To go back to garage use is technically simple. That part is relatively easy to estimate. It's the unknowns that make pricing near impossible without a specification.
Building Regs will have inevitably become much more demanding than they were at the time that it was originally a garage.
They'll need a high level of insulation between the "new" garage, and the rest of the house.
Also, there will have to be a high level of fire-proofing. Because garages contain a car, they are considered a potential hazard for the habitable remainder of the building.
So you can imagine... all thoughts of aesthetics and cost go out of the window until these considerations are figured out.
An architect or building designer would deal with this for you....... or simply put in an application for a "Building Notice". Then you can go through it with the Building Inspector. He'll tell you what's needed. After that it can be priced.