You cannot change the use of your garage without gaining permission from the local authority. We recently received a telephone call from a gentleman who, because he was disabled, had changed his garage into a living area for himself. Someone had reported this to his local authority and they had visited and told him he could not do this without:
Getting their permission
Having the works carried out by a competent builder and passed by them
The type of work that would normally be involved in converting your garage without knocking it down and rebuilding would be:
Removal of the existing garage door and building the wall up at the front of the garage as once you remove the door there will be no wall at the front. This will enable you to fit a new window into the rest of the opening to match the remainder of your property.
At this point if you have no door going through to your existing house, you may want to add a doorway. See above about knocking through your outside load bearing wall, as this is a very technical operation that requires lintels to be placed over doorways to take the weight of everything above.
At this stage you would require the first fit electrics to be carried out. This would involve running wires along walls, vertical and diagonal as required (once again think carefully what you actually want the room to provide). If your house is centrally heated and you are having a radiator put in, this would be the time to run the extra pipework. This should easily be run from existing pipework. Remember to check that anyone doing gas or electrical works has either the CORGI registration certificate or the NICEIC certificate.
The floor of the garage normally runs downward. This would have to be remedied by digging the higher end out (usually with a mechanical digger such as a Kangol) to give you an even surface.
Then a layer of membrane (water proof barrier) and screed will be laid to bring the floor level up to that of the existing house. So as not to form a step. Sometimes forming a step will be unavoidable, but your builder will be able to advise you of this during his site visit.
The next stage is to batten out the walls, this involves fixing 50m X 50m wooden battens to the wall. Once this is complete, the insulation material will be fitted between the battens to retain the heat in the new room.
The next stage is to fit 12.5m plasterboard to the battens, fixed in place by galvanized clout nails. The joints on this plasterboard will be taped over with a webbing. Then the entire walls and ceiling should have a 5m skim of plaster applied to it.
Skirting, door frames and doors will be fitted next. Also the second fit electrics (switches etc) will now be carried out.
The floor will need a self levelling screed to be applied. Your flooring contractor will include for that in his price (check he has).