Costs and value
Does an extension add value to your home?
Short answer
Usually yes, but by less than a simple price per square metre sum suggests. Nationwide's research published on 15 October 2025, "Loft it up", found that, all else being equal, a 10% increase in floor space adds 5% to the price of a typical house. Adding space for an extra double bedroom added 10% to 17%, depending on the type of house. A loft conversion or extension with a large double bedroom and a bathroom could add as much as 24% to a three-bedroom, one-bathroom house.
These are national averages drawn from Nationwide's lending data. They are not a valuation of your home.
What the October 2025 research found
Nationwide correlated the prices paid for homes against their characteristics (type, age, bedrooms, bathrooms, floor area), region and neighbourhood type, using its lending sample from October 2024 to September 2025. It assumed an extra bedroom of 13 m2, and 28 m2 for a large bedroom with a bathroom.
| Value added by an extra bedroom | 2 bed to 3 bed | 3 bed to 4 bed |
|---|---|---|
| Terraced | 13% | 10% |
| Semi-detached | 14% | 10% |
| Detached | 17% | 13% |
Its chart of value added to a typical property gives 4% for an extra bathroom, 5% for a 10% increase in floor area, 13% for an extra bedroom and 24% for an extension or loft conversion with a bedroom and bathroom.
How it compares with 2023
Nationwide ran the same analysis in a special report published on 28 August 2023, using lending in the 12 months to June 2023.
| Measure | 2023 report | 2025 report |
|---|---|---|
| 10% more floor area | 5% | 5% |
| Extra bathroom | 6% | 4% |
| Extra double bedroom on a two-bedroom house | around 14% | 13% |
| Bedroom and bathroom added to a 3 bed, 1 bath house | 25% (22% in 2016) | 24% |
| Terraced, 2 to 3 bed and 3 to 4 bed | 13% and 10% | 13% and 10% |
| Semi-detached, 2 to 3 bed and 3 to 4 bed | 17% and 11% | 14% and 10% |
| Detached, 2 to 3 bed and 3 to 4 bed | 13% and 12% | 17% and 13% |
The floor area figure held steady. The figures by house type moved by up to four points in two years. Treat any single percentage as approximate.
Why price per square metre overstates the value
A common shortcut takes the average price per square metre of the house and multiplies it by the floor area added. The worked example below uses stated assumptions, not data about any real house.
Assume a two-bedroom terraced house of 75 m2 worth £300,000, an average of £4,000 per m2. You add 13 m2, Nationwide's assumed size for a bedroom.
| Method | Value added |
|---|---|
| Shortcut: 13 m2 at £4,000 per m2 | £52,000 (17%) |
| Nationwide, the space creates a third bedroom (terraced, 2 to 3 bed) | £39,000 (13%) |
| Nationwide, the space adds floor area but no bedroom (13 m2 is about 17% more floor area; at 5% per 10%, about 8.7%) | about £26,000 |
The last line scales Nationwide's floor area figure in proportion, which is itself an approximation. Both Nationwide lines come out below the shortcut. The average price per square metre carries everything a buyer pays for once: the plot, the location, the kitchen and bathroom, the street. More floor area adds none of those.
The ceiling price
What your house can sell for is tied to what buyers pay for comparable homes nearby. If your home is already among the largest or most expensive on the street, extra space may add less than the averages above. Nationwide's 2023 report also noted that there is a trade-off between the cost spent on refurbishment and the price someone else is prepared to pay for it, and that its analysis does not take account of gardens. The effect of giving up garden to an extension is not in these figures.
Nothing here is a valuation
A valuation of your home, for a lender or any other purpose, is a separate professional judgement made by a valuer for that purpose. RICS members work to the RICS Valuation Global Standards, known as the Red Book, whose current edition took effect on 31 January 2025. Nationwide's own terms say that anyone relying on its indices does so at their own risk.
What this means for you
Compare the likely value added with the full cost of the project, not the build figure alone: fees, VAT, contingency and the cost of living through the works all count. The structure of that cost is in what a cost plan contains. If the project is really about space, set it against the cost of moving, as in extend or move. A local estate agent's view of the ceiling on your street is worth having before the design is fixed.
Sources
- Nationwide: Loft it up, attic bedrooms can boost the price of a home by a quarter, 15 October 2025 nationwide.co.uk
- Nationwide: Loft it up, report PDF with footnotes nationwide.co.uk
- Nationwide: Bedrooms, bathrooms and extensions, what adds value to your home in 2023?, 28 August 2023 nationwide.co.uk
- RICS: Valuation Global Standards (Red Book), effective 31 January 2025 rics.org