Range from the cheapest building of under stairs storage £783 (Acacia Gardens Ltd based in N13) to the dearest building of under stairs storage £1,435 (Propello based in E17).
Comparing carpenter and joiners estimates for designing under stairs storage 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 under stairs storage in N18 by posting a job with HaMuch and we will send it to carpenter and joiners in N18.

" Under stairs storage - looking to get the existing storage space (which currently has one door) redesigned into something more accessible - ideally with a shoe rack and other pull out storage. It is a standard 1930s house/staircase and there is a small bathroom (loo / sink) in the highest/biggest..."
N13