Depending on the current use of the building you may need to apply for a change of use. You will need to get drawings of the proposal prepared and submit them to the L.A. in the usual fashion. There will be some input from the H.S.E. who will look for for details to show compliance with Evironmental Health regulations and you will also require a fire safety certificate. Depending on when you actually apply you may need to apply for an accessability cert under proposed changes of Part M of the Building Regs.