Will My Extension Pay for Itself?

Does an extension add value? Yes. Houses in England and Wales that were extended between two sales rose in price by about 18% more than similar houses nearby that weren't, which works out at £1,907 per m² of floor area added. That's the good news. The less comfortable part is that building the space typically costs £2,200 – £2,500 per m² before you add a kitchen, so outside London a typical extension raises the sale price by less than it cost you.

The calculator sets those two numbers side by side for your region, house type and size. It shows what extensions like yours have added to sale prices, what yours is likely to cost (or uses your quote if you have one), and how much of that cost you'd recover if you sold. Give it your home's current value and it also prices the alternative: moving to a house that's big enough already.

Where extensions add the most

Location does most of the work. Measured as a percentage, the uplift is fairly similar across the country, so the pound value per m² follows local house prices. The North East sits at the bottom on £1,023 per m². London tops the table on £3,325 per m², and it's the one region where a typical extension tends to cover its cost per m².

House type matters next. Detached houses gain the most from extra space and terraces the least, with semis in between. A detached house in the South East is the other case where a typical extension can earn back what it cost.

Then there's the neighbourhood. Within any region, extensions in the least deprived areas add several times more per m² than the same work in the most deprived ones. Buyers in sought-after streets pay for space, and that shows up in the sale price.

Energy performance and sale price

The extensions that sold best were rarely just extra floor space. Where the house's energy rating also improved between the two sales, the sale price rose by about 24% more than similar unchanged houses. Where the extension went up but the rating stayed put, the rise was nearer 8%.

Part of that gap is the wider renovation that tends to come with a better rating: new windows, a new boiler or heat pump, insulation in the walls and roof. So read it as a picture of what buyers pay for a well-specified extension, not a price tag for insulation alone. Houses that improved their energy rating without extending at all sold for about 5% more, which is a useful baseline for what the energy work on its own is worth.

The calculator shows this split for your region alongside the main figure.

Extend or move?

Most people asking whether an extension adds value are really asking a different question: should we stay and build, or sell up and buy something bigger? Comparing the build cost with the value added only tells half of that story, because moving has costs you never see again either.

Sell and buy and you pay:

• an estate agent, typically 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 bigger home
• conveyancing on both sides, around £2,000 – £2,800
• removals for a three-bed house, around £800 – £1,450

The tax grows with the price of the house you're buying, and the bigger home costs more by roughly the value the extra space would have added. The calculator works it out on the bands for England or Wales.

So the honest comparison is what you lose either way. Extending loses whatever part of the build cost the sale price won't recover. Moving loses the fees, the tax and the van. Whichever loses less is the cheaper route to the extra room, before you weigh up the things no calculator can price: months of living on a building site against leaving a street, a school and neighbours you like.

A worked example

Take a semi-detached house in the South East and a 30 m² extension. The calculator first looks up the typical value added per m² for South East semis and multiplies it by 30. That's the likely lift in sale price, shown with the range covering the middle half of similar extensions.

Next it prices the build. Without a quote, it takes the standard rate of £2,200 – £2,500 per m², applies the regional adjustment for the South East, and multiplies by 30. Put your builder's figure in instead and the answer sharpens straight away.

Dividing one by the other gives the share of the build cost you'd get back on sale. For a South East semi the typical extension falls short of paying for itself, so read the shortfall as the cost of the space you'll use while you live there.

Finally, enter the house's current value. The calculator prices a move to a home worth that plus the value the extension would have added, totting up the agent's fee, Stamp Duty, conveyancing and removals, and sets the total against the build cost you wouldn't recover. The totals for this example appear in the calculator above; change any input and they update.

Where the figures come from

The value figures rest on HM Land Registry sale prices, matched with the floor areas recorded on Energy Performance Certificates. Each house in the data sold twice, with the second sale between 2016 and 2026, and grew by at least 15 m² (and at least a tenth of its floor area) between the two. Its price rise is compared with similar houses in the same council area, of the same type and sold over the same years, that were not extended. What's left over is the value the extension added, divided by the floor area gained. Every figure is a median for a group of at least 100 houses.

Read them with some care:

• They describe past sales across many houses, not a valuation of yours. A local estate agent who knows your street is the better guide to what your house could fetch once extended.
• They include any refurbishment done at the same time as the extension, which accounts for a few percentage points of the uplift.
• Only houses that sold before and after extending are counted, so homes extended by owners who stayed put aren't in the data.
• The spread is wide. A quarter of extensions added less than the lower figure the calculator shows, and some added very little at all.

The build cost side uses the same rate and regional adjustments as the extension cost estimator, so both calculators agree on what the work costs. That estimate leaves out the kitchen and contingency, so hold a buffer on top as set out in budgeting and contingency.