You might want to start with the local (or state) council on aging...they may have some suggestions. Otherwise, certainly most good residential architects should be informed on ADA compliance, which would be a good starting point, as it sounds like you'd need to keep accessibility in mind.

We added a beautiful 1000sq ft plus an elevator onto our home in 2005 for my parents. I have quite a few tips & pointers if you would like to message me :)