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

London home extension construction costs by type in 2025 — IMPORTANT: all costs below are CONSTRUCTION COSTS ONLY (the contractor's contract sum, excluding VAT, professional fees, planning, Building Regulations, and any structural works to the existing house beyond the extension footprint itself): SINGLE-STOREY REAR EXTENSION: the most common extension type in London — a ground-floor addition to the rear of a Victorian or Edwardian terrace or semi-detached house; TYPICAL USES: kitchen-dining room; family room; playroom; home office; TYPICAL DIMENSIONS: 3m-6m deep x 4m-6m wide (the width is typically the full width of the existing rear façade); PLANNING: single-storey rear extensions are permitted development (PD) under Class A up to 4m depth for a terraced or semi-detached house; up to 6m depth via the Larger Home Extension (prior approval) route; CONSTRUCTION COST RANGE: £40,000-£100,000 for a standard single-storey rear extension; WHAT DRIVES VARIATION WITHIN THE SINGLE-STOREY COST RANGE: LOWER END (£40,000-£55,000): simpler specification (brick walls matching existing; flat EPDM or PIR warm roof; standard aluminium bifold doors 3-leaf; no structural wall removal or spine wall opening; straightforward drainage diversion only); smaller footprint (approximately 15-20m² extension floor area); MIDDLE RANGE (£55,000-£75,000): medium specification (matching brickwork; single or double roof lantern or structural glass rooflight; 4-leaf bifold or sliding doors; Part F 2022 compliant kitchen extraction; moderate drainage diversion); footprint approximately 20-28m²; moderate structural opening in existing wall to connect extension to existing rooms; UPPER END (£75,000-£100,000): higher specification (large-format glazing; Crittall-style steel-frame windows and doors; bespoke lantern light; higher specification roofing and insulation beyond Part L minimum; spine wall removal to create fully open-plan ground floor; EV charging provision; higher-specification internal finishes); footprint approximately 25-35m²; DOUBLE-STOREY REAR EXTENSION: adds a floor above the single-storey extension — typically creating an additional bedroom and/or bathroom on the first floor in addition to the enlarged ground floor room; PLANNING: double-storey rear extensions almost always require planning permission (they typically exceed the PD allowances in terms of height and the proximity to the party wall boundary); BUILDING REGULATIONS: more complex than single-storey — additional Part E acoustic requirements between the new upper floor and the existing first floor; Part B fire protection and escape provisions; structural design for the upper floor and the roof; CONSTRUCTION COST RANGE: £80,000-£180,000; WHAT DRIVES VARIATION: LOWER END (£80,000-£100,000): simpler specification; smaller footprint (approximately 15-20m² per floor, 30-40m² total); standard finish; single bathroom on first floor; UPPER END (£140,000-£180,000): larger footprint (25-35m² per floor, 50-70m² total); higher specification; master bedroom with en-suite and dressing room on first floor; premium glazing and finishes throughout; SIDE RETURN EXTENSION: infills the narrow side passage (the "side return") alongside the rear outrigger of a Victorian terrace — a common opportunity in inner London where the side passage is 1.0-2.5m wide but runs for 3-8m along the side of the house; the side return extension widens the kitchen or dining room on the ground floor by the width of the side passage, creating a wider room, typically with a glazed roof above the new side return roof area; TYPICAL DIMENSIONS: 1.0-2.5m wide x 3-8m long; PLANNING: the side return extension is typically within PD allowances (it is a single-storey addition that does not project beyond the rear of the existing outrigger) — BUT check carefully, as some London LPAs treat the side return as a side extension (which has stricter PD limits on width); an LDC is strongly recommended before starting a side return extension without planning permission; CONSTRUCTION COST RANGE: £35,000-£80,000; LOWER END (£35,000-£55,000): narrower side return (1.0-1.5m); shorter run (3-5m); standard rooflight above (Velux or similar); straightforward drainage; UPPER END (£60,000-£80,000): wider and longer side return; large-format structural glazed roof or lantern above the side return; Crittall-style or bespoke glazing; higher specification finishes; WRAP-AROUND EXTENSION: combines a side return extension with a single-storey rear extension — creating an L-shaped addition that wraps around the rear of the Victorian terrace, adding significant floor area to the ground floor; PLANNING: a wrap-around extension typically requires planning permission because the side and rear extensions combined may exceed PD allowances — and the side element may be visible from the public highway; CONSTRUCTION COST RANGE: £75,000-£160,000; the cost reflects the larger floor area (typically 35-60m²); the complexity of the roof (a flat roof with multiple rooflights wrapping around two sides of the house, draining to a single valley or a sump); the structural works (spine wall removal, drainage, services relocation).

The key factors that drive cost variation between London home extension projects within the same basic type and size: STRUCTURAL COMPLEXITY: the structural works required to connect the extension to the existing house are one of the most significant drivers of cost variation that is not immediately obvious from the extension size alone; the most common structural works in a London home extension are: SPINE WALL REMOVAL (creating an open-plan ground floor by removing the central loadbearing masonry wall): adds approximately £4,000-£11,000 to the construction cost (structural engineer fees, steel beam, padstones, propping, masonry removal, making good — see structural alterations guide); EXISTING REAR OPENING (creating a large structural opening in the existing rear wall of the house to connect the existing kitchen to the new extension): a standard 2-3m wide doorway opening in a 9-inch brick wall requires a steel lintel or RSJ, padstones, and temporary propping — adds approximately £1,500-£3,500 per opening; UNDERPINNING (where the extension foundations must be taken deeper to avoid undermining the existing or neighbouring foundations): uncommon for a standard rear extension but more common in: areas of known subsidence risk (particularly on London Clay south of the Thames — Clapham, Stockwell, Brixton, etc.); proximity to existing trees (London Clay is susceptible to desiccation subsidence from tree root moisture extraction); where the Party Wall Act Section 6 applies (excavations within 3m of a neighbouring property's foundations); DRAINAGE AND SERVICES: drainage works for a London home extension are consistently one of the most significant areas of cost variation and cost uncertainty: SEWER BUILD-OVER (where the proposed extension is above or within 3m of a public sewer): Thames Water's Section 185 consent is required before building over or within 3m of a public sewer; the cost of sewer build-over consent and any required sewer protection measures (concrete surround, suspended floor, marker posts) adds approximately £500-£2,500 to the project cost; DRAINAGE DIVERSION (where an existing drainage run — WC soil stack, surface water drain, kitchen waste run — passes through the proposed extension footprint and must be diverted): the cost of draining diversion depends on the depth, length, and material of the existing drainage; a simple surface water gully diversion might cost £300-£600; a WC soil stack diversion can cost £1,500-£5,000; SERVICES RELOCATION (gas meter, electricity meter, water stop cock, external tap, alarm sensors, external lighting — all must be relocated where the extension encloses the area in which they currently sit): add approximately £500-£2,000 per service for relocation; SPECIFICATION OF GLAZING AND DOORS: the single biggest lever on the construction cost of a London home extension after the structural complexity; the glazing specification for a typical rear extension: STANDARD (aluminium bifold 3-leaf, standard flat roof, no rooflight): the lowest glazing cost; MEDIUM (aluminium bifold 4-5 leaf + single roof lantern 1.2m x 2.4m): add approximately £3,000-£6,000 compared with standard; PREMIUM (Crittall-style steel-frame bifolds + structural glass rooflight or bespoke lantern 2m x 3m): add approximately £8,000-£20,000 compared with standard; VERY PREMIUM (full structural glass roof covering the majority of the extension ceiling area, plus Crittall-style steel-frame doors and windows): add approximately £20,000-£60,000 compared with standard; SPECIFICATION OF INTERNAL FINISHES AND KITCHEN: where the extension includes a kitchen (which is the case for most single-storey rear extensions in London Victorian terraces), the kitchen specification is a major driver of total cost; a basic kitchen (IKEA METOD or Wickes — supply and install): approximately £3,000-£8,000; a mid-range kitchen (Benchmarx, Howdens — supply and install with granite or quartz worktops): approximately £8,000-£18,000; a higher-specification kitchen (bespoke hand-painted in-frame kitchen — Smallbone, Humphrey Munson, or similar — supply and install): approximately £20,000-£80,000+; the kitchen specification is NOT part of the structural extension cost — it is a separate element of the total project budget that should be planned separately but simultaneously with the extension design; SITE LOGISTICS AND ACCESS: London construction is significantly more expensive than construction in other UK cities partly because of: restricted access (limited on-road parking for skips, delivery vehicles, and the contractor's vehicles; permit requirements for skip placement and scaffolding in London boroughs); materials cost premium (London materials costs are approximately 15-25% higher than the national average due to logistics and demand); labour cost premium (London construction labour costs are approximately 20-35% higher than the national average).

The total project cost of a London home extension — the methodology for building an accurate all-in budget: the CONSTRUCTION COST (the contractor's contract sum, excluding VAT) is only ONE component of the total project cost; a complete total project budget for a London home extension must include ALL of the following: (1) CONSTRUCTION COST: as per the cost ranges above by extension type; (2) VAT (20%): on ALL construction works; VAT is consistently the most material item after the construction cost itself and the most commonly overlooked; on a £65,000 construction cost, VAT = £13,000; there is no reduced VAT rate for a standard rear extension to an owner-occupied dwelling; (3) ARCHITECT FEES (for planning drawings and/or Building Regulations drawings): approximately £2,000-£8,000 for a standard single-storey rear extension (Stages 1-3, planning only); £5,000-£15,000 for a full service including Building Regulations drawings and construction stage administration; (4) STRUCTURAL ENGINEER FEES: approximately £800-£2,000 for a single-storey rear extension (beam design, floor design, foundation design, structural drawings for LABC); (5) PLANNING APPLICATION FEE (if planning permission is required): £258 (England 2025, Householder); pre-application meeting: £100-£600; planning consultant: £1,500-£4,000 (if appointed); (6) BUILDING REGULATIONS FEES (LABC Full Plans application): approximately £500-£1,500 for a single-storey extension; (7) PARTY WALL FEES: approximately £700-£3,000 per Award (depending on whether the neighbour appoints their own surveyor or agrees to an Agreed Surveyor); (8) DRAINAGE SURVEY AND SEWER BUILD-OVER CONSENT (if a public sewer is within 3m of the proposed extension): CCTV drain survey before planning application: approximately £200-£500; Thames Water sewer build-over application (Section 185): approximately £500-£900 in fees; (9) CONTINGENCY: minimum 10% of the construction cost; 15% for a wider renovation that includes the extension and internal alteration works; (10) FURNITURE, FURNISHINGS, AND CLIENT-SUPPLIED ITEMS: all second-fix materials (kitchen units, sanitary ware, bathroom finishes, light fittings, furniture, curtains/blinds) are client-supplied and are NOT included in the contractor's construction cost; EXAMPLE TOTAL PROJECT BUDGET — standard single-storey rear extension (6m x 4m = 24m² floor area; medium specification; no spine wall removal; planning permission not required — PD; inner London location): Construction cost: £65,000; VAT (20%): £13,000; Architect (planning + BR drawings): £4,500; Structural engineer: £1,200; LABC Building Regulations fee: £700; Party wall (one neighbour, agreed surveyor): £900; Drainage survey: £300; Contingency (10%): £6,500; TOTAL: approximately £92,100 BEFORE client-supplied kitchen, appliances, furniture, and decoration; KEY POINTS TO NOTE: the VAT alone (£13,000) exceeds the combined professional fees and Building Regulations costs in this example; the contingency should be treated as a firm budget provision — NOT as a safety net that is expected NOT to be used (the reality of London Victorian terrace construction is that unexpected items WILL be found during the works, and 10% is the minimum realistic provision).