Converting an attached domestic garage to living space for use incidental to the dwelling is generally exempt, provided the floor area involved is within the extension allowance and the use remains part of the house.
The conversion counts against the same floor area allowance as an exempt extension, so a previous extension may have used it up. Converting the garage into a separate independent living unit is a material change of use and needs permission.
Technically, the works are substantial. Garage floors, walls and roofs are rarely built to habitable standards and typically need insulation, damp proofing and ventilation upgrades.