Extension budget planner
Extension Cost Calculator UK
Get an indicative cost range for a house extension in the UK. Adjust the size, storeys, construction type and specification for a budget estimate based on current UK build rates.
Typical build budget
£30,000–£120,000+
Biggest cost drivers
Size, spec, foundations, glazing
Best use
Early extension feasibility budgeting
Typical aspiration
The sort of mid-range extension result people have in mind before they test the budget properly.
Extension cost snapshot
Size and spec matter — but foundations can swing the project hard
Use this to sense-check the main build budget first, then compare extension vs loft if you are still deciding how to add space.
Remember
Glazing upgrades, kitchen fit-out and difficult ground conditions can push totals well beyond headline per-m² numbers.
Good next step
Read the extension cost per m² guide before paying for drawings or quotes.
Estimated Extension Cost
Indicative range. Not a quote. Excludes architect fees (typically 5–10% of build cost) and structural engineer fees.
Cost Breakdown
| Base Build | — |
| Foundations | — |
| Kitchen Fit-Out | — |
| Wet Services | — |
| Contingency (10%) | — |
| Estimated Total | — |
How We Estimate
- • Estimates assume a straightforward rear or side extension with standard access and conditions.
- • Costs exclude architect fees (typically 5–10% of build cost) and structural engineer fees.
- • Permitted Development applies to many single-storey rear extensions under 60 m². Full planning may be required for double-storey or larger.
- • This is an indicative estimate, not a quote.
- • Last updated: Q2 2026.