To own an old house is to own a window on the past. Emily Eig and Judith Robinson help you open it.

Operating out of a three-room, third-floor M Street walkup sandwiched between the Astor Restaurant and the Pluto Deli, they trace building pedigrees for a increasing number of renovation-minded homeowners, writing the history of the District of Columbia one house at a time.

It's a growing calling. Spurred by the incentives for historic renovation contained in the 1981 tax bill, half a dozen firms and individuals are currently at work in the city sifting deeds and property records for clues to the lives and lessons of Washington lots and row houses.

Their clients are the inner city's swelling affluent minority of childless couples and small families; house-proud bachelors and those eternally curious students of architecture and history willing to pay $300 to $500 to document a piece of the past.

"Actually, it can cost more than that," says Eig, an architectural history graduate of Brandeis and former intern at the National Trust for Historic Preservation. "We're like private detectives. We charge by the hour because we never know how long it will take or what we'll find."

"It's also like doing the family tree," says Ruth Anne Overbeck, who has done similar work on Capitol Hill for a decade. "You stand as big a chance of finding what you don't want as what you do."

Robinson remembers one couple, both lawyers, who renovated a rundown house at 1763 R St. NW, grew curious about it and commissioned a history. They knew the building had been a school at one time and had later been used by the Salvation Army. She and Eig, however, discovered that before that the house had been owned by a Supreme Court justice around the turn of the century--a friend of William Howard Taft, who had been a frequent visitor.

"The lawyer was just delighted," Robinson remembers. "He told us his avocation was reading up on Supreme Court justices of the past, and he was really pleased one had lived in his house."

But they might have found a madam or bootlegger instead. In one search, Eig and Robinson found a published report of a Foggy Bottom body snatcher who mistakenly sold a medical school his wife's still-breathing form.

"One out of 10 houses on Capitol Hill has been pointed out some time by a cabdriver or neighbor as the spot some president visited to see his mistress," says Overbeck, whose firm is Washington Perspectives. "Those reports never check out."

The more common reality, the historians say, is to discover only the names of unchronicled lives--respectable middle- or working-class people, often immigrants, who drifted to Washington in the last half of the 19th Century, built a house and lived and died leaving little record behind.

But sometimes the researchers find extraordinary details in unexpected places. Overbeck remembers finding that one Capitol Hill house was built before the Civil War by a freed black carpenter of obvious and exceptional skill.

"He had been living in the District even before he was freed, probably contracted out by his owner who lived in Maryland. Then he was freed, and we found records of his manumission papers and everything," she said.

One of her clients, Cindy Janke, a director of the Victorian Society, commissioned a history as a 1979 birthday present for her husband, John, a Capitol Hill real estate broker. Their house, a tall brick Victorian at 401 11th St. NE, sits back from the property line and "just looked different from the houses around it," Janke says.

He traced the house history himself through the beginnings of the city's building permits in 1877 without finding any real clue to its age or origins. One puzzle was a pair of enormous windows--42 inches wide by 6 feet high--with a single pane of glass in each sash.

"They just didn't look right," Janke remembers, "but we couldn't imagine what had been there before."

Overbeck's history provided the clue. The house, she found, had been built in 1863 or 1864 by Bernard Van Doren, a Belgian immigrant who with his Dutch-born wife, Mary Catherine, moved to Washington from New York in 1861. The Van Dorens, then in their early 50s and late 40s respectively, set up a tailor shop on 7th Street NW between F and G in what was then the silk-stocking district, catering to the carriage trade.

Living upstairs and prospering making uniforms in the city's Civil War boom economy, they made enough to partially finance the house bought by Janke 112 years later on 11th Street NE--an area then of mostly fields.

In a loan agreement on record with the city, Overbeck found details of the couple's household furnishings, including enough chairs and tableware to seat and feed 12 persons at a time. She also found listings of his mortgaged tailoring supplies, including cashmere, silk and linen fabrics--much of it in the black drab appropriate to the deep Victorian mourning of the war years.

Janke believed the Van Doren's Flemish origins meant the mysterious front windows had originally been large casement windows--like French doors--which were common in the early 1800s in the low countries. Scraping around the windows he and his wife found traces of hardware for casement windows, which they ultimately had reproduced and installed.

Eig and Robinson, whose firm is called Traceries, found history a similar clue to architecture in the case of 2130 Bancroft Place in Kalorama. The two-story flat-front brick building, while elegant, looked distinctly dissimilar from its next- door neighbors. No one seemed to know why. They discovered it had originally been the ballroom of the house next door, which at various times has served as the Costa Rican legation, the embassy of the Congo, and--during World War II--a branch of the War Department. The ballroom was split off as a separate building in the 1960s.

While the main building--built in 1907--had been designed by Frederic B. Pyle, architect of much of Cleveland Park, the ballroom was traced to George Oakley Totten, who designed many of the city's embassies and clubs and in 1908 served for a period as private architect to the sultan of Turkey. The owner who commissioned the ballroom, Nevil Monroe Hopkins, was an engineer, inventor and author who at one time devised a super submarine designed to carry a 16-inch gun.

House researchers find it hard to resist such historical gossip, and in their history of 2130 Bancroft, Eig and Robinson include such trivia as the 1967 death of a Costa Rican diplomat in a jetliner when he failed to fasten his seat belt during air turbulence.

"Obituaries are a gold mind of detail," says Eig. "We just love obits."

But the biggest reward of their work, the researchers say, is the historical context that gradually emerges in their minds--the lives and street scenes and daily human routines of a bygone city at once as close and as distant as memory.

"Washington is not that old, really, and it was not really all that long ago," said Eig, but she wonders at the fruits of that era around the turn of the century.

One of her favorite stories is that of the Mellon Apartments at 1785 Massachusetts Ave., which were built then "just as apartment living was becoming socially acceptable. They were grand vast apartments with oval reception halls, 18-foot ceilings in the living rooms and tons of servants quarters. The servants had seven-foot ceilings."

When Andrew Mellon moved in, she says, an art dealer named Lord Duveen got an apartment under his, just to be near Mellon and all his money. "He covered his walls with paintings and then would invite Andrew down to tea. And it worked! Mellon bought the paintings. And then bought more. Just think of it: Duveen's taste is now the core of the National Gallery."