House Extension Costs in London: What to Budget for a Rear, Side, or Double-Storey Extension

A single-storey rear extension is the most common extension type in London, adding additional ground floor living space — typically a larger kitchen-diner or living room — to the rear of a Victorian or Edwardian terrace or semi-detached house.

Cost per square metre (2024, mid-range specification): £2,200–£3,200 per m² of new internal floor area, including: Demolition of the existing rear wall and any structures to be removed. Foundations (strip or pad foundations, typically 1.0–1.5m deep in London clay — deeper where trees are present or where clay heave risk is elevated). Structural steelwork (RSJs for the opening between the extension and the existing house, and any steels required to support the party wall or party wall return where applicable). External walls (brick and block cavity construction to match the existing house, or structural insulated panels where specified, with insulation to Part L standards). Roof (flat roof with warm deck construction and EPDM or GRP covering, or pitched roof to match the existing — pitched roofs add approximately 10–20% to the roofing cost over a flat roof). External joinery (bifold or sliding doors, rooflights or lanterns, windows) — note: the specification of glazing has a very significant effect on cost. A simple 3m bifold door in aluminium powder-coated: £3,000–£5,000. A 5m bifold with corner glazing and a lantern: £12,000–£18,000+. First fix electrics (lighting circuits, power circuits, boiler/heating connections if affected). First fix plumbing (if the extension includes a utility room or replaces the kitchen). Insulation and plasterboard (walls, floor, ceiling to Part L standards). Plastering (skim and finishing). Second fix electrics (sockets, switches, lights, consumer unit modifications). Second fix plumbing (if applicable). Floor screed or timber floating floor substrate.

Not included in a standard extension build cost: Kitchen units and appliances (see the kitchen guide — add £10,000–£50,000+ depending on specification). Bathroom suite and tiles (if the extension includes a WC or utility). Flooring (tiles, engineered wood, carpet — add £50–£200+ per m²). Decoration (painting and decorating — add £1,500–£3,500 for a typical extension). External landscaping (new terrace, resurfaced path — add £2,000–£8,000). Planning and professional fees.

Example cost ranges for common extension sizes: 12m² (small galley kitchen extension): £26,000–£38,000 build cost. 20m² (medium kitchen-diner): £44,000–£64,000 build cost. 30m² (large open-plan kitchen-diner): £66,000–£96,000 build cost. 35m² (large with roof terrace or garden room elements): £80,000–£115,000 build cost.

A double-storey extension adds new space on two levels — typically a rear or side extension that is two floors tall. The cost per square metre is typically lower than a single-storey extension because the foundation and roof costs are shared across two floors of space.

Cost per square metre (2024, mid-range specification): £1,800–£2,600 per m² of total new internal floor area (sum of both floors).

Example: A 5m wide × 4m deep double-storey rear extension — 20m² on ground floor + 20m² on first floor = 40m² total. At £1,800–£2,600 per m²: £72,000–£104,000 build cost.

What makes a double-storey extension more complex (and more expensive) than a standard rear extension calculation suggests:

Structural connections at first floor: where the new first floor extension connects to the existing house, new structural openings through the existing first-floor rear wall are usually required — more steelwork, more temporary propping, and more disruption to the occupied house.

Bathroom or en-suite on the new first floor: a new bathroom on the first floor adds plumbing, tiling, sanitaryware, and specialist wet area waterproofing — typically an additional £6,000–£12,000 for a mid-range bathroom.

Impact on the existing rear bedroom: in many London terrace properties, the existing rear bedroom window is directly above the area being extended. The window may be lost (bricked up as part of the party wall or the new extension wall), requiring a new window in a different location — additional cost and potentially a planning consideration.

Party wall implications at first floor: where the extension extends alongside a party wall, and the new first floor structure is to be built off the party wall, party wall notice is required. The structural interface between the new extension and the existing party wall at first floor is more complex than at ground floor level.

Permitted development for double-storey rear extensions: In many London boroughs, double-storey rear extensions require full planning permission (they exceed the parameters of permitted development under Schedule 2, Part 1 of the General Permitted Development Order). Always check with the LPA before proceeding.

Side return extension: Many Victorian and Edwardian London terrace properties have a narrow side return — the gap between the main house and the side boundary (typically 1.5–3.0m wide and 3–5m long), often roofed over as part of the original rear outrigger (the single-storey back addition that contains the original kitchen). A side return extension fills in this gap to create a wider, more functional kitchen-diner.

The side return extension is one of the most popular project types for London terraced houses — it is often combined with a rear extension to create a large open-plan ground floor space.

Cost for a side return extension: Because a side return extension involves a relatively small footprint but significant structural complexity (typically requiring the removal of the original outrigger roof and the construction of a new combined roof over the wider space), the cost per m² is often higher than a pure rear extension.

Typical side return extension cost: £35,000–£70,000 for a typical London Victorian or Edwardian terrace, depending on size, specification, and what other works are included.

When combined with a rear extension (wrap-around), the combined project often achieves economies in the fixed costs (scaffolding, site management, professional fees) compared with the two projects individually.

Wrap-around extension: A wrap-around extension combines a rear extension and a side return extension to create an L-shaped addition that maximises the ground floor footprint. It is typically the most transformative single structural change to a London terrace — adding 30–55m² of new floor space and creating a large, light, open-plan kitchen-diner-living space.

Cost for a wrap-around extension (2024): £60,000–£120,000 build cost depending on size and specification. For a premium specification (large lantern, bifold doors, bespoke kitchen, premium flooring), total project cost including kitchen fit-out can reach £130,000–£180,000.

In South and South West London (Clapham, Battersea, Wandsworth, Richmond) and West London (Chiswick, Ealing, Islington) where Victorian terraces are large and property values are high, wrap-around projects at this level of expenditure typically show a strong return on investment — often adding more to the property value than the cost of the works.

1. Ground conditions: London clay is expansive — it shrinks in dry weather and swells in wet weather, causing seasonal ground movement. Trees on or adjacent to the site extract moisture from the clay and can cause significant differential settlement. Building close to trees in London clay typically requires deeper foundations (pile and beam or enhanced strip foundations to 2.0–3.0m depth) rather than standard strip foundations at 1.0–1.5m. The cost premium for piled or enhanced foundations can be £8,000–£20,000 over standard strip foundations.

2. Party wall complexity: Where the extension is being built adjacent to a party wall or requires works to the party wall (cutting into it, building off it, or underpinning below it), the party wall process adds cost: surveyor fees (£1,500–£3,500 total), potential Schedule of Condition costs, and possibly additional structural measures to protect the adjoining property.

3. Size and specification of glazing: The glazed area is one of the largest single variables in extension cost. A basic 3-panel bifold door (2.4m wide) at standard specification: £2,800–£4,500. A 5m+ bifold with structural glass corner, overhead rooflight lantern, and concealed drainage gutter: £18,000–£30,000. Choosing a simpler glazing solution significantly reduces cost without compromising the usefulness of the space.

4. Existing services: Where the extension footprint overlaps with existing drainage runs, gas mains, or electricity supplies, these must be diverted before construction. Drainage diversion in particular can be expensive — a sewer build-over or drainage diversion can add £3,000–£8,000 to the project.

5. Access to the site: London terraced properties with no side access (rear access only possible through the house) are more expensive to build than those with good side gate access. Materials handling is slower, more expensive (smaller loads, more trips), and limits the plant that can be used. Properties with no rear access at all (a terrace backing onto another terrace with shared rear lane access or no rear access) are the most expensive to extend.

6. London labour market: London construction labour costs are consistently 20–35% higher than the national average. The combination of high land costs, skilled labour demand, and the need to source specialist subcontractors in a busy market means that London extensions consistently cost more than equivalent projects in the Midlands or North.

When comparing extension quotes from different contractors, the price is only meaningful if you are comparing the same scope of works. Common areas where quotes differ:

VAT: construction work on an existing dwelling is charged at the standard VAT rate (20%). A quote that does not include VAT will look significantly cheaper than a VAT-inclusive quote. Always confirm whether the quoted price includes VAT.

Structural work: some quotes list steelwork as "provisional" or "excluded until structural engineer's design is confirmed." A quote that excludes structural steel is less complete than one that includes it (even on a provisional basis).

Drainage and services: if the new extension affects the existing drainage layout, the cost of drainage works must be included. Check whether the quote includes drainage, sewer surveys (to confirm the position of the sewer), and any build-over approval fees.

Finishes: check whether flooring, tiling, decorating, and kitchen fit-out are included. Most construction-only quotes do not include these — but a turnkey contractor's quote will. Comparing a construction-only quote with a turnkey quote directly is misleading.

Provisional and prime cost sums: legitimate quotes often include provisional sums for items whose cost is not yet confirmed (the structural steel, the kitchen, the glazed doors). Provisional sums are not fixed — the final cost will adjust to reflect what is actually supplied. A quote full of very low provisional sums may look cheap but will increase significantly when the provisionals are replaced by actual costs.

What a reliable extension quote should always include: A detailed written scope of works — not just a lump sum. A materials specification (type and quality of brickwork, roof covering, insulation specification). Structural engineer's initial assessment or a provisional sum for structural works. Clarity on VAT inclusion. Clear statement of what is excluded. A programme (timeline for the works) with key milestones. Details of the contractor's public liability and employer's liability insurance. Evidence of Building Regulations compliance route (Approved Inspector or Local Authority Building Control).