Single or double-storey extension: which is better value?

Short answer

A double-storey extension usually gives better value per square metre because the foundations and roof serve two floors. A single-storey extension is quicker, often permitted development, and suits open-plan kitchens. The right choice depends on what space you need and what planning will allow.

Cost

Single-storey extensions typically cost £1,950–£2,900 per m² of floor area and double-storey £1,650–£2,500 per m² of floor area (both floors), both including VAT. The total for a double-storey is higher, but you get roughly twice the floor area.

Planning

Many single-storey rear extensions are permitted development. Most double-storey extensions need a householder application, and councils look carefully at the effect on neighbours’ light.

Disruption

Both are built mostly outside the house. A double-storey takes several weeks longer on site and involves opening up the first floor.

Future plans

If you might want a bedroom later, building the foundations for two storeys now can save money, even if the upper floor is added later.

Sources and further reading

GOV.UK: Permitted development rights for householders, technical guidance

This guide is general information, not legal or tax advice. Rules change and depend on your property; we confirm what applies to your project at the survey.