I just had my extension done, didnt go double story as my house would have looked horrible. Builder said to me that he roughly charges about £14000 for a single and double that for double. I would say though, always put another £5 - 10,000 away as you dont know what problems you will get below the soil. My place is an end of terraced and when they were doing the footings they came across a storm drain pipe which served the whole road and this run through the center of my footings. The builders had to dig down over 6ft (mostly done by hand) and then use 2 RSJ's in a cross formation. These were supported on two padstones which were about a meter square each... cost me an extra 5 grand.