Extensions - single storey at rear

[Barquentine]

Does anyone know what a simple single storey flat roof extension at the back of a terraced house in London ought to cost? We've got some quotes which have made my eyes water! Also, does anyone know by how much we could haggle the price down? I'm sure we could get some taken off, but I want to be realistic. Many thanks.

[pug100]

Depends on the area (sq. m) that you plan on having built, what height, bricks or rendered blocks how many and size of windows doors etc.

[buildersmate]

Well you live in London and there is no such thing as "how much should a project cost" anyway.
Prices for this sort of thing are driven by supply and demand. At the moment there are lots of people doing up houses so demand outstrips supply.
The way you assure yourself you are getting the best value in the given circumstances / economic conditions is by opening the thing to competitive tender with three (or four) tenderers.
The way to work out a very, very, very rough idea of what it MIGHT cost is to use say �1400 per square metre of floor area. I am only suggesting this figure for you to work out whether it might cost you �20k, �50k, or �150k. And hence whether you might be able to afford it, rather than wasting your time. Please do not try and use in any other way - namely, 'well the bloke on AB said it should cost me �40k so you're too expensive'.