How much does a single-storey extension cost in North London?

Typically £2,500–£3,500 per m² to build, excluding VAT, with a typical single-storey rear or side-return extension at £80,000–£120,000 builder only.

Is a double-storey extension cheaper per m²?

Usually, because foundations and roof are shared across two floors. The total is higher, but you get more space for each pound.

Does a double-storey extension need planning permission?

Almost always. Permitted development allows some two-storey rear extensions, but the limits are tight and rarely suit North London terraces and semis.

Which adds more value?

Extra bedrooms and bathrooms usually add more to a home's value than extra living space, which is why double-storey can pay back well.

What else should I budget for?

Structural engineer £1,500–£3,000, Building Control £1,000–£1,500, party wall surveyors £1,000–£1,500 per neighbour, a 10% contingency and VAT at 20%.

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Cost guide · Single vs double storey

Single or double storey extension: which is better value?

People often assume a two-storey extension costs twice as much. It doesn't. The foundations and roof, two of the most expensive parts, are shared, so the cost per square metre usually comes down. Planning is the bigger question.

Written by Dor Cohen, RIBA Chartered Architect, ARB Registered

Last updated 11 November 2026

A North London home designed by DOR Architects

Is a double storey extension cheaper per m\u00b2 than a single storey?

Usually, yes. A double-storey extension shares one set of foundations and one roof across two floors, so its cost per m² is typically lower than a single-storey extension. In North London, single-storey extensions typically cost £2,500–£3,500 per m² to build, excluding VAT. The catch is planning: double-storey extensions almost always need a full planning application and get closer scrutiny for neighbours' light.

Single-storey rear extension with wide sliding doors onto a London garden — A good extension is mostly about light and how the kitchen meets the garden.

The key differences

Here's how the two compare in practice.

Cost per m²Single storey: £2,500–£3,500 per m² excl. VAT. Double storey: usually lower per m², higher in total.
PlanningSingle storey can often use permitted development. Double storey nearly always needs an application.
NeighboursA second storey affects light and outlook much more, so the design needs more care.
DisruptionDouble storey usually means working on the upstairs too, so you may need to move out for a while.

How to decide

Do you need bedrooms?If the house lacks bedrooms or a bathroom, double storey is usually the better investment.
How much garden?Going up rather than out keeps more of your garden.
Where you liveConservation areas and tight terraces may rule out a second storey.
Your total budgetSingle storey costs less in total, which can matter more than value per m².

Don't just compare price per m². Compare what each option does for the way you live, and what the council is likely to accept.

Dor Cohen, RIBA Chartered Architect

Questions

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Typically £2,500–£3,500 per m² to build, excluding VAT, with a typical single-storey rear or side-return extension at £80,000–£120,000 builder only.

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