Nearly 29,000 of the approximate 82,000 properties are residential homes on the Register of HIstoric Places. Old does not necessarily mean "historic" to view a complete criteria go to www.nps.gov/nr/listing.htm
Many people are looking to buy within historic districts and gain access to federal tax easements that can be applied to their homes.
One way owners can obtain such easments they can donate the facade of a building (by title only) the building stays in tact, to an approved IRS "qaulified organiztion" which adds that portion to thier collection. The home owner then gets a tax credit for the appraised value of the donated portion of the structure.
For more information on this and other programs here are some websites you can go to:
National Park Service guideliens and standards
National Register Owner Information
National Register of Historic Places
Federal Tax Easement Regulations
State Historic Rehabilitation Tax Credit