Have given up trying to sell our house and have decided to stay put and do some renovations instead.
Top of the list is we'd like to convert our double garage into a home office for DH. We'd like a couple of sky lites, and large, sliding glass doors to replace the garage door. Would also need insulating and plaster boarding inside. It already has power and lighting.
I'm no expert, but we are currently getting our single garage (which is part of the house) converted into a living room including additional electrics and plumbing and a new internal door. Materials and work look like they're going to come out close to £10k. Your space presumably is twice ours, and obviously a lot depends on how fancy things like doors and fittings are, but it feels like it's going to be somewhere over £15k, and maybe closer to £20k. But treat that as a rough starting point! We're in East of England - rates may vary round country?
Yes you wil need to contact the planning dept to find out rules for coversion.
We're in a conservation are so needed to follow specific guidelines.
We did ours about 10 years ago and local builders did it for about 5,000- insulation, plastering, a false ceiling, electrics, putting in another window. (We didn't need a new door as already one on there) but 20,000 sounds a lot of money! (We're in the south) I'd get several quotes if I were you.