Blair House, the president's 100-room guest house for visiting heads of state at 1650 Pennsylvania Ave., is to be remodeled, redecorated and endowed at a cost of $12.6 million, U.S. protocol chief Selwa Roosevelt said following an advisory group meeting yesterday.

Mark Hampton and Mario Buatta of New York will be the interior designers for the redecoration, under the direction of Clement Conger, curator collectively of Blair House, the White House and the State Department. Work is expected to begin immediately and be finished in 18 months.

The work will include a new curved wing, about 40 by 40 feet, at the rear of the house. The lower floor will be a garden party room, the upper floor a maximum-security three-room suite for visiting kings, queens and presidents. New air conditioning, plumbing (for all 30 bathrooms and the kitchen) and wiring will be installed for all four floors. The many small stairways between different levels will be eliminated. A new basement with kitchens and pantries will be added.

Impetus for the remodeling came after the chandelier in the principal bedroom crashed onto the bed about three and a half years ago. Fortunately no one was then in residence. Shortly afterward, the furnace failed, and though it was soon patched back together, Roosevelt said, "We couldn't take any more risks." The house was then closed to overnight visitors. State guests have been staying in hotels instead. However, the entertaining areas of Blair House -- the living rooms and dining rooms -- had continued to be used for receptions until it was closed Jan. 22.

The construction will be paid for from an $8.6 million congressional appropriation. Remodeling architects are Mendel, Mesick, Cohen, Waite and Hall of Albany, N.Y.

The redecoration, including restoration and reupholstery of the furniture plus some new furniture, rugs and curtains, will be financed by a $4 million fund to be raised from private contributions over the next two years, Roosevelt said. "When we took down the curtains they all fell apart," she said. "And not a single piece of furniture had decent upholstery. Some chairs even were missing legs."

Conger said: "When the government bought Blair House in 1941 for $142,000 it came with the Blair collections, including some late 18th and early 19th century American and English antiques. These will all have to be restored and reupholstered. We hope people will donate furniture of the same period as well as oriental rugs and paintings."

Cochairmen of the 40-member National Council for the Blair House Restoration Fund are former ambassador to Great Britain Anne Armstrong of Texas and Robin Chandler Duke, wife of former chief of protocol Angier Biddle Duke, of New York. Roosevelt will serve as honorary chairman. A Washington committee, headed by Oatsie Charles, Mary Jane Wick and Ceci Carusi, will hold a major fund-raising event in the fall of 1986.

Conger said he'll be relieved when the new fire stairs go in at Blair House. "Once, while the king of Saudi Arabia was staying in Blair House," said Conger, "the housekeeper smelled smoke. She followed her nose to the hall outside the king's bedroom. His servant had put an incense burner on a tapestry-covered chair. The chair was blazing. The housekeeper pulled the blankets off the king's bed, smothered the fire and saved the house."

Actually, Blair House is four houses, two on Pennsylvania Avenue and two on Jackson Place. The two on Pennsylvania Avenue are the original Blair House and Blair-Lee House. The Jackson Place houses are later Victorians. All together, the houses have about 100 rooms.

Blair House has housed history for all its years. The original house was built at 1651 Pennsylvania Ave. in 1824 by Joseph Lovell, the U.S. Army's first surgeon general. On Dec. 6, 1836, Francis Preston Blair, a Kentucky newspaperman, bought the house. He moved to Washington to establish a pro-Andrew Jackson newspaper called The Globe. Blair's house became the "Kitchen" for Jackson's so-called "Kitchen Cabinet," his group of unofficial advisers.

In 1843, Blair built a house next door at 1653 Pennsylvania Ave. for his daughter Elizabeth, who married Samuel Phillips Lee, a cousin of Robert E. Lee. At the beginning of the Civil War, Blair, in his house, speaking for President Abraham Lincoln, offered Gen. Lee command of the northern armies. Lee refused. Another Civil War general, William T. Sherman, married his own foster sister, Ellen Boyle Ewing, at Blair House at one of the great society weddings of the era. She was the daughter of Sen. Thomas Ewing (secretary at various times of the departments of both the treasury and interior), who rented the house during the 1840s.

The federal government bought Lee House for offices in 1941 and Blair House in 1942. In 1943, doors were cut through, joining the buildings.

Following Franklin Delano Roosevelt's sudden death, Harry Truman and his wife lived in Blair House for a week, as a courtesy to Eleanor Roosevelt. After they moved to the White House, Margaret Truman's piano fell through the floor above the East Room. So in November 1948, the Truman family moved back into Blair House while the White House was renovated. They lived there until February 1952. During John F. Kennedy's administration, two more houses on Jackson Place were acquired to enlarge the visitor facilities.

In recent years, another house, adjacent to the Blair-Lee complex on Jackson Place, also has been used as a grace-and-favor house for past and future presidents. President-elect and Mrs. Reagan stayed there before he took office