When Hamilton Morton, a restoration architect from Washington, D.C., was planning work on the Delaplane House (c. 1850) in Virginia, he ran across a mystery. ''We wondered why the walls on the first floor were 16 inches thick in the back part of the house,'' he said during a two-day seminar here sponsored by the National Building Museum and National Preservation Institute, ''and the walls on the second floor were only six inches thick.''
Tearing away part of the wallboard and floor, he says, they found a log cabin first built on the site by white settlers, and later incorporated into the much larger house.
Such surprises are common in older homes, says the architect, who has spent 23 years working on restoration, preservation, and adaptive use projects on the East Coast. ''So you need a contractor who is sympathetic to this type of work. I've had some good contractors tell me they won't work on historic projects because they're afraid their crews will butcher the place.''
Before the owner ever gets to the contractor stage, however, Mr. Morton advises a lengthy study and planning period. Look for records on the house in deeds, wills, newspaper articles, and historical societies, he advises.
Check also with the Historic American Buildings Survey (HABS) documentation program, which has deposited measured drawings, photographs, and histories of 17 ,000 buildings at the Library of Congress. ''One firm sent two architects down to a house to do measured drawings and then found the plans at the library,'' laments Mr. Morton.
The whole house should then be gone over carefully with a measuring tape, and small pieces of wall and floorboard removed and checked. ''You're looking for the condition of beams and joists, for the composition of the plaster, for covered-up walls, ceilings, doors, for evidence of past use,'' says the architect.
Surveys should also be taken of the heating, water, and electrical systems (if any), the landscape, any archaeological evidence (the Delaplane House had everything from old bottles of vanilla extract to notes scratched on the walls), and any building code deficiencies.
''Most building codes now say that if renovative work is undertaken which will cost more than 25 percent of the value of the house, then the house must be brought up to current building standards,'' says Mr. Morton. There are occasional exceptions to this rule with historic homes, however.
With all this information, Mr. Morton says, the owner can see what he's up against and begin planning his restoration - a job the architect says is ''75 percent deferred maintenance,'' repairing and replacing old, worn-out building materials.
While most owners of a historic house want the home to take on the appearance of a particular period, they also want to incorporate modern heating, lighting, and kitchen facilities. Making sure these modern additions don't harm or significantly alter the structure is the task of a restoration architect, Mr. Morton says.
Often old houses have energy-conscious features already built in, he points out. Early homes had many energy-conserving features out of necessity, because of the inefficiency of heating with fireplaces and the lack of artificial cooling - features like the large, cool porches around Natchez, Miss., and the slate roof and dark-stained clapboards of the House of Seven Gables in Massachusetts that absorb heat from the sun. Learning to reuse these features is one of the advantages of living in an older home, Mr. Morton says.
Once you have a good idea of what you want, how you want it done, and how much you're willing to pay for it, the owner and architect can draw up contract documents and obtain bids.
There are many ways to arrange payment for a contractor, but Mr. Morton advises giving him a fixed fee and specific allowance limits on materials and labor because ''there are so many surprises in an old house, and when you find some of these things, you may want to save them.''
That's what happened in Delaplane, the architect reports, where the kitchen is now housed in the log cabin, authentic down to the wood.