What Does a Rear Extension Cost in London in 2026?

By specification level (per m², construction only, excluding VAT):

Standard — solid build, standard glazing, basic finishes: £2,000–£2,500/m²

Mid-range — bi-fold or sliding doors, roof lantern, good-quality finishes: £2,500–£3,200/m²

High-spec — premium glazing, underfloor heating, bespoke joinery: £3,200–£4,000+/m²

These are build-only figures. Add VAT at 20%, professional fees (architect, structural engineer, party wall surveyor where applicable), and a 10–15% contingency to arrive at your total project budget. The “What’s Included — and What Isn’t” section below breaks all of that down.

What’s Included — and What Isn’t

This is where most online guides go wrong. Here’s what a properly scoped quote should cover.

Typically included:

Groundworks and foundations Structural frame, walls, and roof Glazing (to agreed spec) Electrics, plumbing rough-in, and plastering Internal decoration to first-fix standard

Typically excluded — budget for these separately:

Architectural fees: £1,200–£5,000 Structural engineer fees: £1,200–£3,500 Party wall surveyor (if required): £1,000–£2,500 per neighbour Planning application (if required): £548 Kitchen fit-out: £15,000–£50,000+ depending on range Landscaping and external works VAT at 20% on all construction

What Drives the Price Up in London Specifically

London extensions cost 25–40% more than the national average. Here’s why.

Labour rates are higher. Skilled tradespeople in London command a premium. There is no way around this — and attempting to cut it by hiring cheap labour is the single biggest source of extension disasters we see.

Sites are tighter. Terraced housing, shared boundaries, restricted rear access, and basement drainage all add complexity. More planning, more time, more cost.

Party wall obligations. If you share a wall with a neighbour — which most London homeowners do — you are legally required to serve party wall notices. This adds process time and professional fees.

Borough matters. Inner London boroughs (Islington, Wandsworth, Lambeth) tend to run 10–15% above outer London areas like Bromley, Sutton, or Merton. Conservation area properties add another layer of cost regardless of borough.

Planning Permission: Which Route Does Your Extension Need?

Most single-storey rear extensions in London fall into one of three routes.

Route 1 — Permitted Development (no application needed) You can build without any planning application if your extension is: no more than 3m deep for a terraced or semi-detached house (4m for detached), no higher than 4m at the ridge, and not in a conservation area or on a listed building. This is the simplest and fastest route. Get a Lawful Development Certificate anyway — it costs around £103 and protects you at the point of sale.

Route 2 — Prior Approval (larger single-storey extensions) The Prior Approval process allows terraced and semi-detached houses to extend up to 6m, and detached houses up to 8m. Your council notifies neighbours and has 42 days to respond. There is no design assessment — only neighbour objections are considered. This is the most common route for a 4–5m rear extension in London.

Route 3 — Full Planning Permission Required if you’re in a conservation area, exceeding prior approval limits, or building a two-storey rear extension. Expect a 8–12 week determination period and more detailed design scrutiny. Budget £3,000–£6,000 for architect fees at this stage.

Does a Rear Extension Add Value?

Yes — consistently. A well-executed rear extension in London typically adds 10–20% to the property’s value, depending on size and location. In boroughs like Wandsworth, Merton, and Lambeth, where the average semi-detached home sits at £700,000–£900,000, that means a potential uplift of £70,000–£180,000 from an investment of £65,000–£95,000.

Compare that to moving. Stamp duty, legal fees, agent fees, and removal costs on a £800,000 property can easily top £50,000 — before you’ve even gained a square metre. For most London homeowners, extending makes better financial sense.

How to Get an Accurate Quote (and Avoid the Traps)

Three rules:

Get at least three quotes — and make sure they’re like-for-like. A quote that looks 20% cheaper is often scoping 20% less. Check what’s included: foundations, structural openings, glazing spec, finishes, and sign-off.

Don’t choose on price alone. Ask for references from similar projects in London. Ask how long they’ve been operating. Ask if they’re FMB-registered. A builder who has been delivering extensions in London for 20+ years will cost a bit more per day — and save you significantly more in avoided problems.

Get the spec locked before the contract is signed. Variations mid-build are the primary driver of costs running over. The more detail in the specification at the start, the fewer surprises at the end.

Blueprint Builders is a Federation of Master Builders (FMB) member with over 20 years of experience delivering extensions across London and the Home Counties. Our 5-star Google rating reflects how we work: transparent pricing, detailed specifications before any work begins, and open communication throughout the project.

Plan Your Rear Extension with Blueprint Builders

If you’re planning a rear extension in London — whether in Wandsworth, Merton, Lambeth, or anywhere across the Home Counties — Blueprint Builders can give you a clear, detailed quote with no obligation.

We cover the full project: design coordination, planning, structural work, build, and fit-out. No hidden costs. No vague ranges.

Get in touch with Blueprint Builders for a free consultation →