Thanks for all the feedback guys, yes
@Folkman I will be concentrating on the conversion first although as there is rooms above the garage and I wouldn't be on for trying to make it bigger I am restricted to a certain extent, but I'll definitely be looking at all the things you mentioned in your post first and foremost.
I'll probably opt for windows on the frontal aspect of the same size and spacing as the front of the house ( it's an old stone house with sash windows) , as to make it look like less of a garage conversion , there only is one other small gable end window which I'd like to commission an encapsulated stained glass pane for, I'm undecided about whether to close up the rear pedestrian door, it may be handy to keep it for access although the garage is accessible from within the house.
Insulation and heating will be a priority, I'm considering an underfloor heating system for this space but may opt for radiators, perhaps traditional cast radiators, insulation will depend upon the existing construction, I'm not sure if the adjoining garage is a later addition to the property, I think it is, hopefully it will have cavity walls, if not I will dry line the walls with sufficient insulation and perhaps use a variety of cladding or facing materials, perhaps natural stone, etc.
I will install a dedicated mains ring and have the power outlets positioned appropriately for the system. Luckily I'm capable of doing all the construction related work myself to high standard as I've 30 years experience in construction and I'm super fussy.