What does a rear extension cost?

Figures per square metre get quoted a lot, and they’re useful for one thing: telling a porch from a rebuild before you spend any time on it. They don’t tell you what a particular job will cost.

One worked example

This is a made-up job, not a market rate: a single-storey rear extension of about 20 m², with a flat roof, a roof light and folding doors.

Groundworks and drainage	£5,613
Walls and structure	£11,973
Roof and glazing	£4,121
Finishes and fit-out	£9,205
Building work	£30,912
Preliminaries at 10%	£3,091
Overheads and profit at 15%	£4,637
VAT at 20%	£7,728
The price	£46,368

Change the rates to your own and this total moves. So it should.

What moves the price most

Ground. Normal ground and short footings are one job. Clay near trees, a drain in the way or a need for piling is another, and you won’t know from the drawings.

The opening into the house. Taking out the back wall means a steel, an engineer and making good. The wider the opening, the more it costs.

Roof and glazing. A flat roof with one roof light is simpler than a pitched roof tied into the house, and big folding doors can cost as much as a good part of a trade.

The kitchen. Whether it’s in your price, and which one, can swing the total more than the building does. Say clearly whether it’s included.

Access. Clear side access is cheaper than carrying everything through the house.

What usually isn’t included

Say so on your quote if these aren’t in your price: building control fees, a party wall surveyor, the structural engineer, and moving the boiler. They’re the things that cause arguments later.

How to give a figure without getting caught out

Early on, give a range and say what it’s based on. Once you’ve seen the drawings and the site, give a price with your assumptions written down. The client gets a number they can plan around, and you’ve got something to point to when the ground turns out to be different.