Rear Extension Cost London 2025: What You'll Really Pay

A rear extension quote could reasonably be anywhere from £45,000 to £200,000 for the same notional project — and both quotes could be entirely valid. The variation comes from six main dimensions:

1. Size: a 3m x 4m footprint is very different from a 6m x 7m footprint 2. Structural complexity: removing the rear wall and installing steelwork varies significantly 3. Roof type: flat roof (EPDM) vs pitched vs lantern vs full glazed roof 4. Finish specification: kitchen range, floor type, bifold door brand 5. Site constraints: access, neighbouring properties, drainage 6. Whether the extension is single or double storey

The London premium adds to all of these. London labour — carpenters, roofers, plasterers, bricklayers — runs 15–25% above the UK national average. Scaffolding costs more in dense urban streets. Planning and party wall processes add fees that do not exist to the same degree in less constrained locations.

A single-storey rear extension is the most popular project type in London. Typical depths are 3–6m, widths 3–8m depending on the property. The most common configuration connects to the existing kitchen, creating an open-plan kitchen-diner.

• What is typically included in this price range:
• •Demolition of existing rear elevation and single-storey outbuilding where applicable
• •Foundations (strip or pad foundations depending on ground conditions)
• •External walls (blockwork cavity or ICF)
• •Flat roof with EPDM membrane, or pitched tiled roof
• •Structural opening from existing house into extension (steel beam, padstones, temporary support)
• •External doors (bifold or sliding — see separate cost note below)
• •Internal plasterwork, first and second fix electrics, plumbing connections
• •Underfloor heating (wet or electric depending on floor construction)
• •Building Control sign-off
• •Planning (if required)

Bifold door cost note: bifold doors are a significant cost item that is sometimes quoted separately or underspecified. A 3-panel aluminium bifold: £3,500–£5,500. A 5-panel bifold: £5,500–£9,000. Premium German-engineered bifolding or sliding systems: £8,000–£20,000+. This cost is material to the total budget.

• Single-storey extension price ranges:
• •Small (3m x 3m, flat roof, no kitchen): £45,000–£58,000
• •Standard (4m x 4m, flat roof, kitchen connection): £58,000–£72,000
• •Mid-size (5m x 5m, pitched/lantern roof, kitchen): £72,000–£90,000
• •Large (6m+ wide, premium glazing, high spec): £90,000–£130,000

A double-storey extension adds both a ground floor room (usually kitchen-diner) and a first floor room (usually a bedroom with or without en-suite) at the rear of the property. It is significantly better value per square metre than a single-storey extension — you get twice the floor area for a 40–60% price premium.

Double-storey extensions usually require planning permission (they exceed the permitted development threshold for rear extensions). This adds planning fees, architect fees, and timeline (typically 8–13 weeks for a decision from a London borough).

• What drives cost above single-storey:
• •Structural complexity: first floor structure above the ground floor extension
• •Additional exterior finish: brickwork or render on all four walls of the upper floor
• •Staircase impact: if the existing rear bedroom is modified to create the new extension footprint
• •En-suite at first floor: add £8,000–£16,000
• •Planning process: £3,000–£8,000 in architect and application fees

• Double-storey extension price ranges:
• •Standard (4m x 4m, both floors, no bathroom at first floor): £90,000–£115,000
• •With en-suite at first floor: £105,000–£135,000
• •Large (5m+ wide, high spec kitchen + full bathroom upstairs): £130,000–£165,000
• •Premium spec (bespoke kitchen, zinc cladding, aluminium windows): £150,000–£200,000+

The roof specification significantly affects both the price and the feel of the extension. The main options for a London rear extension:

Flat roof (EPDM single-ply membrane): the most cost-effective. EPDM is durable, 25-year lifespan, and allows maximum internal ceiling height. It is the default specification for most single-storey London extensions. Cost: included in standard extension price.

Pitched tiled roof: more traditional in appearance, matching the main house. Preferred by some planning authorities in conservation-sensitive areas. Requires greater structural depth (roof pitch cannot be as shallow as front elevation) and reduces the ceiling height at the rear of the extension. Add £4,000–£9,000 versus flat roof.

Roof lantern: a glazed lantern (pyramid or rectangular) on a flat roof, introducing natural light into the centre of the extension. Transforms the feel of a deep extension. Costs £2,500–£6,000 for a standard lantern (1.5m x 1m). Add structural upstand and specialist fitting.

Full glazed roof (structural glass, frameless): high-end specification. Creates a greenhouse effect — light-filled but also thermally challenging without solar control glazing. Cost: £8,000–£25,000+ for the glazed roof system alone.

Walk-on roof (for terrace access): uses structural walk-on glass or decking over the extension flat roof. Requires upstands, drainage details, and balustrade. Add £6,000–£15,000.

In terms of planning acceptability: most London boroughs accept EPDM flat roofs on single-storey rear extensions under permitted development. Conservation area properties may be required to use a pitched roof or specific covering material — always check before specifying.

One of the most significant and most underestimated cost items in a rear extension is the structural opening from the existing house into the new extension.

• The existing rear wall of the house is load-bearing (it carries the floor above and potentially the roof). To create an open-plan connection between the extension and the main house, this wall must be partially or fully removed and replaced with a structural steel beam (RSJ or UC section). The process involves:
• •Temporary support to the structure above (acrow props, needles)
• •Careful demolition of the wall section
• •Construction of padstones on piers to receive the beam ends
• •Installation of the steel beam by crane or manually depending on size
• •Fire protection to the steel (intumescent paint)
• •Making good to brickwork, plaster, and finishes

• Cost of a standard structural opening in a London terrace:
• •3m opening (most common): £4,500–£7,500
• •4m opening: £6,000–£9,500
• •5m opening (full-width): £8,000–£14,000

This should be clearly itemised in any extension quote. If a contractor does not mention it or includes it in a vague 'structural works' line without detail, push for clarification. Structural work that is poorly executed is the most expensive defect to remedy.

Most London rear extensions are driven by the desire for an open-plan kitchen-diner. The extension itself (the structure) is one cost — but the kitchen that goes into it is a separate budget that many homeowners underestimate.

• Kitchen costs are entirely separate from the extension build:
• •Budget kitchen (IKEA, B&Q): £5,000–£15,000 supplied and installed
• •Mid-range (Howdens, Wren): £12,000–£25,000 supplied and installed
• •High-end (bespoke joinery, Siematic, Roundhouse): £30,000–£80,000+

Appliances are in addition to the above for bespoke ranges. A range cooker (AGA, Falcon) alone can be £3,000–£20,000.

Underfloor heating: wet underfloor heating (connected to the boiler) is the premium option for an extension — warm and energy-efficient. Cost: £3,000–£6,000 for a standard extension floor area. Electric underfloor heating costs less to install but more to run — better suited to bathrooms than whole rooms.

Flooring: an extension floor requires a decision on whether to match the existing house or use a new material. Matching existing Victorian floorboards can cost £2,000–£8,000 (restoration + extension). New large-format porcelain tiles: £4,000–£10,000 supplied and fitted for a standard extension.

The realistic total for a rear extension with a new mid-range kitchen, good glazing, and underfloor heating in London is £85,000–£130,000 — not the £50,000 that headline quotes suggest.