I used to see some sites and advertising for "Cave Homes". I would
like to build a retirement home in a side of a hill. Where can I get
information and plans, and, possibly, a contractor?
One of the true "guru"s of underground homes is Mike Oehler. Although he is more than a little bit off beat, his designs and engineering make great sense and have been proven in use for many years. He can be reached through: Mole Publishing Co., Rt. 4 Box 618, Bonners Ferry, Idaho 83805. I highly recomend digesting his books before commiting to anyone else!
Here is a "Cave Home" builder in IL:
http://daviscaves.com/
Another alternative is Monolithic (see the thread "Monolithic Domes"). These need very little change in design to be buried.
For general info: http://www.monolithic.com/
A list of builders using the same techniques: http://www.domebuilders.com/