16k to tank with sump pump drainage for a 20sq metre. Up to 70 - 100k to dig down if reduced height and all the legal fees for change of use, party walls, decorating, fixtures/fittings. Be warned - most outfits are cowboys using very cheap labour. We went for the 18k tanking, with huge sump pump, backup battery, drainage, electrics and professional plastering, through a highly reputable company, keeping the limited height 5.5 ft high, as will be just for storage. But, although looking good now, mainly due to our DIY efforts after the cowboys had finished, the whole process was awful. Dust, dirt, took 5 more days than quoted 5 days quoted, mainly because they turned up late, had a 2 hour lunch break and left early, resulting in a 4 hour working day, they threw all their left over paint and rubbish in one of our built-in cupboards in the cellar- and that was with one of us on site to try and site-manage. The sub-contracted electrician was the only highlight, he even re-wired some faults from a previous cowboy for free. My advice is closely manage, write in the contract that they have to take a fixed amount of money off the final cost for every day over - and check around before paying a penny over the deposit. We have had to do all the painting, skirting, flooring - so add about just below 1k to make it a room. It is looking good now, so do it - but be prepared to turn to a lot of gin to get you through the mayhem.