Converting a garage is usually one of the cheaper ways to add habitable space, because the structure, roof and foundations already exist. The main costs are bringing the floor, walls and roof up to current insulation standards, providing ventilation and daylight, and altering the front elevation where the garage door is removed.
A garage was not built as habitable space, so the conversion has to address damp proofing, floor and wall insulation, ventilation, natural light, heating and electrical provision. Where the garage is attached to the house, fire separation and structural checks on the existing opening are also relevant.
The change of use from garage to habitable room can be exempted development in certain circumstances, but exemptions are conditional and can be removed by planning conditions on the original permission, so the position should be checked with the local authority before work starts.