Free estimator
House 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.
It multiplies floor area by a London build-cost band, then adds the costs that sit outside the builder’s price: drawings, structural design, the statutory application fee and building control. It returns a range rather than a quote, because ground conditions, drainage diversions and party wall work move the total more than floor area does.
| Cost that sits outside the build | Amount | Basis |
|---|---|---|
| Householder planning application | £548 | Statutory, England, 2026 rate |
| Lawful development certificate | £274 | Statutory |
| Larger home extension prior approval | £249 | Statutory |
| Extension planning drawing set | from £1,225 | Our fixed fee |
| Building regulations package | from £840 | Our fixed fee |
| Party wall award | surveyor’s fee per adjoining owner | Party Wall etc. Act 1996 — not a statutory scale |
Sources: statutory application fees are set by the Town and Country Planning (Fees for Applications) (England) Regulations 2012 as amended, 2026 rates; our own fees are the published fixed fees on this site; both retrieved 1 September 2026. Build-cost bands are our 2026 London range and are an estimate, not a published statistic — treat the output as a budget bracket, not a tender.
Worked examples
This tool estimates the build cost of a single-storey rear or side extension in London. As a London market range, expect £2,200–£3,200 per square metre, so a 20 m² extension usually falls between £44,000 and £64,000 before fit-out. That is a budgeting range, not a quotation.
Three sets of cost sit outside the builder’s price and only one of them is fixed nationally. The statutory fee is: £548 for a householder planning application, £249 for larger home extension prior approval, £274 for a lawful development certificate. Building control is set locally and runs somewhere around £500 to £1,200 across London. Professional fees are the third set, and the ones most often left out of a first budget: drawings, structural design, and a party wall surveyor for each adjoining owner served with notice.
The choice between those first two fees is not simply £249 against £548. London councils determined 14,643 larger home extension prior approvals in the twelve quarters to March 2026 and refused 3,646 of them — close to one application in four (MHCLG planning application statistics, PS1, 33 London boroughs pooled, retrieved 1 September 2026). A refused prior approval leaves you paying the £548 afterwards anyway, several weeks later. On a deep single-storey rear projection it is worth asking which route your own borough actually rewards before the cheaper fee decides it for you.
As an envelope to design inside, not a target to hit afterwards. The order is what matters. A scheme drawn without a budget gets value-engineered after consent, and by then a material change is a fresh application rather than a phone call. Fix the range, design to it, then put one defined drawing package in front of two or three contractors so that what comes back is comparable.
Build a little slack into the programme as well as the money. Across the 33 London boroughs, 56,291 of 163,144 decided applications had an extension of time agreed with the council rather than being determined inside the statutory period — 34.5%, or roughly one application in three (MHCLG planning application statistics, PS1, twelve quarters to March 2026, retrieved 1 September 2026). Eight weeks is the target, not the median experience.
The per-square-metre bands above are our own reading of London domestic tender prices in 2026. They are an estimate rather than a published statistic, and nothing here is a quotation. Statutory application fees are the England rates in force in 2026 under the Town and Country Planning (Fees for Applications) (England) Regulations 2012 as amended. Building control charges are set by each local authority or by a registered approver and differ from borough to borough, so check the published scale for yours rather than the middle of our range.
Pick a slot that suits you. No obligation and no sales pressure, just straight answers on your planning route.
