Range from the cheapest single storey extension £43,800 (J&K Expert Building Services based in EN5) to the dearest single storey extension £54,360 (MJR Construction Ltd based in WD23).
Comparing extension builders estimates for fitting a downstairs toilet gives you an idea of who will be better value but the best way to compare costs is to get actual quotes.
What to do - Get rates and quotes to build a single storey extension in WD6 by posting a job with HaMuch and we will send it to extension builders in WD6.

"Single storey extension"
BOREHAMWOOD