The major focal point in the planning of my retirement home is accessibility. This comes by way of seeing the limitations/problems that are evident in my parent's home. When confined to a wheelchair or walker, one quickly sees the hurdles that standard hallways/doorways and stairs present. There is a book entitled, "Building for a Lifetime" by Margaret Wylde. I don't have a copy but check ebay on a regular basis. Also check with the ADA for recommendations.
