Extending over a garage (already part extended)

I’m looking at buying a detached house with a 3m wide garage attached to the side. I have posted a separate question about the same house re the loft. The garage has been built on at some point in the past to creat a first floor bedroom and en-suite. The floor level (and roof level) is about 80cm lower than the first floor of the rest of the house. The roof at the front elevation is sloping and there is a single velux. At the rear (above the garage) there is a full brick built extension with a large window. We could gain quite a lot more space and make this extension more fluid with the rest of the house by raising the floor level to meet with the existing house and by either putting a dormer on the front or by building a proper 2nd story with a pitched roof instead of the sloping roof at the front. Question: - so long as building control sign off was given for this extension, can I assume that the foundations of the garage are adequate to allow the for extra weight of raising the floor (and ceiling as a result) and either adding a dormer to the front or a ‘full’ extension with a pitched roof in line with the roof of the main house. - does having this part done extension save on costs vs just building over a garage with nothing on top? The house is in Cardiff. I hope I’ve made this clear Thank you in advance