Many of the old houses in the Washington region, particularly in once-rural areas, are a mish-mash of time periods and styles--perhaps a pre-Revolutionary core, some additions that survived the Civil War, an early-20th-century modernization to add electricity or plumbing, plus a 1950s update just because.

So how do their contemporary owners decide what to restore or renovate, assuming that historical accuracy is a concern?

"Even with the sites we've had in our collection for a very long time, we grapple with the question of what period to restore it to," said William Dupont, an architect who oversees historic sites for the National Trust for Historic Preservation. The 50-year-old D.C. group manages such grand old places as Montpelier, James Madison's home north of Charlottesville, and Oatlands, the former plantation near Leesburg.

While few private citizens own buildings that are historically important, "it's instructive to anyone to look at how we make decisions at the National Trust," he said.

The first step, he said, is "a tremendous amount of research." At the trust, the research team might include architects, historians, archeologists and conservators.

After researching a property, Dupont said, the trust's specialists use that information to consider why it is significant. For instance, Montpelier is important because Madison lived there. Another local Trust property, the Pope-Leighey house in Fairfax County, is important because it was designed by architect Frank Lloyd Wright.

The significant aspect, he said, will become "what we're most respectful of" as the renovation proceeds.

There are also levels of significance within a building. For instance, he said, if one room has greater importance than other rooms because of events that happened there, "that's the room you won't want to change."

With that information as a basis, Dupont said, "then we make an informed decision about what's the best period to show the public."

Dupont said a homeowner can trace a building's chronology through "simple things." Among them:

* Do a deed and title search. Courthouse real estate records show when properties changed hands and who the buyers and sellers were.

* Check other legal records. Birth, death and marriage certificates, insurance records and probate records may all contain valuable information. For instance, the old wills in probate court can include lists of household items.

* Contact the former owners or their descendants. They may have pictures or stories.

* In the library, check old newspapers, photo collections, atlases and city directories.

* Examine any architectural plans you can find--they may be available through the building permit office or from previous owners.

In addition to examining records, Dupont said, examine the house, "looking for clues that are in the way it's built." For instance, there might be changes in trimming or hardware that show where additions were built.

When the trust is restoring a property, it follows what are known as the Secretary of the Interior's Standards for Rehabilitation--10 general principles for balancing preservation and modernization. To be eligible for federal tax incentives, renovations of "certified historic structures" must follow these standards. Homes in some historic areas are also bound by them.

But what if you just want to live in your old house? Dupont said you can still balance your desire for modern conveniences with respect for the old structure. Kitchens and baths, he said, usually require major changes to bring them up to current levels, but "it's always possible" to meld modern plumbing with the old building.

He said, "I've never hit a situation where we've said, wow, no compromise is achievable."

There is a wealth of information available, printed and electronic, on old houses and how to restore them. Here are a few places to start. Most of these publications and Web pages include helpful bibliographies and links where you can learn more.

* For a quick overview of how to plan a historic home rehab, see the National Trust for Historic Preservation's pamphlet "The New Old House Starter Kit." For the $6 pamphlet, or a catalogue of trust publications, call 202-588-6286 or see the group's Web site at www.nthp.org.

* The bimonthly magazine Old House Journal is available at well-stocked newsstands; its Web page is www.oldhousejournal.com. The Web page includes free electronic versions of the federal government's booklets on common preservation and repair problems. See www.oldhousejournal.com/notebook/ npsbriefs/index.asp for a full list. (There's no link from the magazine's home page directly to the list.)

* Many areas and even neighborhoods have their own historic societies that specialize in the quirks of local buildings. Every state and territory also has an official state historic preservation office. In the District, call 202-442-4570; in Maryland, 410-514-7600; and in Virginia, 804-367-2323.

* Information about preservation tax incentives for investors, a copy of the Secretary of the Interior's Standards for Rehabilitation and a variety of other preservation publications are available through the National Park Service's Heritage Preservation Services division, at www.cr.nps.gov or 202-343-9573. Publications can be purchased through the Government Printing Office, at 202-512-1800.

* Old house voyeurs might want to check out www.thisoldhouse.com, Web site of the long-running public TV show. If nothing else, its reviews of past projects remind you that even the pros go overbudget--almost all the time.