Thanks – fully understood on the gas pipe.
Hadn’t got as far as thinking about ventilation but it’s a good point – would a simple wall mounted xpelair be sufficient?
The ceiling… I know that it is part of the fire compartment, it already has a light fitted to it, presumably this is ok? I was intending to build cupboards in place as per this guide: http://www.familyhandyman.com/garage/storage/installing-large-garage-cabinets/step-by-step will it cause a problem with the fire compartment?
Floor I’m planning to use rubber interlocking tiles to cushion the weights.
I did think about just insulating the external walls and plasterboard on the internal walls for aesthetics.
It will always be a garage and not a habitable room – if we ever go down this route in the future it will be a proper conversion.