Free Tool
House Extension Cost Calculator
Get an instant cost range for your extension based on size, type, location, and finish level. Based on real London project data.
Estimate Your Extension Cost
Adjust the inputs below for an instant indicative range based on London & South East project data.
Good quality — mid-range materials, decent kitchen/bathroom, solid finish
Indicative Cost Range — Standard
£56k – £71k
30m² · Single storey · Outer London · approx £2k/m²
Cost Benchmark Comparison
Basic
£49k – £63k
Standard ✓
£56k – £71k
High Spec
£72k – £91k
Estimated Cost Breakdown — £63,750 midpoint
* Indicative split based on standard specification. Actual proportions vary by project.
Want a QS-reviewed estimate?
From £95 — based on your actual drawings and spec.
* Indicative ranges only. Actual costs depend on site conditions, structural requirements, specification, and current material prices. Not a formal quotation.
What Affects Extension Costs in London?
Structure & foundations
Ground conditions, existing structure type and the proximity to neighbours all affect foundation and structural costs significantly.
Specification
Glazing, flooring, kitchen, bathrooms and finishes vary enormously. A glass-roof kitchen extension can cost 60% more than a standard tiled-roof one at the same m².
Planning conditions
Listed building consent, conservation area restrictions, or unusual permitted development constraints can all add cost and time.
Access & logistics
Narrow London terraced streets, no parking for skips, basement access — all increase prelim costs that the calculator cannot account for.
Ready to Get a Proper Number?
Our QS-reviewed estimate service starts from £95 and gives you a detailed cost breakdown suitable for planning and budgeting.
See Estimating Packages from £95