Extension value calculator
Build cost per m² against local value per m² — does the extension pay?
Not sure? Your area’s £/m² is on our area pages.
Watch the ceiling value — every street has a price the best house rarely breaks, however much you spend. Budget for planning permission, building regs and party-wall agreements too. Illustration only.
The core arithmetic of extending is one comparison: what a square metre costs to build versus what a square metre is worth in your postcode. Building currently runs roughly £2,000–£3,500 per m² for a straightforward single-storey extension (higher in London and for complex or high-spec work), plus VAT, professional fees (architect, structural engineer, building control — typically 10–15% on top), and a kitchen or bathroom if the new space contains one. If local homes sell around £4,500/m², a 20m² extension costing £55,000 that adds £90,000 of floor-area value looks compelling; at £2,200/m² locally, the identical project destroys £10,000–£15,000.
The complication is the ceiling: every street has a price band the best house on it struggles to escape, because buyers with bigger budgets buy on better streets. Value added is not linear in metres — the extension that takes a three-bed to a four-bed with a second bathroom can jump the home into a new buyer bracket and outperform the £/m² sum, while the one that makes an already-large house larger often underperforms it badly. Check what the biggest, best homes on your street have actually sold for; that is your ceiling, and money spent pushing past it is mostly gone.
Two honest reminders. Budget with a 10–20% contingency — extensions meet the ground, the drains and the unknown, and quotes are opening positions; and remember that not extending is a competitive option: the difference between extending and moving is the extension cost versus stamp duty, agency and moving fees plus the price gap to a bigger home, and in high-duty brackets extending wins that comparison more often than it used to. If you would enjoy the space for years, value “for us” legitimately counts — just don’t book it as profit.
Common questions
How much does an extension cost per square metre?
Roughly £2,000–£3,500/m² for a standard single-storey extension in 2025/26, before VAT and fees — so a 20m² kitchen extension typically lands at £50,000–£80,000 all-in once professional fees, building control and fitting-out are counted. London, difficult sites and high specifications push it higher. Always price from itemised quotes, not averages.
How much value does an extension add?
As a first pass: the metres added times your local sold £/m² — then adjusted for what the space achieves. Adding a genuine bedroom-and-bathroom (a three-bed becoming a four-bed) often beats the £/m² estimate; enlarging already-generous space, or pushing past the street’s ceiling price, often falls short of it. The local ceiling is the binding constraint.
Do I need planning permission to extend?
Many single-storey rear extensions fall under permitted development (within size limits — typically up to 3–4m projection, more via prior approval), but conservation areas, listed buildings, flats and already-extended homes usually need full permission. A lawful development certificate is worth getting even when permission isn’t needed — buyers’ solicitors ask for it.
Is it cheaper to extend or move?
Compare the extension’s all-in cost against the full cost of moving (stamp duty, agency, legal, removals — often £15,000–£30,000+ for movers in higher price brackets) plus the price difference to a home that already has the space. Extending keeps transaction costs at zero, which is why high stamp duty has quietly shifted this comparison towards improving rather than moving.
Numbers are half the story. Check the home itself.
One search pulls the official record on any address in England & Wales — value, flood risk, schools, noise and more, scored 0–100.