Cost calculator
Double storey extension cost calculator
A two storey extension is the best value per square metre in domestic building, because the expensive elements — foundations and roof — are shared between two floors. It is also more disruptive and more likely to need planning permission.
Combined new floor area across both storeys.
Broad trade availability keeps day rates near the national average.
Estimated range, ex VAT
£69,000 – £115,000
- Labour (typical)
- £50,500
- Materials (typical)
- £41,500
Guidance only. A real quotation depends on site conditions, specification and access — which is exactly what an independent review of your own quote will tell you.
What affects the cost
Foundations
Two storeys impose more load — deeper or wider footings, and possibly piles near trees.
Roof integration
Tying a new roof into the existing one is more complex than a lean-to and often needs valley or hip work.
Structural openings
Ground floor knock-throughs plus first floor openings mean multiple beams.
First floor use
A bedroom is cheap; an ensuite or family bathroom above adds drainage, waterproofing and ventilation.
Planning route
Two storey rear extensions often exceed permitted development, adding time and design cost.
Regional pricing differences
Materials cost roughly the same everywhere in the UK; labour does not. Use the region selector above to apply a local index — London and the South East typically run 15–30% above the national average, while the North East, Northern Ireland and Yorkshire sit below it.
Labour vs materials
Roughly 55% of this type of work is labour and 45% materials. Bricklaying, scaffolding and carpentry lead labour. Steel, roof coverings and glazing lead materials.
Common hidden costs
- Party wall awards for both storeys, usually more involved than single storey.
- Scaffolding for a longer period and to a greater height.
- Rebalancing the heating system — more radiators may need a larger boiler or better distribution.
- Redecoration and flooring across the connecting rooms on both floors.
How to save money
- Build up rather than out where the site allows — you get double the space per pound of foundation.
- Stack bathrooms above kitchens or existing bathrooms to keep drainage runs short.
- Keep the roof form simple and matched to the existing house.
- Appoint an architect and engineer before pricing so all builders quote the same scheme.
Frequently asked questions
How much does a double storey extension cost?
£1,650–£2,650 per m² ex VAT nationally in 2026, so roughly £70,000–£110,000 for 40m² across two floors.
Is a two storey extension cheaper per m² than single?
Yes — usually 20–35% cheaper per square metre, because foundations and roof are shared.
Do I need planning permission?
Often yes. Two storey rear extensions have tighter permitted development limits, and side extensions usually need consent.
How long does it take?
Sixteen to twenty-six weeks depending on complexity and weather.
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