Nick,
is the garage integral with the house, or free standing?
Free standing garages, even if attached to the house but not part of it, are not required to have any more than single skin brick walls, are not required to have a DPC and are not required to have any insulation under the roof tiles.
A garage built this way will be likely be damp and need dehumidifying. All the houses we had before this one were so.
This time our garage is part of the house, there is a room above (spare bedroom) and behind (the utility room) and so it has all the insulation, including cavity wall and double glazed window, of a normal dwelling room, including the floor slab. It is bone dry and if I turn on the radiator, as warm as the rest of the house.
