The conversion of a garage to any kind of living space is treated as a change of use of part of a building. An application for approval under the Building Regulations must be made before work starts. The submission of a Building Notice is an appropriate way to achieve this.
Permission will need to be sought in all cases and the following points will be considered before permission can be granted. As well as the regulations being met, it is as well to consider the type of flooring you intend to use.
It will be necessary to make sure the walls and roof are insulated to industry standards. Windows should achieve a 'U' value of 2.0 W/m2/K, Wall insulation value should be equal to 0.35 W/m2/K.
Windows must incorporate opening vents of an area equal to 1/20th of the floor area of the room, and trickle vents to provide background ventilation of 8000mm2. A window suitable for escape purposes will be required if the new room can only be accessed through another room.
If the existing wall is single leaf construction with piers it must be checked for stability and freedom from defects. If satisfactory it is likely the wall would be considered suitable.
A foundation must be provided to carry any additional masonry loads such as a new inner leaf to external walls or to the garage door infill. The condition and suitability of the existing foundation/floor can be checked when the door infill area is excavated.
A wall of single leaf construction must be treated to provide satisfactory resistance to the passage of moisture. A waterproofing compound applied on the internal face and incorporated into a floor membrane may be an effective way of achieving this. The provision of an inner leaf to create a cavity wall will require a cavity tray at the base of the wall.
Floors (new and existing) should also have a suitable damp proof membrane. See PDF on suitable garage flooring which can be laid directly on existing garage floor slab.