Garage Conversion

Fibreglass or rockwool, both fire retardant and both work very well, along with them being as cheap as chips, although you can expect a volley of suggestions to use celotex or other types but they will be 3 times the price of fibreglass and there's little *real* difference in heat retention, despite the claims.

not really, no....you will gain almost nothing by doing this, but if you dot & dabbed plasterboard onto the bare walls prior to building the studding, you would gain substansially more

Sounds reasonable - but don't forget to check on building regs. Being a detached garage, it was probably exempt - but won't be when part of it is living accommodation. If the other part is still a garage - or even if it may only be used for storing flammable materials - you'll need suitable fire protection between the garage and office parts.

I'm not sure whether you'll also need planning permission for change of use. It may be as well to check.

if its for habitation, you'll want insulation in your new wall cavity and also under the floor.

NT

I would use Kingspan K17 boards. This has the advantage of not requiring the wooden frame to be built and you don't need plasterboard, either.

Yes.

Christian.