Interactive calculator with real prices adjusted by UK region. Select your scope, pick your region, get your instant estimate.
1. Choose Scope of Work
2. Select Your Region
What Affects the Cost?
Size and footprint
Single vs double storey
Foundation requirements
Roof type
Internal finish spec
Kitchen or bathroom in extension
Structural steel
Planning and Building Regs
Frequently Asked Questions
How much does an extension cost per sqm UK?
Extensions typically cost £1,500-£2,500 per sqm depending on location and specification. London and the South East are at the higher end. Basic shell construction is cheaper; full fit-out with kitchen or bathroom is more expensive.
Do I need planning permission for an extension?
Single storey rear extensions up to 4m (detached) or 3m (semi/terrace) usually fall under permitted development. Larger extensions, side extensions, and two-storey extensions typically need planning permission.
How much value does an extension add?
A well-designed extension typically adds 75-100% of its cost in property value. A single storey extension adds 5-10% to property value. A double storey extension adds 10-20%.