I am not sure if my studio building experience can help you but I built mine in 2011 using off the shelf kits. I didn't know anything about room size ratios so never took those into account and it has not been an issue because I put in so much equipment in there. There are no bare walls to cause issues. I also have a pitched roof which isn't an issue. My best advice would be to build as big as possible as it is surprising how quickly you fill the space up. Here in the UK we use concrete slabs due to proximity of tree ( roots ) and building regulations suggest that concrete is less likely to move over time. I planned my studio internally to use screens and booths which could be moved around inside the space rather than use it as one big space. I only record individual artists, not complete bands, and I don't do live drums that works for me. I don't get any issue with sound being transmitted through the slab, it's too big and absorbs sound being very dense and solid. I insulated my building using modern foam for heat and noise which has worked well. I had to pay special attention to doors and windows though. I double glazed the windows and if I did it again I would triple glaze to keep noise both in and out. Doors probably need to be doubled as well, e.g. a set with in a set to give a second barrier for sound. In general a brick/stone building will always work better than a timber building for noise absorption, that said I am very happy with what I can achieve in my own self built studio.