBLE-STOREY REAR EXTENSION COST RANGE (LONDON 2025 — TYPICAL 3M DEEP × 4M WIDE = 12M² PER FLOOR = 24M² TOTAL NEW FLOOR AREA): approximately £80,000-£130,000 (standard specification); approximately £120,000-£200,000+ (high specification — render cladding, high-end windows, roof terrace to first floor extension); COST PER M² (TOTAL NEW FLOOR AREA — LONDON 2025): approximately £3,000-£5,000/m² (standard); approximately £5,000-£8,000/m² (high specification); WHAT INCREASES THE COST OF A DOUBLE-STOREY EXTENSION: STRUCTURAL FRAME: the first floor of the extension must be structurally supported — typically by: CONCRETE BLOCK WALLS (standard residential) or STEEL FRAME (if large open spans or heavy loads are involved); the first floor structure adds cost versus a single-storey extension; FLOOR STRUCTURE TO FIRST FLOOR: a structural floor (typically 200mm solid concrete slab or engineered timber joists + 22mm chipboard decking) spanning between the extension walls; FIRST FLOOR WINDOWS: the first floor external walls require windows (adding cost vs a single-storey roof without windows); PARTY WALL FEES: a double-storey extension typically triggers BOTH SECTION 2 (work to party wall) AND SECTION 6 (excavation within 3m of neighbour's structure) Party Wall Act notices — increasing the cost and complexity of the party wall process; SIDE RETURN EXTENSION — LONDON 2025 COSTS: WHAT IS A SIDE RETURN EXTENSION: a SIDE RETURN is the NARROW PASSAGE that runs alongside the main body of the Victorian terrace (typically 0.8m-1.5m wide, between the side of the house and the boundary wall); it connects the rear garden to the front of the property; infilling this side return with a new extension adds approximately 5-12m² of floor area to the ground floor, typically widening the kitchen by the width of the side return passage; SIDE RETURN EXTENSION COST RANGE (LONDON 2025 — TYPICAL 4M LONG × 1M WIDE = 4M² SIDE RETURN INFILL): approximately £25,000-£50,000 (side return only — typically combined with the rear extension as a COMBINED REAR AND SIDE RETURN project); COMBINED REAR AND SIDE RETURN (THE MOST COMMON LONDON KITCHEN EXTENSION FORMAT): approximately £70,000-£130,000 for a 5m-deep rear extension plus full side return infill (total floor area approximately 25-35m²); the side return rooflights (typically a FRAMELESS ROOFLIGHT OVER THE SIDE RETURN to bring light into the extended kitchen) are a significant cost element: approximately £3,000-£8,000 for a full-length side return rooflight (aluminium-framed walk-on glazing or pitched polycarbonate glazing).

Wrap-around extension costs, what is excluded from typical extension quotations, how to read and compare quotations, and cost control strategies for London home extensions in 2025: WRAP-AROUND EXTENSION (COMBINED REAR AND SIDE EXTENSION — FULL WRAP) — LONDON 2025 COSTS: WHAT IS A WRAP-AROUND EXTENSION: a wrap-around extension combines a FULL-WIDTH REAR EXTENSION with a FULL SIDE RETURN INFILL, creating a continuous new floor area that wraps around the rear and one side of the property; for a typical Victorian terrace: the ground floor footprint is significantly increased; the new floor area is typically 30-50m² (the largest single-storey extension possible within the property boundaries); the kitchen is typically relocated into the new full-width open-plan rear space; a LARGE ROOF LANTERN or MULTIPLE ROOFLIGHTS over the new extension bring natural light deep into the plan; FULL WRAP-AROUND EXTENSION COST RANGE (LONDON 2025): approximately £90,000-£170,000 for the build (excluding professional fees and kitchen fit-out); WHAT DRIVES THE WIDE RANGE: SIZE OF THE WRAP (depth of the rear extension × total width including side return); SPECIFICATION OF THE ROOF GLAZING (a large frameless rooflight or a complex roof lantern can cost £8,000-£30,000 for the glazing element alone); EXTERNAL CLADDING SPECIFICATION (brick-clad to match existing — standard cost; zinc or copper cladding — premium; rendered — medium cost); BIFOLD OR SLIDING DOOR SYSTEM SPECIFICATION (standard aluminium bifold — £5,000-£12,000 for a 4-panel system; premium lift-and-slide aluminium — £8,000-£20,000 for the same opening width); WHETHER THE PROJECT INCLUDES UNDERFLOOR HEATING IN SCREED (ADD £2,500-£5,000); INTERNAL SPECIFICATION (plaster direct to existing brick vs full boarding and skimming of the existing walls that are opened up); PROFESSIONAL FEES FOR A WRAP-AROUND EXTENSION (SEPARATE FROM BUILD CONTRACT): ARCHITECT (PLANNING AND BUILDING CONTROL DRAWINGS): £4,000-£8,000 (a wrap-around is typically larger and more complex than a simple single-storey rear extension — more drawing time, more coordination, often requires a design-and-build approach); STRUCTURAL ENGINEER (FOUNDATIONS, WALLS, ROOF STRUCTURE, STEEL BEAMS FOR OPENINGS): £1,200-£3,500; PARTY WALL SURVEYOR: because a wrap-around extension typically works along BOTH SIDE WALLS (one side wall is the party wall with the neighbour; the opposite side wall is at or near the garden boundary — typically a shared boundary with a rear neighbour), MULTIPLE PARTY WALL NOTICES may be required: a SECTION 2 NOTICE (for the work to the party wall side wall) to the SIDE NEIGHBOUR; a SECTION 6 NOTICE (for the foundations within 3m of the rear neighbour's property) to the REAR NEIGHBOUR; total party wall surveyor fees: £2,000-£6,000 (if both neighbours require surveyors); BUILDING CONTROL (FULL PLANS APPLICATION): £600-£1,500; PLANNING APPLICATION (FULL PLANNING PERMISSION TYPICALLY REQUIRED FOR A WRAP-AROUND — THE COMBINED SIDE AND REAR EXTENSION CHANGES THE MASSING OF THE PROPERTY): £258 (application fee) + architect's time for heritage/design statement if in a conservation area; TOTAL PROFESSIONAL FEES (WRAP-AROUND): approximately £8,000-£20,000; WHAT IS NOT INCLUDED IN A TYPICAL EXTENSION QUOTATION (AND WHAT YOU SHOULD CHECK BEFORE COMPARING): this is one of the most important sections of this guide — a QUOTATION THAT APPEARS CHEAP may simply have EXCLUDED the most expensive items; TYPICALLY EXCLUDED FROM LONDON EXTENSION BUILD CONTRACTS: PROFESSIONAL FEES: all of: architect fees, structural engineer fees, party wall surveyor fees, Building Control fees, Thames Water build-over application fees (if applicable) — typically NOT included in the build contract price; these may add £6,000-£20,000 to the total project cost; KITCHEN FIT-OUT: the kitchen units, worktop, appliances, and installation are almost always excluded from an extension build contract and are a separate specification and contract (add £5,000-£80,000+ depending on specification — see kitchen renovation guide); FLOOR FINISHES: the build contract typically ends at a SCREEDED FLOOR (or POWER-FLOATED CONCRETE SLAB) — the final floor covering (engineered wood, porcelain tiles, LVT) is a separate allowance (add £40-£120/m² for supply and install); DECORATION: internal painting and decorating may or may not be included; CHECK THE SCOPE CAREFULLY (some builders include 2-coat emulsion on walls and ceilings; others leave the room in plaster only and the client arranges their own decorator); EXTERNAL WORKS: reinstatement of the garden after construction (paving, lawn, planting) is typically not included in the build contract; in a rear extension, the construction traffic and storage will damage the garden — budget £2,000-£10,000+ for garden reinstatement depending on the size and specification; GARDEN WASTE REMOVAL BEYOND STANDARD SKIPS: large volumes of excavated material (London Clay from new foundations) generate SEVERAL SKIP LOADS of spoil; check the build contract to confirm how many skips are included and what the rate is for additional skips (London skip hire: approximately £200-£350 per 8-yard skip, plus tip charges for excavated material which is significantly more expensive to dispose of than general waste); CONTINGENCY: construction projects routinely encounter UNFORESEEN CONDITIONS — especially in Victorian terrace foundations (made-up ground, historic buried drainage, soft spots in London Clay, previous building activity); a CONTINGENCY of 10-15% of the build contract value is STRONGLY RECOMMENDED; HOW TO READ AND COMPARE EXTENSION QUOTATIONS: STANDARDISE THE SCOPE: ask all contractors to quote from the SAME SCOPE OF WORKS document (ideally prepared by the architect); a quotation without a schedule of works is almost meaningless — you cannot tell what is included or excluded; CONFIRM WHAT IS INCLUDED: specifically ask: are professional fees included?; is the kitchen included?; is the floor covering included?; is the decoration included?; is garden reinstatement included?; HOW MANY SKIPS ARE INCLUDED?; BREAKDOWN BY ELEMENT: ask for the ELEMENTAL BREAKDOWN (how much for foundations? how much for the flat roof? how much for the bifold doors?) — this allows you to compare like-for-like and identify where one quotation is cheaper (is it because the specification is lower, or because the contractor is more efficient?); LOW TENDER — RED FLAGS: a quotation that is significantly lower than the other quotations is usually lower because: the contractor has ASSUMED A SIMPLER SPECIFICATION than the others (e.g. cheaper bifold doors, thinner concrete, shallower foundations, lower-quality steel); the contractor has EXCLUDED ELEMENTS that others included; the contractor is BUYING THE WORK (pricing at below cost to secure the contract, intending to increase the cost later via VARIATIONS); a 10% difference in quotations between reputable contractors is normal; a 30-40% difference should prompt a detailed scope comparison and a very careful assessment of what the cheaper quotation has excluded or assumed; COST CONTROL FOR LONDON HOME EXTENSIONS: (1) DECIDE YOUR SPECIFICATION BEFORE INVITING QUOTATIONS: the most expensive change on a construction project is the change made AFTER THE CONTRACT IS SIGNED — a VARIATION INSTRUCTION (VI) changes the scope and is priced by the contractor at a rate that reflects their negotiating position once the project is underway; every significant specification decision (roof glazing type, external door specification, floor finish, wall materials, kitchen specification) should be made BEFORE THE BUILD CONTRACT IS SIGNED; (2) AGREE A FIXED-PRICE CONTRACT WITH A CLEAR VARIATION PROCEDURE: a JCT Minor Works Building Contract (or an equivalent simple fixed-price contract) sets out the original contract price, the process for authorising changes, and the mechanism for pricing variations; without a formal contract, cost escalation is uncontrolled; (3) RETAIN A BUILDER'S DEPOSIT HOLDBACK: a standard retention of 2.5-5% of the contract value is held back until the MAKING GOOD OF DEFECTS PERIOD has expired (typically 6 months after practical completion) and any snagging issues identified after completion have been rectified; (4) BUDGET SEPARATELY FOR ASSOCIATED WORKS: ELECTRICAL WORKS IN THE EXISTING PART OF THE HOUSE (the extension may trigger a consumer unit upgrade and an electrical inspection of the existing installation — costs: £1,500-£5,000); INTERNAL REFURBISHMENT OF THE ROOMS ADJOINING THE EXTENSION (the rooms that open onto the extension will typically need replastering, new skirting and doors, and redecoration — budget £5,000-£15,000 for the adjoining internal works).