House Extension Cost Calculator UK
Quickly estimate your extension budget. Enter extension area, choose storey and foundation types for an instant cost forecast.
How It Works
Total cost = (area × storey rate) + (area × foundation rate). Adjust the rates if you have bespoke quotes for your project.
For full planning advice, see our How to Plan a House Extension guide.
Planning & Permits
Always check permitted development rights or apply for planning permission. Our Planning Permission Guide helps you navigate approvals.
Material Selection
Brick, timber or blockwork walls affect costs. See our Best Extension Materials 2025 for pros & cons.
Budgeting Tips
Include 10% contingency for surprises. Learn more in our Extension Budgeting Tips.
Project Breakdown
| Entity | Attribute | Value |
|---|---|---|
| Extension | Area | m² |
| Extension | Storey rate | £/m² |
| Extension | Foundation rate | £/m² |
Frequently Asked Questions
What's the difference between single and double storey extensions?
Single-storey adds one level; double-storey adds two levels and costs more per square metre.
How do I choose foundation type?
Standard slab for firm ground; piled foundations for weak soils or heavier loads.
Does this include planning or consultancy fees?
No—this covers construction only. Add planning, architect and consultancy fees separately.
How accurate is this estimate?
It uses typical UK build rates; replace the defaults with your contractor quotes for best accuracy.
