depends if you can get permission really
then look out as the skys the limit

don't forget you need to do walls and fire safety and access etc to factor in before you start putting in a stronger floor etc and you've also got to get the rest of the structure restructured to support the shell of the foor

neighbour done it for just under £16k with a toilet and shower in there as well as extrending the foor to be square at the rear