Free estimator
Basement extension cost calculator
Estimate drawings + build cost in seconds. Borough-specific. Saved in your URL — share with anyone.
Estimate drawings + build cost in seconds. Borough-specific. Saved in your URL — share with anyone.
The consent and the neighbours. A new basement always needs planning permission at £548, and excavating within three metres of an adjoining owner’s foundations triggers a Party Wall notice with one month’s notice. Several boroughs also apply a specific basement policy on top.
| Item | Amount or period | Basis |
|---|---|---|
| Householder planning application | £548 | statutory, England 2026 |
| Party wall notice, excavation within 3m | 1 month | Party Wall etc. Act 1996, s.6(1) |
| Party wall notice, excavation within 6m | 1 month | Party Wall etc. Act 1996, s.6(2) |
| Structural and waterproofing design | engineer’s fee | not a statutory scale |
| Basement applications, Kensington and Chelsea | 1,027 | 96% approved |
| Basement applications, Islington | 10,989 | 61% approved |
Sources: fee from the Town and Country Planning (Fees for Applications) (England) Regulations 2012 as amended, 2026 rate; notice periods from the Party Wall etc. Act 1996; application counts from the Planning London Datahub, retrieved 15 August 2026, which phrase-matches published descriptions and is comparable within a borough rather than between boroughs. Build costs the calculator returns are our own 2026 London range and are an estimate, not a published statistic.
Worked examples
This tool estimates the build cost of a basement conversion or excavation in London. As a London market range, expect £4,000–£6,500 per square metre. That is a budgeting range, not a quotation.
Statutory fees go to the council and are set nationally for England: a householder planning application is £548 determined in 8 weeks, larger home extension prior approval is £249 over 6 weeks, and a Lawful Development Certificate is £274. Building control is separate again at typically £500 to £1,200 in London.
As a budget envelope before design, not as a target after it. Build cost should drive the design rather than follow it: a scheme drawn without a budget is a scheme that gets value-engineered after planning consent, which usually means a second application. Fix the range first, design to it, then price the defined drawing package with two or three contractors.
Ranges are indicative London market rates observed across domestic projects and are not a quotation. Statutory figures are the England rates in force in 2026 under the Town and Country Planning (Fees for Applications) (England) Regulations. Building control charges are set by each local authority or registered building control approver.
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