Cost per m2 including loft rooms?

[Rachieble]

I’m working out the costing to build a house, and have been given a rough figure by the architect and builder per m2. I’m aware this includes habitable rooms and the finished roof. But we plan to put rooms in the loft right from the beginning so I’m trying to work out how to cost for the extra to build them. The initial cost per m2 includes the roof work and structure, so building these rooms can’t be as much per m2 as the main build as it’s just extra windows, stairs and internal fit out. How much extra per m2 should I allow for the loft rooms then? I need a costing for the borrowing. 

Edited March 3 by Rachieble

[Dave Jones]

probably cost more per m2 than a normal room as they will need more labour as the ealls wont be plumb, boards need cutting, insultion etc etc

 

add 30-40% more per m2 to your existing m2 price.

[saveasteading]

3 hours ago, Rachieble said:

building these rooms can’t be as much per m2 as the main build as it’s just extra windows, stairs

Yes. To some extent you are right if there is lots of spare height, and building as part of the project is much cheaper than later conversion.

However, the roof slope probably reduces the practical area of floor. The stair takes out a large proprtion of floor x 2 storeys. Divide the cost by this reduced floor area and £/m2 is probably high.

And the perceived value of attic rooms is less.