Are the existing foundations deep enough for a habitable room?

Detached garages were often built on shallow footings and a thin slab, because the original garage only ever had to carry a car. Building control will sometimes want a trial hole dug so the depth can be checked, and where footings fall short they can usually be underpinned rather than started again. Any structural work of that kind is shown on the building regulations drawings and signed off as the job goes along, so the converted garage is properly documented when you come to sell. Our guide to costs, planning and design for living space conversions covers what to expect.