House Extension Cost in London 2026: Rear, Side Return and Wrap-Around
House extension costs in London for 2026: single-storey rear £78k–£140k, double-storey £120k–£200k. What's included, planning notes and how to compare quotes.
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A single-storey rear extension in London costs £78,000–£140,000 in 2026, and a double-storey £120,000–£200,000. Size, specification and postcode move the figure inside that range, and the design and structural work sit inside it rather than on top — our extensions page sets out what a build covers, the cost hub breaks it down by area, and the estimator prices your own footprint.
A rear extension is London’s most popular home improvement. Done right, it transforms a dark Victorian terrace into a contemporary open-plan home — and adds substantially more value than it costs.
This guide sets out real 2026 costs from 2VP’s completed projects.
Extension Costs at a Glance (London 2026)
| Extension Type | Typical London Cost | Notes |
|---|---|---|
| Single-storey rear (small, ≤15m²) | £45,000 – £80,000 | Infill or modest addition |
| Single-storey rear (standard, 20–30m²) | £78,000 – £140,000 | Most common; open-plan kitchen |
| Side return only | £55,000 – £90,000 | Narrow infill; big impact on light |
| L-shaped (rear + side return) | £95,000 – £165,000 | Full open-plan transformation |
| Wrap-around | £110,000 – £185,000 | Maximum ground-floor footprint |
| Double-storey rear | £120,000 – £200,000 | Adds bedroom above kitchen |
All prices include design, structure, building regs, labour, materials and VAT. Kitchen and fitted furniture are not included unless specified.
What Makes a House Extension Cost More?
Size
Cost per m² tends to decrease as extension size increases — the costs that barely change with size (groundworks, roof structure, steelwork) are spread across more usable area.
Structural complexity
Most Victorian terraced homes have shallow strip foundations. If ground conditions require piled foundations, costs increase by £8,000–£15,000. Basement extensions are a different product category entirely.
Glazing specification
The biggest single variable in an open-plan extension budget is the rear glazing. Standard aluminium bifold doors (3m–4m opening): £3,500–£6,000. Premium sliding glass doors with structural glass roof: £12,000–£22,000. Full-width rooflights and structural glazing can add £15,000–£30,000 to the project.
Kitchen and finishes
2VP builds the shell and services. What you spend on kitchen installation, worktops and appliances is separate — plan £20,000–£60,000 for a quality kitchen in a London extension.
Postcode uplift
As with loft conversions, prime London postcodes carry a 20–40% cost premium over outer boroughs — driven by labour rates, access constraints and specification expectations.
The Victorian Side Return: London’s Most Overlooked Space
Most Victorian terraced homes in Fulham, Chelsea, Battersea and Clapham have a narrow passage running alongside the ground floor — typically 900mm to 1.5m wide. It’s usually wasted space: a dark corridor to the garden.
A side return extension fills this passage, pushing the rear of the house to the full width of the plot. Combined with a rear extension (an L-shape), it:
- Doubles the ground-floor width at the kitchen end
- Maximises natural light with a large rooflight running the full length of the extension
- Creates a genuine open-plan space — kitchen, dining and living in one continuous room
The side return element itself typically costs £55,000–£90,000 as a standalone project. As part of an L-shaped extension, the additional cost of incorporating the side return is proportionally lower (the mobilisation, groundworks and roof are already priced for the rear extension).
Planning Permission for Extensions in London
Permitted Development
A single-storey rear extension up to 3m (semi-detached or terraced) or 4m (detached) depth, maximum 4m height, can proceed under Permitted Development Rights — no planning application, just a Certificate of Lawful Development.
The Neighbour Consultation Scheme (Prior Approval) extends this to 6m (semi-detached/terraced) or 8m (detached) for single-storey extensions.
When you need Full Planning Permission
- The extension is to the side elevation visible from the street
- You want a double-storey extension (side or rear)
- The property is in a conservation area or Article 4 Direction area
- The property is listed
2VP manages the full planning process in-house.
What’s Included in a 2VP Extension Contract?
A typical 2VP extension contract covers:
Pre-contract
- Measured survey and topographical survey
- Architectural drawings for building regulations
- Structural engineer’s calculations and specifications
- Planning application (if required)
- Building regulations submission and approval
- Party wall notice service
Structure and fabric
- Excavation and new foundations
- New structural walls (brick, block or structural frame)
- Steel beams and posts
- Roof structure: flat roof (warm deck) or pitched
- External windows and doors (aluminium as standard)
- Insulation to Part L
Services
- Extend central heating (new radiators, underfloor heating if specified)
- Electrical first and second fix
- New WC if included in the scheme
Finishes
- Plasterboard and skim throughout
- Floor screed or timber deck ready for floor finish
- Internal paint (white as standard)
Not included: kitchen, furniture, flooring finish, external landscaping.
How to Read a Quote
When comparing extension quotes, ensure you are comparing like-for-like:
- Structural glazing included? Many “low” quotes price sliding doors at minimum spec. Ask what bifolds or sliders are specified.
- Is underfloor heating included? Often priced separately.
- What foundation depth is assumed? If the quote doesn’t mention a site investigation, it may be assuming easy conditions that your site doesn’t have.
- Is the kitchen included? Almost never — but confirm.
- Is VAT included? Standard extensions are subject to 20% VAT.
2VP prices against a JCT Minor Works contract, itemised stage by stage, with a site risk allowance already inside the rates rather than added at the end. The scope you sign is the scope we price, and nothing is added to it without your written approval first.
Before you compare quotes at all, though, it is worth knowing which procurement route produced them: design and build versus a traditional contract decides who pays when the drawings and the building disagree, and that is a larger number over a project than any single line above.
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- Extend or Just Renovate: One House, Two PricesWe priced the same Battersea terrace twice — with the rear extension and without. The gap was £38,532 and four weeks, and 68% of it sat in two trades.
- Loft Conversion Cost in London 2026: The Complete GuideLoft conversion costs in London for 2026: Velux £35k–£60k, dormer £55k–£95k, mansard £80k–£140k. Real prices, planning notes and what moves the final bill.
- Hammersmith Loft Conversions: What W6 Allows40 cubic metres on a terrace, 50 on anything else — the Class B limits, what Hammersmith's conservation areas remove, and what the Article 4 does not touch.