Do building regulations differ from a normal garage conversion?

Yes. A conversion that stays part of the house is assessed as building work. A conversion that creates a dwelling is a material change of use, which brings in additional requirements around fire safety, sound insulation, resistance to moisture and contaminants, ventilation and energy efficiency. In practice that usually means a higher specification of floor build up, fire resisting separation between the flat and the main house, designed extract ventilation, and drainage that satisfies building control rather than simply working.