Extensions / Cost calculator
Extension cost calculator
Choose the type of extension, its size and the spec, and see a guide price for London including VAT. It takes under a minute and you don’t need to give us any details to use it.
Guide price, including VAT
£55k to £63k
Single-storey rear extension, 18 m², standard spec
- 18 m² at £2,880 to £3,480 per m² including VAT
- Lifted to our single-storey rear starting price of £55k. A small extension still needs the foundations, the opening and the roof, so it never costs much less.
A guide range, not a quote. Your price is fixed and itemised after we’ve surveyed the house. Not included: kitchen, scaffolding, architect and engineer fees, and party wall costs.
Get this as a fixed quote
Leave your number and we’ll call to book a free site visit. Add your email and we’ll send you this estimate too.
No deposit. You pay weekly in arrears for work done. See our privacy notice.
How the calculator works it out
The rate per square metre
Published London guides put a single-storey extension at about £1,900 to £3,400 per m², mostly before VAT. HomeOwners Alliance gives £2,300 to £3,400 for London, and Resi gives £1,900 to £3,000 for London and the South East. The calculator splits that spread into three spec levels and adds 20% VAT.
Our starting prices
A small extension still needs foundations, a structural opening and a roof, so the estimate never drops below the bottom of our guide range for each type. Our ranges, for a mid-terrace including VAT:
- Single-storey rear: £55k to £110k
- Side return: £45k to £85k
- Wrap-around: £85k to £170k
- Double-storey: £110k to £200k
Checked against our own quotes
Our recent single-storey extension quotes, from 2025 and 2026, came in at about £56k to £90k including VAT. A typical permitted development extension on a north London terrace is 3m deep and 15 to 20 m².
What sits on top
- Architect or designer, about £4,500 to £6,500
- Structural engineer, about £1,500 to £2,500
- Building Regulations, about £1,200
- Party wall agreement, about £1,000 per neighbour
- Kitchen, scaffolding and landscaping, priced separately
Sources and dates are in our guide to rear extension costs in north London.
Questions about extension costs
It gives a guide range, not a quote. It uses the London cost per m² published by HomeOwners Alliance and Resi, adds VAT, and never goes below our own starting price for each type. Ground conditions, drains, trees, steel beams and glazing can all move a real price, which is why we fix the price after a survey.
Yes. The range includes VAT at 20%. Many cost guides online quote before VAT, so add 20% to theirs before comparing.
The kitchen units and appliances, scaffolding, architect drawings, structural calculations, Building Regulations fees, party wall costs and landscaping outside the new walls. Our rear extension cost guide lists typical figures for each.
The foundations, the opening into the house and the roof edge have to be built whatever the size. That's why the first metre is the expensive one, and why the calculator never shows less than our starting price for each type.
No. There's no deposit. We invoice every Friday for the work done that week, so you pay weekly in arrears.