How much does a luxury house extension cost in 2026?
The short answer: large, design-led extension projects in London and Hertfordshire run from £150,000 to £500,000 and beyond in 2026. A large glazed single-storey or a straightforward double-storey scheme sits at the £150,000 to £250,000 end. Double-storey plus internal reconfiguration, or a wrap-around with a full kitchen wing, occupies £250,000 to £500,000. Basements and whole-house transformations go beyond £500,000. Every figure on this page is an all-in build cost excluding VAT, professional fees and kitchen fit-out, and this page was last updated in July 2026.
At this level the honest currency is the square metre. Expect £2,800 to £4,200 per square metre as the London baseline, rising to £3,500 to £5,500 per square metre in the premium postcodes, where houses are bigger, specifications higher and the glazing, stone and joinery that suit them cost real money.
The three bands, honestly
| Band | What it typically buys | Typical schemes |
|---|---|---|
| £150,000 to £250,000 | 40 to 70m² of new space at a high specification | Large glazed rear with structural glazing · double-storey rear · wrap-around with kitchen fit-out |
| £250,000 to £500,000 | 60 to 120m² plus reworking of the existing house | Double-storey extension with internal reconfiguration · wrap-around plus full ground-floor remodel · modest basement |
| £500,000 + | Major below-ground work or the whole house rethought | Basement leisure floor · whole-house transformation combining extension, reconfiguration and full refit |
Two things to read alongside that table. First, the bands overlap on purpose: a 45m² extension finished in book-matched stone can cost more than a 70m² one finished simply. Second, the band is set less by floor area than by how much of the existing house the project touches. The £250,000-plus projects are rarely just extensions; they are extensions plus the house reorganised around them.
What actually drives the cost
Structural glazing. Slim-framed sliding systems and structural glass are the signature of the premium extension and the single biggest line-item surprise. A full-width sliding system with a structural corner can carry a five-figure cost on its own, before the steels that hold the opening up.
Below-ground work. Basements carry excavation, underpinning, waterproofing warranties, structural engineering and party wall procedures. That is why they are priced on design rather than by the square metre, and why a basement is almost always a £500,000-plus programme once finished space is included.
The specification. Bespoke joinery, natural stone, handmade kitchens and honest materials are what these houses deserve, and they compound: the difference between a good finish and a luxury one across 80m² is regularly six figures by itself.
How much of the house the project touches. Moving stairs, opening structural walls, rewiring and replumbing while the family stays in residence: reconfiguration work is what separates the middle band from the first.
The site itself. Access for machinery and skips, mature protected trees, sloping plots and conservation-area or green-belt constraints all shape the programme. On the tightest sites, logistics alone can move a project between bands.
Professional fees and VAT. Design fees typically run 8 to 15 percent of build cost, and planning, structural engineering, party wall surveyors and building control all sit on top of the build figure, as does VAT. We itemise every one of these lines in the quote, so nothing arrives as a surprise.
What the figures exclude
Consistency matters when you compare quotes, so we band the same way everywhere on this site: build cost excluding VAT, professional fees and kitchen fit-out. When we quote your project, all of those appear as their own lines with real numbers against them. If another quote looks dramatically cheaper, the first place to look is what it quietly leaves out.
Why the cheapest quote is the expensive one
At this end of the market a low price is a risk signal, not an attraction. The most common story we hear from people who have built at scale before is not that the job was slow; it is that the final bill bore no resemblance to the estimate. Our answer is structural: a fixed, itemised proposal, line by line, VAT stated, and the number you sign is the number you pay. Changes happen only when you choose to change the scope, priced and agreed in writing first.
Costs on London’s finest streets, area by area
Every area guide in the premium cluster carries its own 2026 cost table, tuned to the local stock: Hadley Wood, Arkley, Totteridge, Hampstead, Highgate, Mill Hill and the rest of the luxury extensions hub. If your project sits in the green belt, read our guide to extending in the green belt before you set a budget; the constraint shapes the design, not the ambition.
Luxury extension cost FAQs
How much does a luxury house extension cost per square metre in London? Plan on £2,800 to £4,200 per square metre as the 2026 London baseline, and £3,500 to £5,500 per square metre in the premium postcodes such as Hadley Wood, Totteridge and Hampstead. Specification is the swing factor: structural glazing, stone and bespoke joinery push towards and beyond the top of the range.
What does a £150,000 extension buy in 2026? Roughly 40 to 70 square metres of new space at a high specification: a large glazed single-storey rear, a straightforward double-storey rear, or a wrap-around including kitchen fit-out. Excluding VAT and professional fees, and depending heavily on glazing and finish choices.
How much does a basement cost? Basements are priced on design because excavation, underpinning, waterproofing and engineering vary so much between plots. As a planning figure, a finished basement leisure floor is realistically a £500,000-plus programme. We give a fixed, itemised price after survey and design.
Do these projects need planning permission? The larger schemes almost always do: double-storey rears, wrap-arounds and basements go to full application, typically around eight weeks to determine. Some large glazed rears still fit permitted development, up to 8m on detached houses with prior approval. We manage whichever route your project needs and secure a Lawful Development Certificate for anything built under PD.
Is the price you quote really fixed? Yes. After a design visit you receive an itemised, line-by-line proposal with VAT stated, and that figure is what you pay. Scope changes are priced and agreed in writing before any extra work proceeds.
Working out a budget for something substantial? Book a private design visit → and get a fixed, itemised figure for your actual house, or start with the luxury extensions hub. 020 3051 9430