House Extension Cost London — Honest 2025 Guide
Real prices from real London projects. Rear extensions, side returns, wrap-arounds, kitchen extensions and double-storey — what they actually cost, what's included, and what changes the price.
Extension costs by type
All figures include labour, materials, structural steelwork, structural engineer coordination, and Building Regulations. Kitchens, second-fix finishes, and planning fees are separate.
Single-Storey Rear Extension
£45,000 – £90,000The most common London home improvement. Extends the rear of the property to create a larger kitchen, open-plan kitchen-diner, or additional living space. Typically 3m–6m projection, full-width or part-width.
Typical use: Kitchen-diner or open-plan living. Most single-storey rear extensions are permitted development (up to 3m for terraced/semi, 4m for detached).
- ✓Foundations and ground works
- ✓External walls — brick or block to match
- ✓Structural openings and steel beams (RSJ)
- ✓Flat or pitched roof with insulation
- ✓Bi-fold or sliding door opening
- ✓First-fix plumbing and electrics
- ✓Underfloor heating (if specified)
- ✓Plastering throughout
- ✓Building Regulations — structural engineer and applications
- ✗Kitchen units, worktops, and appliances (client-supplied)
- ✗Second-fix materials (tiles, flooring, ironmongery, fittings)
- ✗Carpet
- ✗Planning permission if required (typically for >3m PD limit)
Side Return Extension
£35,000 – £65,000Infills the narrow side passage common in Victorian and Edwardian terraces, typically 1m–2m wide. Often combined with a rear extension to create an L-shaped footprint. Transforms the ground floor layout dramatically.
Typical use: Combined with kitchen knock-through. No planning permission in most cases — side return infill is generally permitted development where it doesn't exceed property width.
- ✓Side return ground works and foundations
- ✓External walls or glazing
- ✓Party wall works where required
- ✓Structural opening and RSJ into main house
- ✓Roof — typically flat with lantern or rooflight
- ✓Plastering and first-fix services
- ✗Party wall surveyor fees if required (typically £600–£2,000 per neighbour)
- ✗Kitchen units and appliances
- ✗Second-fix materials
- ✗Carpet
Wrap-Around Extension
£65,000 – £120,000Combines a rear extension with a side return in a single continuous structure. Creates a large L-shaped or near-square footprint at ground floor — transforming the entire rear of the property.
Typical use: Open-plan kitchen-dining-living for family living. Usually permitted development. Creates genuinely large spaces on modest-sized terraced properties.
- ✓Combined rear and side return structure
- ✓All ground works, foundations, external walls
- ✓Structural steelwork (typically multiple beams)
- ✓Flat or mixed roof with glazing
- ✓Full first-fix M&E
- ✓Plastering throughout
- ✗Kitchen (client-supplied)
- ✗Second-fix materials
- ✗Carpet
- ✗Party wall fees if required
Kitchen Extension
£45,000 – £70,000A focused single-storey rear extension specifically designed to expand and reconfigure the kitchen — often with an opening into the existing rear reception room. Usually includes a structural opening and new bi-fold or sliding door to garden.
Typical use: The most popular home improvement in London. Fixed price includes structure, opening, first-fix, and plastering — not the kitchen itself.
- ✓Extension structure (foundations, walls, roof)
- ✓Structural opening and steel beam(s)
- ✓New rear glazing or door
- ✓Electrics — sockets, extraction point, lighting
- ✓Plumbing — relocation or extension if required
- ✓Plastering and boarding
- ✗Kitchen units, worktops, appliances (client-supplied)
- ✗Second-fix materials
- ✗Carpet
Double-Storey Extension
£80,000 – £160,000Extends both ground and first floors — typically to the rear. Adds kitchen extension at ground level and an additional bedroom (often with en-suite) on the first floor. Maximum value added per square metre of all extension types.
Typical use: Popular on semi-detached and detached properties. Usually requires full planning permission (not permitted development for upper floor). Programme: 16–24 weeks.
- ✓Ground and first-floor structure
- ✓Staircase alterations where required
- ✓First-floor bedroom and en-suite (first-fix)
- ✓All structural steelwork
- ✓Roof — tiled to match existing where possible
- ✓Full M&E first-fix both floors
- ✓Plastering throughout
- ✗Planning permission fees (usually required)
- ✗Party wall fees if required
- ✗Second-fix materials throughout
- ✗Kitchen (client-supplied)
- ✗Carpet
About these figures
These are ballpark ranges from completed London projects as at 2025. They are not formal quotations. Actual costs depend on your specific property, ground conditions, structural requirements, and specification choices. A detailed estimate is only produced after a site survey. Costs may refine up or down once we have assessed your project properly.
Extension cost — frequently asked questions
How much does a house extension cost in London?
A single-storey rear extension in London typically costs £45,000–£90,000. A side return extension costs £35,000–£65,000. A wrap-around (combined rear and side) costs £65,000–£120,000. A double-storey extension ranges from £80,000–£160,000. These are real London figures based on projects completed by RCB Design & Build — not national averages which typically understate London costs.
Do I need planning permission for a rear extension in London?
Most single-storey rear extensions in London are permitted development — meaning no planning application is required. The key limits are: 3m projection for terraced and semi-detached properties, 4m for detached. Width should not exceed half the width of the original house. Height should not exceed the height of the existing eaves. Properties in conservation areas, Article 4 direction areas, and listed buildings may have restricted permitted development rights. We check this at survey stage for every project.
How long does a house extension take?
A standard single-storey rear extension takes 14–20 weeks from contract sign to completion. This includes approximately 2–4 weeks of pre-construction (structural engineer details, Building Control application, material procurement) and 10–16 weeks on site. A wrap-around extension typically takes 18–24 weeks. A double-storey extension runs 22–30 weeks on site. We provide a programme before work starts so you know the timeline for your specific project.
Why do extension costs vary so much between quotes?
Several reasons: specification level (what materials are included, whether underfloor heating is specified), scope boundaries (some quotes include kitchens and finishes, others don't), assumptions about structural conditions (what the ground conditions will require), and commercial strategy (some contractors quote low to win the job, then raise the price through variations). Our scope of works documents exactly what is included, so you can compare like-for-like.
What is a structural opening and is it included?
A structural opening is the removal of part of a load-bearing wall to connect the extension to the main house. It requires a structural engineer's calculation and a steel beam (RSJ) to carry the load above. A structural opening is included in all our extension prices — it is not an extra. The scope of works specifies the beam size and opening dimensions.
Does the extension price include the kitchen?
No. Our extension prices cover the building structure — foundations, walls, roof, structural openings, first-fix plumbing and electrics, and plastering. The kitchen (units, worktops, appliances) is client-supplied. We install your chosen kitchen if you have one, or can help coordinate supply through trade suppliers. This keeps our fixed price based on what we can control, and gives you freedom over the kitchen specification.
Get a free estimate for your extension
Tell us about your property and what you're planning. We respond within a few hours with a realistic budget range and can arrange a free site visit when you're ready.