A garage was built to keep a car dry, not a person warm. Turning one into a room means the floor, the walls, the roof and the old door opening all have to be brought up to the standard of the rest of the house, and building control check each of them.
The roof is often the weak point. Many attached garages have a flat roof with no insulation in it, and converting the room underneath without dealing with the roof means building a new room under an old problem. We look at the roof first, because that is our trade.
Where the garage door was, a new wall goes in, usually with a window in it, built on a foundation of its own rather than on whatever the door frame stood on.