DUPONT CIRCLE stands on the edge of Boundary Street (now Florida Avenue), once the line between the city of Washington and the rest of the District of Columbia.

In the 1870s, according to the Dupont Circle Citizens Association, Judge J.C. Hillyer, a mining speculator, built his country home where the Cosmos Club is now. Hillyer Place is named after the carriage drive to his house. Massachusetts Avenue NW at that time didn't go all the way to Florida Avenue.

The Blaine Mansion at 2000 Massachusetts Ave. NW was built in 1881 by James G. Blaine, always a presidential candidate, never a president. Judge Hillyer subdivided his property in 1882-3 for 11 houses on Hillyer Place. In the 1890s the romantic Victorian castle-style houses were built along the 1600-1700 blocks of 22nd Street. The great mansions such as the Codman house in the Beaux Arts style (named for the Paris Ecole des Beaux Arts , where everyone studied) came after the turn of the century.

Two of these great mansions are on the Dupont Circle Citizens Association house tour today, the Codman house (see story on this page); and the Columbian Embassy, one of the six fine mansions designed by Jules Henri de Sibour (1872-1938) that are now embassies. Tea will be served from 3 to 5 p.m. at the Columbian Embassy. Tickets ( $8) are on sale at the Codman House, 2145 Decatur Place, from 1 to 5 p.m. Profits from the sale benefit the citizen's association projects. The area included is bounded by Connecticut and Massachusetts Avenues, R and 22nd Streets.

The most important face about the Dupont Circle area is that mixed in with these grandiose houses are handsome, livable smaller houses. For a few years it was profitable and popular to tear down these charming townhouses to build high-rise apartment buildings. Now, fortunately for the city, these row houses with their towers, wide glass windows, handsome woodwork and neat brick exteriors, are being remodeled or restored at high prices. Ten other houses, with varying degrees of change, are on the Dupont Circle Tour today.

The home of Dudley Cannada, an architect with Design Environment Collaborative, and James Pewett, a lawyer, was built in 1897. Its redesign shows how the houses can be made to function in a day when the servants' quarters usually have to be rented out to pay the utilities bill.

Cannada designed their house to have three quite separate apartments. "The back of the house, with its porches up and downstairs, faces Florida Avenue. So that worked out very well to make a separate two-story unit with its own entrance. Out of the old dining room, I made two kitchens, one of our apartment and one for the back one. A bedroom on the back of the second floor also became part of that unit. It's closed off from our part now by our laundry, made out of the old hall." The English basement has another rental unit. The house is heated by three heat pumps.

Cannada took out the walls, which had once made a large entry and stairhall, and made this a part of the living/dining room. The beautiful old oak pocket doors slide into the wall between the main room and the new kitchen.

"I kept the kitchen deliberately dark," said Cannada, "so you don't notice it from the dining room. Actually, we never use the kitchen, we have it for resale."

The custom cabinets were made for the house by Kidder Brothers Construction, the contractors for most of the work. Cannada and Pewett did some of the work themselves, including burning off the old paint. "I'd never do that again," said Cannada.

The men bought the house in May 1977 for $90,000. "We paid for it, as we say in Mississippi, 'by hook and crook and a good cotton crop.' "He figures they have put in about $75,000 in the remodeling.

They made a den, guest bedroom and bath on the second floor, and two bedrooms and a bath on the third. The second is still under construction, but it will eventually have a roof deck atop the back rental unit.

Much of the house is furnished from Cannada's grandmother's attic, positioned against Pewett's new slick modern furniture. Both look quite at home.

Other houses on the tour today are those of Mr. and Mrs. Darwin Scher, Moustafa Soliman and Lynn Skzynear, Geoffrey Fox and Philip Melkye, Elizabeth Adams, William Chewning, Camille Bullock, Wolf Von Eckardt, Mr. and Mrs. Edward S. Cogen and Mr. and Mrs. Sabin Chase.