Not had one myself but this is how you would do it.
Check with planning autority you can convert under permitted development rights (pd's). New houses tend to have these rights removed as houses need a certain number of parking space. If pd's are removed, may get planning anyway if they you can get and extra car on driveway, or they will accept the loss of one space.
Construction simple. remove garage door, build up front brick in matching material, with new window, alter the masonry detail over the garage door to suit the window width.
Insulate and floor board (sand cement screed) over the insulation to lift the garage floor the same level as main house. May need to insulate the existing garage walls \ roof, 1st fix extra electrics,plumbing, plaster, decorate
Easy peasy
Cost as a rough stab for single garage
Brickwork £600-750
Window £500
Floor insuation and boarding £250
Insulate walls with pumped cavity insulation(one wall assumed as integral garage) £250
Electrics (lights, power, tv, telephone) circa £500
Extend Pipework and radiator £250-300
Plastboard £600
Skirting and architraves £200-250
Total 3,500 plus 15% profit to builder
£4,000 total plus VAT on refurb
just over £4,500
and screw the builder down on price.