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by Sean Ronnie Hill on Feb 21, 2025
Moving house in London is expensive and stressful, which is why so many of our clients decide instead to make the home they already own work harder. The first question they ask is nearly always the same one: what will an extension cost?
The honest answer is that it depends on what you're building, how complicated your site is, and the standard you're building to. But "it depends" isn't much use for planning a budget, so here are the real numbers we're seeing on London projects, and the factors that move them up or down.
Design visualisation of a rear extension in London, its arched glazing opening to a garden with a linear pond lit by rooflights beneath the water. RISE Design Studio.
The headline figures
As a starting point for build costs in London:
- Single-storey extension: £2,300 to £3,400 per square metre
- Two-storey extension: add roughly 50 to 60% to the single-storey figure
- Basement extension: £4,400 to £5,500 per square metre, reflecting the engineering involved in excavating London clay
These are build costs only. Professional fees, VAT at 20%, surveys and contingency all sit on top, and we cover them at the end of this post. Treat the ranges as a sense-check rather than a quote: the same floor area can land at either end depending on the decisions below.
What the space is for changes the number
An extension's purpose drives its cost more than its size does, because some rooms carry far more services and joinery than others.
Kitchens are the most common reason to extend and the most expensive space per square metre, because they concentrate plumbing, electrics, appliances and joinery in one room. As a guide, around £15,000 buys a well-designed kitchen with good finishes, £25,000 to £30,000 brings in better appliances, custom cabinetry and smarter storage, and £40,000 and up moves you into stone worktops and premium brands. Islands, rooflights and slimline sliding doors each add their own line to the budget.
Bathrooms look simple and aren't. Drainage, waterproofing and ventilation all have to be right, and a new en-suite or downstairs WC often means rerouting plumbing. Budget from £5,000 for a straightforward bathroom and £10,000 upwards for something more refined, before underfloor heating and high-end finishes.
Living spaces are more flexible, and the money goes into structure, glazing, heating and insulation rather than fittings. This is where a rear or side-return extension earns its keep: done well, it changes how the whole ground floor works, not just its area.
Home workspaces range from £5,000 to £8,000 for a compact, well-lit study to £15,000 and more for a fully fitted room with storage and acoustic treatment. If what you really want is separation from home life, a garden studio is sometimes the better investment than an extension.
Whatever the room, the connection matters as much as the addition. A new space has to work with the old plan: how rooms flow into each other, whether existing rooms lose light, whether walls or stairs need to move. The best extensions read as though they were always part of the house, and that quality is designed, not bought by the square metre.
Build complexity: where budgets actually move
Two identical-looking extensions can differ by tens of thousands of pounds because of what you can't see in the photographs.
Ground conditions come first. Clay soils, nearby trees or a floodplain can mean deeper foundations or piling. Access is next: a detached house with a driveway is straightforward, while a mid-terrace with no rear access means every material comes through the house, slowly and by hand. Older housing stock brings its own surprises; Victorian and Georgian structures often need more remedial work than anyone expects until the walls are opened up. And if you're in a conservation area or a listed building, permissions take longer and the palette of acceptable materials narrows.
If you share a wall with a neighbour, the Party Wall etc. Act will likely apply, which means formal notices and usually a surveyor, sometimes one for each side. If a neighbour dissents, allow extra weeks in the programme. More broadly, neighbours can object to planning applications on daylight, privacy and massing grounds, so talk to them before the council does; a conversation over the fence early on prevents most objections later.
Structural work scales costs quickly. Removing a rear wall for an open-plan kitchen needs steelwork; adding a storey can mean strengthening foundations; large glazed openings need reinforced support, and high-performance glazing itself costs more but repays the difference in comfort and running costs. A structural engineer's input early is money well spent.
Finally, plan for what can't be planned. Rotten joists, asbestos, drainage that needs rerouting, material price movements: something always turns up. We advise a contingency of 10 to 15% of build cost, held back and ideally never spent.
Materials and specification
Materials decide how the extension looks, how it performs, and how it ages, and they're where cost and quality trade off most visibly.
For structure, brick and block remains the London standard: cost-effective, durable and easy to match to existing fabric. Timber frame builds faster and performs well thermally. Cross-laminated timber offers low embodied carbon and beautiful exposed interiors but needs specialist design. Steel and glass suit large open spans, at a premium in both cost and embodied carbon.
If you upgrade one thing beyond the regulations, make it the fabric. Building regulations set a minimum for insulation and energy performance, and the minimum is exactly that. Better insulation, an airtight structure and triple glazing cost more once and pay back every year in lower bills and a home that holds its temperature in January and July alike. Add MVHR, which recovers 75 to 90% of the heat in outgoing air, and the extension can be the best-performing part of the house. This is the approach we take on our low energy projects as standard.
Glazing and external finishes are the visible end of the same trade-off. Aluminium slimline frames cost roughly double standard UPVC; steel-framed windows can reach £2,000 per square metre. London stock brick blends with period streets, timber cladding is sustainable but needs maintenance, zinc and copper last decades at a price, and render is economical but shows its age sooner. Orientation matters too: a fully glazed south-facing room will overheat without shading designed in from the start, not bolted on afterwards.
The team, and what each member protects
The right team protects your budget as much as your design. An architect turns the brief into a buildable scheme, steers it through planning, and produces drawings detailed enough that builders price facts rather than guesses. A structural engineer makes sure the opened-up plan stands, and is essential the moment walls come out or storeys go on. On projects above roughly £250,000, a quantity surveyor earns their fee several times over by turning a hoped-for budget into a real cost plan and tracking it through the build. The builder matters most of all: choose on recommendation, a detailed transparent quote and how well they work with your architect, never on the lowest number alone. On complex builds, dedicated project management keeps the moving parts aligned; on smaller ones, your architect or builder can carry that role.
When extensions go wrong, it's almost always because one of those roles was missing or the team wasn't talking. When they go right, the saving isn't visible anywhere on an invoice: it's the expensive mistake that never happened.
The other costs to budget for
Beyond the build itself, allow for:
- Planning fees: a householder application is currently £528; a lawful development certificate for a proposed use costs half the application fee
- Party wall costs: surveyor's fees where the Act applies, typically per adjoining owner
- Professional fees: architect, structural engineer and, where appointed, QS and approved inspector
- Insurance: standard home insurance won't cover major works, so you'll need specific cover during the build
- VAT: 20% on most residential work, though a reduced 5% rate can apply where a home has been empty for two years or more
More space, or better space
The extensions that repay their cost, financially and in daily use, are the ones designed around a clear purpose: where the light should fall at breakfast, where the household actually gathers, what the existing house does badly that the new space will fix. Size is the easy part. Getting the plan, the fabric and the team right is what separates an extension that merely adds area from one that improves every day you live there.
If you're weighing up an extension and want a realistic picture of costs for your particular house, we'd be glad to talk it through. Please do get in touch and we can discuss your brief, your budget and the options for your site.
→ Email us at architects@risedesignstudio.co.uk
→ Or call the studio on 020 3947 5886
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