I hate to say it, but this isn't as simple as it sounds. It depends on thestructural system and condition of your roof and the hidden extras extend as far as having the doors on the lower floor fireproof to a specific standard. Then there's the position of your drains and water supply to consider and whether you need dormer windows or not. You can also lose quite a lot of existing space installing stairs. There is no substitute for getting a number of specialists in to give you quotes. Mine cost around �8k but that was nearly 15 years ago. Having said that it meant that I was able to separate brothers who shared a bedroom and fought for hours at bed time. It was worth every penny and has increased the value of the house. BTW If your flat is leasehold you will need to check the terms of your lease.