Extension Value Calculator UK 2026
Instantly estimate how much value a home extension could add to your property. A free UK tool for homeowners to weigh up added value, new property price and return on investment before building.
Extension Value Projection
Enter your property and extension details to estimate the added value
The current estimated market value of your property before extending.
Regional property values affect how much each square metre adds.
Different extension types add value at different rates.
The total new internal floor area created by the extension.
Your total budgeted or quoted cost for the build, including fittings.
Your Extension Value Estimate
Added value, new property price and ROI breakdown
Enter your property and extension details above and click Calculate Added Value to reveal your value projection.
Extension Value Benchmarks
Quickly reference typical value uplift ranges for common types of UK home extensions, based on general market benchmarks.
| Extension Type | Typical Value Uplift | Notes |
|---|---|---|
| Single-Storey Rear Extension | ~5–10% | Extends kitchen or living space |
| Kitchen Extension | ~6–10% | Popular for open-plan living |
| Wrap-Around Extension | ~8–12% | Combines rear and side space |
| Double-Storey Extension | ~10–15% | Often adds a bedroom upstairs |
| Loft Conversion | ~10–20% | Frequently adds a bedroom and bathroom |
Extension Value FAQ
Everything you need to know about how home extensions affect property value, typical costs, and what a healthy return on investment looks like.
On average, a well-built extension in the UK adds between 5% and 15% to a property’s value, though this varies significantly by region, extension type and quality of finish. A single-storey rear extension typically adds a smaller percentage than a double-storey extension or loft conversion of comparable size, because it creates more usable floor area and, in many cases, an additional bedroom.
Costs vary widely by region and specification, but a single-storey extension typically costs between £1,800 and £3,000 per square metre, while a double-storey extension or loft conversion can cost £2,000 to £3,500 per square metre. London and the South East generally sit at the higher end of these ranges due to labour and material costs.
No. Loft conversions and double-storey extensions tend to offer the strongest value uplift because they often add a bedroom or bathroom, which buyers value highly. Kitchen extensions and wrap-around extensions also perform well, particularly when they create open-plan living space, while smaller single-storey additions typically see a more modest uplift per square metre.
It depends on the numbers. Extending can be more cost-effective than moving once stamp duty, estate agent fees, legal costs and the price differential between properties are taken into account. Comparing the projected added value and ROI of an extension against local moving costs is a useful starting point before deciding.
A ROI of 20% or more, where the added property value exceeds the build cost by a healthy margin, is generally considered a strong outcome. Many well-planned extensions achieve a ROI between 0% and 20%, effectively paying for themselves in added value, while poorly specified or over-built extensions can sometimes fail to recoup their full cost.
Yes, significantly. Property values per square metre are much higher in London and the South East than in many other parts of the UK, so the same extension can add considerably more monetary value in those areas, even though build costs are also higher. Local buyer demand for extra space and bedrooms also plays a role.