UK extension costs · 2026
Double storey extension cost in the UK (2026 prices)
A double storey extension gives you roughly twice the space for far less than twice the price, because the foundations and roof are shared between both floors. The figures below show the total cost for both floors combined on a semi-detached house with a standard finish.
Typical double storey extension cost by size
| Size | Indicative cost |
|---|---|
| Small (12 m² per floor) | £33,500 – £42,500 |
| Medium (20 m² per floor) | £55,500 – £70,500 |
| Large (30 m² per floor) | £83,000 – £106,000 |
| Very large (40 m² per floor) | £111,000 – £141,000 |
Indicative only, based on typical UK build rates for a standard finish. Your own quote depends on your property, access and specification.
Why a second storey is better value per m²
The most expensive elements of any extension are the groundworks and the roof. Building upwards reuses both, so the added floor typically costs around 15% less per square metre than the ground floor.
If you have any thought of needing more bedrooms later, it is almost always cheaper to build both floors now than to return for a second project.
Planning and party wall considerations
Two storey extensions are more likely to need full planning permission, particularly where they come close to a boundary or affect a neighbour's light and outlook.
If you are building on or near a shared boundary you will normally need a party wall agreement with the adjoining owner before work starts.
What affects the price most
- Floor area in square metres — the single biggest driver of the total price.
- Access to the rear or side of the property for machinery, deliveries and waste.
- Ground conditions, drainage runs, manholes and any nearby trees affecting foundation depth.
- Structural openings: a wide steel beam into the existing house costs more than a standard doorway.
- Glazing choice — bifold or sliding doors, roof lanterns and large windows add several thousand pounds.
- Internal fit-out: kitchens, bathrooms, underfloor heating, flooring and decoration.
- Professional fees: architect or designer, structural engineer, building control and any planning application.
- Where you are in the UK — London and the South East typically run 10–20% above national averages.
Frequently asked questions
- Is a double storey extension worth it?
- In most cases yes — you gain a bedroom or bathroom upstairs for a modest uplift on the single storey cost, which usually shows well in the resale value.
- How long does a two storey extension take?
- Typically 16–24 weeks on site, depending on size, access and how much internal reconfiguration is involved.
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