London Extension Cost Per Square Metre Guide 2025: Extension Costs by Type, Specification Level, and London Zone

London extension cost-per-square-metre breakdown for single-storey rear extensions, double-storey extensions, side extensions, wrap-around extensions, and kitchen extensions in 2025 — at standard, good, and excellent specification levels — and what drives the cost difference between specification tiers in London residential extension construction in 2025: LONDON EXTENSION COSTS PER SQUARE METRE BY EXTENSION TYPE AND SPECIFICATION LEVEL IN 2025 (BUILD CONTRACT COST ONLY — EXCLUDING PROFESSIONAL FEES, VAT, AND CONTINGENCY): SINGLE-STOREY REAR EXTENSION (THE MOST COMMONLY CONSTRUCTED EXTENSION TYPE IN LONDON RESIDENTIAL IN 2025 — ADDED BEHIND THE REAR ELEVATION — TYPICALLY 3-6m PROJECTION WITH A FLAT ROOF OR LOW-PITCH ROOF — MOST COMMON DRIVER IN LONDON IS KITCHEN-DINER ENLARGEMENT OR ADDITIONAL RECEPTION ROOM): STANDARD SPECIFICATION (FAIR-FACED BRICKWORK MATCHING EXISTING IN COLOUR BUT NOT HANDMADE OR STOCK BRICK; STANDARD DOUBLE-GLAZED PVCU OR ALUMINIUM WINDOWS AND DOOR; FLAT EPDM ROOF; STANDARD BLOCKWORK CAVITY WALL; BASIC KITCHEN FIT-OUT EXCLUDED — SHELL ONLY; STANDARD ELECTRICAL AND HEATING; NO UNDERFLOOR HEATING): £2,200-£2,800/m² INNER LONDON 2025; GOOD SPECIFICATION (STOCK BRICK OR FACING BRICK MATCHING EXISTING MORE CLOSELY; ALUMINIUM BIFOLD OR SLIDING DOORS SMARTS VISOFOLD 1000 OR SCHUCO SYSTEM; EPDM FLAT ROOF WITH GRP FIBREGLASS PARAPET FINISH; CAVITY WALL INSULATION EXCEEDING PART L; UNDERFLOOR HEATING (WET UFH) MOST INSTALLED IN GOOD-SPECIFICATION LONDON SINGLE-STOREY REAR EXTENSIONS IN 2025 — POLYPIPE OR HONEYWELL WETROOM UFH SYSTEMS MOST INSTALLED IN LONDON RESIDENTIAL EXTENSIONS IN 2025; LANTERN ROOFLIGHT — KORNICHE ALUMINIUM ROOF LANTERN MOST SPECIFIED LONDON RESIDENTIAL EXTENSION ROOFLIGHTS IN GOOD SPECIFICATION IN 2025): £2,800-£3,800/m² INNER LONDON 2025; EXCELLENT/HIGH SPECIFICATION (HANDMADE IMPERIAL FORMAT LONDON STOCK BRICK (BULMER BRICK MOST SPECIFIED HANDMADE STOCK BRICK LONDON CONSERVATION AREA EXTENSIONS IN 2025; SHIRE HANDMADE CLAY BRICK SECOND MOST SPECIFIED); STRUCTURAL GLAZING (SCHUCO FW 50+ OR ALUMIL M11000 MOST SPECIFIED STRUCTURAL ALUMINIUM GLAZING FOR PREMIUM LONDON RESIDENTIAL EXTENSIONS IN 2025); SEDUM GREEN ROOF (BAUDER SEDUM BLANKET MOST INSTALLED LONDON RESIDENTIAL EXTENSION FLAT ROOFS AT PREMIUM SPECIFICATION IN 2025); WET UFH THROUGHOUT WITH THERMOSTAT ZONING; PREMIUM LANTERN ROOFLIGHT (ATLAS ROOF SYSTEM MOST SPECIFIED PREMIUM FLAT ROOF LANTERN LONDON RESIDENTIAL EXTENSION IN 2025); POLISHED CONCRETE OR LARGE FORMAT PORCELAIN (800mm×800mm+) FLOOR SLAB; DESIGNER INTERNAL JOINERY; INTEGRATED AV/SMART HOME FOUNDATION): £3,800-£6,000+/m² INNER LONDON 2025; DOUBLE-STOREY EXTENSION (A TWO-STOREY EXTENSION ADDING ACCOMMODATION ON BOTH THE GROUND FLOOR AND FIRST FLOOR — TYPICALLY A GROUND-FLOOR KITCHEN EXTENSION WITH ONE OR MORE BEDROOMS OR BATHROOMS ABOVE — MOST COST-EFFICIENT EXTENSION TYPE PER SQUARE METRE OF ADDITIONAL FLOOR AREA ADDED IN LONDON RESIDENTIAL IN 2025 BECAUSE THE ADDITIONAL FIRST-FLOOR ACCOMMODATION IS BUILT ON THE SAME FOUNDATIONS AND EXTERNAL SHELL AS THE GROUND FLOOR — THE INCREMENTAL COST OF THE UPPER STOREY IS LOWER THAN THE COST OF THE GROUND FLOOR): £1,800-£2,800/m² FOR THE UPPER STOREY ADDITIONAL COST OVER AND ABOVE THE SINGLE-STOREY EQUIVALENT BUILD COST (INNER LONDON 2025 — STANDARD TO GOOD SPECIFICATION — TOTAL DOUBLE-STOREY EXTENSION COST PER m² CALCULATED AS: (SINGLE-STOREY GROUND FLOOR RATE × GROUND FLOOR AREA) + (UPPER STOREY RATE × UPPER FLOOR AREA)); SIDE EXTENSION (EXTENDING OUTWARD TO THE SIDE OF THE PROPERTY — TYPICALLY IN A SEMI-DETACHED OR DETACHED LONDON HOUSE WHERE A SIDE PASSAGE OR GARDEN AREA CAN BE BUILT OVER — THE COST PER m² FOR A SIDE EXTENSION IS TYPICALLY SIMILAR TO OR SLIGHTLY HIGHER THAN A SINGLE-STOREY REAR EXTENSION DUE TO THE STRUCTURAL AND FOUNDATION REQUIREMENTS OF BUILDING AGAINST AN EXISTING FLANK WALL AND THE TYPICALLY TIGHTER ACCESS ON A SIDE EXTENSION SITE): £2,300-£4,000/m² INNER LONDON 2025 (STANDARD TO GOOD SPECIFICATION); WRAP-AROUND EXTENSION (A COMBINATION OF SIDE AND REAR SINGLE-STOREY EXTENSION THAT "WRAPS" AROUND THE REAR CORNER OF THE PROPERTY — THE MOST FREQUENTLY REQUESTED EXTENSION TYPE FOR LONDON SEMI-DETACHED AND END-OF-TERRACE HOUSES IN 2025 — MORE STRUCTURALLY COMPLEX THAN EITHER A SINGLE REAR OR SINGLE SIDE EXTENSION DUE TO THE CORNER JUNCTION — TYPICALLY REQUIRES MORE STRUCTURAL STEEL AT THE CORNER JUNCTION): £2,500-£4,200/m² INNER LONDON 2025 (STANDARD TO GOOD SPECIFICATION); KITCHEN EXTENSION (WHERE THE EXTENSION IS SPECIFICALLY A KITCHEN-DINER SPACE WITH A FULL FITTED KITCHEN INCLUDED IN THE CONTRACT — MOST LONDON SINGLE-STOREY REAR EXTENSION CONTRACTS IN 2025 ARE QUOTED AS A SHELL ONLY WITH THE KITCHEN CONTRACT SEPARATE — WHERE THE KITCHEN IS INCLUDED IN THE EXTENSION CONTRACT THE COST-PER-SQUARE-METRE FIGURE INCREASES SUBSTANTIALLY BASED ON THE KITCHEN SPECIFICATION): ADD £500-£3,000/m² TO THE BASE EXTENSION RATE ABOVE DEPENDING ON KITCHEN SPECIFICATION (HOWDENS BUDGET THROUGH TO BESPOKE HANDMADE KITCHEN SPECIFICATION AS DESCRIBED IN THE KITCHEN INSTALLATION GUIDE ABOVE).

Inner London vs outer London extension cost differential in 2025, how professional fees, VAT, and contingency add to the build contract cost to produce a total project budget for a London residential extension, and the most reliable framework for planning a total London extension budget in 2025: INNER LONDON VS OUTER LONDON EXTENSION COST DIFFERENTIAL IN 2025: THE INNER LONDON COST PREMIUM (APPROXIMATELY LONDON TRAVEL ZONES 1-3) OVER OUTER LONDON (ZONES 4-6) IN RESIDENTIAL EXTENSION CONSTRUCTION IN 2025 IS DRIVEN BY: LABOUR COSTS (LONDON CONSTRUCTION WORKERS IN INNER LONDON ZONES 1-3 COMMAND HIGHER DAY RATES DUE TO TRAVEL, PARKING, CONGESTION CHARGE, AND ULEZ CHARGE COSTS — IN 2025, THE INNER LONDON CONSTRUCTION LABOUR PREMIUM OVER OUTER LONDON IS APPROXIMATELY 15-20% ON MOST TRADES); SITE ACCESS AND LOGISTICS (INNER LONDON TERRACE PROPERTIES TYPICALLY HAVE MORE RESTRICTED SITE ACCESS — NARROW SIDE PASSAGES, NO FRONT DRIVE — INCREASING PLANT AND LABOUR COSTS; SKIP AND DELIVERY PERMIT COSTS ARE HIGHER IN INNER LONDON WHERE ALL SKIPS ON THE PUBLIC HIGHWAY REQUIRE A COUNCIL SKIP PERMIT — IN INNER LONDON THE PERMIT COST IS TYPICALLY £30-£75/WEEK FROM THE LOCAL AUTHORITY IN 2025); CONSERVATION AREA AND MATERIAL MATCHING REQUIREMENTS (INNER LONDON HAS A HIGHER PROPORTION OF CONSERVATION AREA PROPERTIES WHERE HANDMADE STOCK BRICK, LIME MORTAR, AND PERIOD-APPROPRIATE MATERIALS ARE REQUIRED BY PLANNING — THESE MATERIALS COMMAND A SIGNIFICANT PREMIUM OVER STANDARD MULTI STOCK OR ENGINEERING BRICK COMMONLY USED IN OUTER LONDON RESIDENTIAL EXTENSIONS); THE MOST SPECIFIED COST DIFFERENTIAL FACTOR FOR LONDON EXTENSION COST PER SQUARE METRE IN 2025: INNER LONDON (ZONES 1-3) IS TYPICALLY 15-20% HIGHER THAN OUTER LONDON (ZONES 4-6) FOR A COMPARABLE SPECIFICATION — FOR PLANNING PURPOSES: OUTER LONDON 2025 STANDARD SPEC SINGLE-STOREY REAR EXTENSION: £1,900-£2,400/m² (BUILD ONLY); OUTER LONDON 2025 GOOD SPEC: £2,400-£3,200/m²; OUTER LONDON 2025 EXCELLENT SPEC: £3,200-£5,000+/m²; TOTAL PROJECT COST FRAMEWORK FOR A LONDON RESIDENTIAL EXTENSION IN 2025 (STARTING FROM BUILD CONTRACT COST PER m²): STEP 1 — DETERMINE EXTENSION AREA (m²) AND APPLY THE APPROPRIATE BUILD COST RATE (STANDARD/GOOD/EXCELLENT BY EXTENSION TYPE FROM ABOVE); STEP 2 — ADD KITCHEN CONTRACT IF APPLICABLE (BUILD RATE + KITCHEN SPECIFICATION RATE × KITCHEN FLOOR AREA); STEP 3 — ADD PROFESSIONAL FEES (AS DESCRIBED IN THE RENOVATION BUDGET GUIDE ABOVE — ARCHITECT 10-12% OF BUILD COST; STRUCTURAL ENGINEER £1,500-£3,500; CDM PRINCIPAL DESIGNER £500-£2,500; PARTY WALL SURVEYOR £1,600-£3,000 WHERE APPLICABLE FOR LONDON TERRACE WITH TWO ADJOINING OWNERS); STEP 4 — ADD STATUTORY FEES (PLANNING FEE £258; BC FULL PLANS £400-£1,200); STEP 5 — ADD VAT AT 20% ON BUILD CONTRACT (MOST LONDON RESIDENTIAL EXTENSION WORKS ARE STANDARD RATED); STEP 6 — ADD CONTINGENCY AT 10-15% OF BUILD CONTRACT (AS DESCRIBED IN THE RENOVATION BUDGET GUIDE ABOVE); WORKED EXAMPLE — INNER LONDON SINGLE-STOREY REAR EXTENSION 2025 GOOD SPECIFICATION 25m² FLOOR AREA: BUILD CONTRACT: 25m² × £3,200/m² = £80,000; STRUCTURAL ENGINEER: £2,500; ARCHITECT STAGES 1-4: £5,000; CDM PRINCIPAL DESIGNER: £1,500; PARTY WALL SURVEYOR (TWO ADJOINING OWNERS): £2,500; PLANNING: £258; BC: £800; VAT ON BUILD CONTRACT: £16,000; CONTINGENCY (12% OF BUILD): £9,600; TOTAL PROJECT BUDGET (EXCL. KITCHEN): £118,158 (APPROXIMATELY £120,000 ROUNDED — EQUIVALENT TO £4,800/m² TOTAL PROJECT COST vs THE HEADLINE BUILD CONTRACT RATE OF £3,200/m²); THE HEADLINE BUILD COST PER m² AND THE TRUE TOTAL PROJECT COST PER m² ARE THEREFORE SIGNIFICANTLY DIFFERENT — PROFESSIONAL HOMEOWNERS AND RENOVATORS IN LONDON IN 2025 BUDGET AT THE TOTAL PROJECT LEVEL, NOT THE BUILD CONTRACT LEVEL ALONE.