Ideas on building a listening room!!

As an Architect I first say make it beautiful. As an audio buff, I say make it work. It is a issue of whether you design from the inside out or the outside in. Since you say that you have lots of room to build, I take it you are not in on a cramped city lot and do not have to worry about disturbing the neighborhood. If you have a basement in the existing house you will want at least a crawl space below your addition if not a full basement. It will be much easier to run wires and experiment. A slab would give you tighter bass response though. Hard surface is not all bad. Do you want windows? Do you have a view? I personally would look at the orientation that you want and determine how that fits with the existing house. Do you want it to be a retreat of solitude, or a gathering space for all your family? You can always cure a bad sounding room with surface modifications, but you can only plant ivy if it does not look good from the outside!