House Extension Cost London 2026 — What to Budget Primary keyword: house extension cost London 2026 URL slug: /blog/house-extension-cost-london-2026 Word count: approx 1,020
House Extension Cost London 2026 — What to Budget
If you're researching house extension costs in London for 2026, you'll find a wide range of figures online — and that range reflects genuine variation rather than vague estimates. Extension costs in London are influenced by a large number of factors, and understanding what drives those differences will help you plan your project and budget with far greater accuracy.
This guide covers typical cost ranges for the main extension types, explains the key cost drivers, and highlights the pitfalls to watch out for when comparing quotes.
Typical House Extension Costs in London 2026
The figures below are based on Greater London market conditions as of 2026. They assume a well-specified build using reputable contractors — not the cheapest tender on the market, and not the most premium specification in every category.
Single-storey rear extension: £40,000–£75,000
The most common extension type in London. Typical schemes add a kitchen-diner or enlarged living area to the rear of the house. Lower figures apply to smaller, simpler schemes: a 3-metre depth addition, standard materials, no major structural opening into the main house. Higher figures reflect larger footprints, bifold or sliding doors, quality finishes, and significant structural alterations.
Double-storey rear extension: £65,000–£120,000
Adds accommodation on both ground and first floors simultaneously. Generally the most cost-efficient way to add significant space, since the cost of foundations, roof, and external envelope is shared across two levels. Planning periods may be longer for more substantial schemes.
Side return extension: £35,000–£65,000
Common on Victorian and Edwardian terraced and semi-detached London houses, where a narrow passage or return often sits beside the kitchen. Filling this space creates a wider kitchen or open-plan ground floor. Smaller footprint than a rear extension, but structural openings, drainage, and new rooflights can push costs meaningfully higher.
Wrap-around extension: £80,000–£150,000
Combines rear and side return into a single L-shaped or wrap-around footprint. Creates genuinely transformational ground floor space but involves greater structural complexity, more external envelope, and a longer build programme.
[link: /services/extensions]
What Drives the Cost Variation?
Structural Openings and Steels
Creating large open-plan space means removing internal walls and inserting structural steel beams (RSJs) to carry the load above. A single straightforward steel opening might add £3,000–£6,000 to your project costs. Multiple openings, or load-bearing walls in awkward positions, add considerably more. The structural engineer's specification has a direct effect on price.
Drainage and Sewer Build-Over
Many London houses have drainage runs and sewer pipes close to — or directly beneath — the proposed extension footprint. Where a drain is affected, a build-over agreement with Thames Water may be required, and drainage redesign can add both time and cost to the project. A thorough pre-construction survey identifies these risks before they become on-site surprises.
Specification and Finishes
The internal specification is one of the largest variables in any extension budget. A kitchen extension with standard cabinetry, solid worktops, and mid-range tiles will cost substantially less than one with hand-painted units, quartz worktops, underfloor heating, and bespoke joinery.
At RCB Design & Build, our proposals make material responsibilities explicit from the outset. Where second-fix materials — flooring, sanitaryware, kitchen units, tiling, lighting — are client-supplied, this is clearly stated so there is no ambiguity when pricing.
Party Wall Matters
If your extension sits within three metres of a neighbouring property's foundations — as is regularly the case in London — the Party Wall etc. Act 1996 applies. You will need to serve notice on your neighbours, and if they dissent, appoint a party wall surveyor. Surveyor fees typically run to £700–£1,500 per adjoining owner, and sometimes more where the matter is complex.
Building Control and Professional Fees
Building Regulations approval is required for any extension. Local authority Building Control fees typically range from £600–£1,500 depending on scheme size. If you are using an architect for design and planning coordination, allow an additional 5–12% of construction cost depending on their fee structure and scope of service.
Contingency
A properly managed project budget should always include a contingency allowance — typically 10–15% for extension and refurbishment works. Unforeseen conditions are common in London's older housing stock: hidden drains, unexpected structural elements, poor previous construction. Having a contingency allows decisions to be made based on what is right for the project, not what is cheapest under pressure.
Why Three Quotes Often Doesn't Work
The instinct to get three quotes and choose the middle one is understandable. It only works if all three contractors are pricing the same scope. In practice, contractors price what they choose to include. One quote might cover demolition, steels, and drainage as a matter of course. Another might exclude all three. The cheapest quote is often the one that leaves the most out — and leaves the most room for variation orders once work has started.
The most effective protection against this is a detailed, written scope of works produced before pricing begins, so every contractor is working from identical information. At RCB Design & Build, we invest in pre-construction scoping precisely because it produces more reliable pricing, cleaner comparisons, and smoother delivery.
Getting an Accurate Budget for Your Project
For accurate budgeting, you need a project-specific figure. The right approach is:
- Define your scheme clearly — size, type, key design features
- Understand your planning route
- Obtain architectural drawings before any detailed pricing
- Have a pre-construction review with your principal contractor
- Agree a detailed, written scope before pricing begins
We offer structured pre-construction support for London extension projects, helping homeowners move from initial idea to a reliable budget figure with confidence and clarity.
Ready to Get Started?
Call 07359 872594 or email contact@rcbgroup.co.uk