Build costs

[Footsoldier888]

The Homes and Renovating magazine and website is very good for this sort of project.

A designed project has a ball-park figure of 1800 per sq metre currently as a very rough guide. Yes you can get that down with Diy but the Vat is a bit of a killer on extensions.

 

[John D v2.0]

There's a big difference between shell and 1st fix, shell is much more doable on a budget.
Don't underestimate the cost of plumbing, electrics, plasterboarding etc.
For ours we did basically to first fix but we took on water and heating plumbing and all electrics ourselves. We are finishing off the kitchen at the moment. This probably saved us 50k but will probably take 18 months longer.