Ask an estate agent whether an extension adds value and you'll get a percentage. Ask a builder and you'll get a bigger one. Nearly every figure in circulation comes from one of two places: a lender's statistical model of a hypothetical three-bedroom house, or an opinion repeated often enough to pass for data.
We wanted to know what happened to real houses. So we took every house in England and Wales we could trace that sold, was extended, then sold again, matched each sale to the floor area on the house's Energy Performance Certificates, and compared its price rise with similar houses in the same council, of the same type, sold over the same years, that were left alone. More than a hundred thousand extended houses, measured against around one and a half million that weren't.
The answer has two halves. Extensions add value. In most of the country they also add less than they cost.
How the Sales Were Matched
The sale prices come from HM Land Registry Price Paid Data. The floor areas come from the Energy Performance Certificates lodged with MHCLG, joined to the sales through Ordnance Survey's unique property reference numbers. A house is counted as extended when a new certificate issued between its two sales records at least 15 m² more floor area, and at least a tenth more than before. Houses whose floor area and energy score barely moved between sales are the comparison group.
For each extended house, the question is simple. How much more did its price rise than the typical unextended house of the same type, in the same council, sold in the same pair of years? Divide that extra rise by the floor area added and you get a value per m². Every figure below is a median across a group of at least a hundred houses, and the comparison group, by construction, centres on zero. The full method, limitations and a CSV of every table are on our extension value research page.
What an Extension Adds, Region by Region
| Region | Houses | Value added per m² | Middle half | Build cost per m² | Recovered |
|---|---|---|---|---|---|
| London | 12,362 | £3,325 | £1,146 to £5,846 | £2,820 | 118% |
| South East | 23,160 | £2,414 | £701 to £4,141 | £2,470 | 98% |
| South West | 14,968 | £2,027 | £465 to £3,705 | £2,470 | 82% |
| East of England | 13,058 | £2,015 | £358 to £3,601 | £2,470 | 82% |
| West Midlands | 9,201 | £1,607 | £170 to £3,047 | £2,350 | 68% |
| East Midlands | 8,819 | £1,557 | £248 to £2,877 | £2,350 | 66% |
| Yorkshire and The Humber | 11,497 | £1,356 | £97 to £2,787 | £2,000 | 68% |
| North West | 11,831 | £1,352 | £22 to £2,883 | £2,000 | 68% |
| Wales | 4,111 | £1,181 | £31 to £2,725 | £2,120 | 56% |
| North East | 3,623 | £1,023 | £29 to £2,450 | £2,000 | 51% |
Two things stand out. The percentage barely changes as you move around the country: an extension adds something close to 18% to the sale price almost everywhere. The pounds change a great deal, because a fifth of a London house is a lot more money than a fifth of a house in Sunderland. Across England and Wales the median is £1,907 per m² added. London sits at £3,325, more than three times the North East's £1,023.
So the value of your extension is set mostly by where you already live. The builder's price is not.
Does It Pay for Itself?
Building a single-storey extension costs £2,200 – £2,500 per m² per m² all-in before you add a kitchen, with London and the South paying more and the North paying less. Put that beside the value column and the picture is blunt.
London is the only region where a typical extension adds more to the sale price than it cost to build. The South East is the one other place where the median comes out close to level, and that is carried by its detached houses. Everywhere else, the typical extension hands back a share of its cost when the house sells, and that share shrinks as you head north and west. In the North East and Wales a typical extension returns well under its build cost.
Then there's the kitchen. The data measures floor area, and the most expensive thing most people put inside that floor area is invisible to it. On the documented 55m² kitchen extension this site is built around, the kitchen alone came to £59,766. A buyer will pay for the room. Whether they pay extra for your worktop depends on whether they happen to like it. The higher the kitchen specification, the smaller the share of your total spend that the sale price is likely to return.
Warning
Extend or Move? The Fair Comparison
Nobody asking this question is choosing between extending and doing nothing. They're choosing between extending and moving to a house that already has the space. Judged against 100% payback, most extensions fail. Judged against moving, the arithmetic changes.
Sell and buy and you lose, for good:
- the estate agent's fee, around 1.2-1.8% of your sale price on a sole-agency deal
- Stamp Duty Land Tax in England, or Land Transaction Tax in Wales, charged in bands on the price of the home you buy, so a bigger home means a bigger bill
- conveyancing on both sides, roughly £2,000 – £2,800
- removals, about £800 – £1,450 for a three-bed house
And the bigger house is priced to include the space you want. You pay the market's value for those extra square metres whichever route you take. The difference is that extending leaves you with a shortfall between cost and value, while moving leaves you with fees and tax you never see again. Whichever shortfall is smaller is the cheaper way to get the room. The extension value calculator prices both for your own house, region and quote.
What Moves the Number
House type
| House type | Houses | Typical m² added | Sale-price uplift | Value added per m² |
|---|---|---|---|---|
| Detached | 43,777 | 38 m² | +22.5% | £2,250 |
| Semi-detached | 37,708 | 28 m² | +18% | £1,825 |
| Terraced | 31,145 | 26 m² | +12.1% | £1,412 |
| All houses | 112,630 | 30 m² | +17.8% | £1,907 |
Detached houses gain the most per m², semis sit in the middle, terraces gain the least. Part of that is location, since detached houses cluster in pricier streets. Part is the ceiling effect: a terrace on a street of similar terraces can only climb so far above its neighbours, however good the extension. And a terrace extension often eats the only garden, which buyers notice.
Cross the two and the interaction matters more than either on its own.
| Region | Detached | Semi-detached | Terraced |
|---|---|---|---|
| London | 113% | 107% | 125% |
| South East | 115% | 93% | 63% |
| South West | 97% | 75% | 58% |
| East of England | 92% | 81% | 55% |
| West Midlands | 88% | 64% | 32% |
| East Midlands | 80% | 59% | 23% |
| Yorkshire and The Humber | 96% | 74% | 29% |
| North West | 98% | 73% | 26% |
| Wales | 74% | 53% | 34% |
| North East | 76% | 55% | 27% |
Every house type in London comes out ahead. Detached houses in the South East do too, and detached houses in the South West, the North West and Yorkshire come within sight of their build cost. Terraces in the Midlands and the North are the other end of the table: a typical terrace extension there recovers a small fraction of what it cost, and a quarter of extended terraces in those regions sold for no more than their unextended neighbours.
Energy work done at the same time
The extensions that sold best were rarely extra floor space on its own. Where the house's energy rating also improved between the two sales, the uplift was about 24%. Where the extension went up but the rating stayed put, it was nearer 8%. Houses that improved their rating without extending at all sold for about 5% more.
Read that as a picture of what buyers pay for a properly finished house. New windows, insulation, a new boiler or heat pump and a fresh certificate tend to travel with a well-specified extension. If you are opening up the roof and walls anyway, the energy work is cheap to add and the data says buyers notice it.
Size, lofts and the market you sell into
Bigger extensions add more per m², not less. Larger additions are more likely to create rooms that change how the house is described (a fourth bedroom, a second bathroom), while the smallest are often conservatories and lean-tos that buyers value less. Size helps on the cost side too: foundations, steel and a new roof are spread over more floor.
Roof rooms behave much like ground-floor extensions. Houses whose certificate recorded a new loft room between sales gained about 23%, and per m² they land close to a rear extension.
The year you sell matters as well. Extended houses whose second sale fell between 2016 and 2021 gained 19-20% over their neighbours; those sold more recently gained 14-16%, drifting down as house prices flattened after the 2022 peak. Extra space is worth most in a rising market.
Neighbourhood and ground conditions
Across England and Wales, extensions in the least deprived areas added several times more per m² than the same work in the most deprived. Buyers in sought-after streets pay for space; buyers in cheaper streets have cheaper alternatives.
Clay soil, radon areas and flood zones make almost no difference to the value added. They make a difference to the cost, through deeper foundations, membranes and drainage. They move the build side of the comparison and leave the sale side alone.
The One Build We Can Vouch For
The documented build behind this site was a 55m² kitchen extension on an early 1980s detached house in Oxfordshire. That is a South East detached house, the one combination outside London where the data says a typical extension roughly pays for itself.
Its construction cost per m², before the kitchen, came to £2,221. That figure sits inside the standard build-cost range the tables above use, which is why the cost side of this analysis rests on a real ledger as well as a model. We make no claim about what it added to the house's value. A median across ten thousand South East detached houses is not a valuation of one.
Read the Figures With Care
These are medians from past sales, not a valuation of your house. The better guide to what yours could fetch once extended is the sold price of the largest comparable house on your street, because valuers and surveyors rely on completed sales of similar homes nearby. Ask a local agent for that figure before you commit to a design. If the biggest house on the road sold for less than your current value plus the build cost, the street has already told you the answer.
Only houses that sold twice are counted. Owners who extended and stayed are invisible to the data, and they may be the ones who got the most out of it.
Refurbishment rides along with extension. Houses that got a new certificate between sales but whose recorded floor area fell, our placebo group, still gained about 3% over unchanged houses. Treat that as the part of the headline uplift that is a new kitchen, new carpets and a new certificate rather than extra space.
The spread is wide. A quarter of extensions added much less than the typical figure, some added nothing, and the middle-half ranges in the tables show how far apart the outcomes sit. EPC floor areas are assessor estimates, which adds noise of its own.
And the figure you may have seen elsewhere answers a different question. Nationwide's October 2025 report found that adding a large double bedroom and bathroom can add as much as 24% to the value of a three-bedroom, one-bathroom house. That is a lender's statistical model of a best-case extension on one house type. The median here covers every extension of 15 m² or more, on every house type, and it comes from sales that happened. An older Nationwide figure of 23% still circulates and refers to earlier work.
Put Your Own Numbers In
The extension value calculator takes your region, house type and size, shows what extensions like yours added to sale prices with the middle-half range, prices the build (or takes your builder's quote), and works out how much of the cost you would recover on sale. Give it your home's current value and it prices the alternative too: the agent, the tax, the solicitor and the van.
If you don't have a quote yet, the extension cost estimator uses the same build-cost basis, so the two agree with each other and with the tables on this page.
Extend because you want the space and will use it for years. Do it with the numbers in front of you.
Frequently Asked Questions
How much value does an extension add?
Houses in England and Wales extended between two sales gained about 18% more than similar unextended houses, or £1,907 per m² of floor area added. The percentage is similar everywhere; the pounds follow local house prices.
How much value does a single storey extension add?
The data measures floor area rather than storeys, so the figures above apply to a single-storey rear or side extension of 15 m² or more. What changes the answer is region and house type: London and detached houses gain the most per m², northern terraces the least.
How much value does a kitchen extension add?
The same as any other extension of the same size, because buyers pay for the room rather than what's fitted in it. A high-specification kitchen is the part of the spend least likely to come back on sale, so budget for it as something you will enjoy rather than as an investment.
Is an extension worth it financially?
Outside London, a typical extension adds less to the sale price than the £2,200 – £2,500 per m² per m² it costs to build. It is worth it if you will live in the space for years and the alternative is moving, because moving has its own unrecoverable costs: the agent's fee, Stamp Duty or Land Transaction Tax, conveyancing and removals.
Is it cheaper to extend or move?
Compare the money you lose either way. Extending loses the share of the build cost the sale price does not recover. Moving loses the agent's fee of around 1.2-1.8%, the purchase tax on the bigger home, about £2,000 – £2,800 in conveyancing and £800 – £1,450 in removals. The calculator prices both for your own region and house.
Does a loft conversion add more value than an extension?
Houses that gained a roof room between sales rose about 23% more than unextended neighbours, a touch above the 18% for extensions in general, mainly because loft rooms tend to be larger. Per m², the two are close.