The Mount Pleasant duplex where the worst fire in the city's history killed 10 mental outpatients last spring put up for sale this week for $375,000, a 79 percent increase over its purchase price 18 months ago.

Still showing damage from the April 11 blaze, the house at 1715-17 Lamont St. NW was listed with Galkin & Associates, a Silver Spring real estate firm.

A spokesman for the firm said a "For Sale" sign was posted at the property "just a couple days ago."

The asking price for the damaged and partly boarded-up building suprised the District of Columbia's chief residential tax assessor. "It sure as hell isn't worth that much," said the official, George Altoft. After the fire, the house was assessed for 1980 tax purposes at $140,000, he said.

The owners of the house, John G. Carleton and Mark E. Brodsky, paid $210,000 for it in February 1978, according to D.C. land records. The house was operated as a group home for St. Elizabeths Hospital outpatients by the Volunteers of America, a national social service organization.

For the $375,000 asking price, the house offers a total of 21 rooms in two wings. About half the room -- those in the east wing -- are heavily damaged and have not been repaired, according to the real estate agent.

The house is surrounded by a chain link fence. Three small cottages stand in the rear of the house, where there is access to an alley.

The garden that was once tended by Nancy Inman, a resident who died during the fire, is overgrown with weeds. the window that she jumped from is boarded from the inside, and jagged shards of glass still line the sill. reached by telephone yesterday Brodsky would not confirm that the house is for sale, despite the fact that a Galkin official said that Carleton and Brodsky had placed the listing.

"I really have no comment on anything to do with that property, even (on) a simple question," Brodsky said.

One real estate analyst who asked not to be identified said that the asking price for the house might be related to its potential for condominium conversion.

D.C. housing officials said that no certificate of eligibility has been issued for the house. A spokesman for the condominium office said that, since the Lamont Street duplex has never been operated as an apartment house and is vacant, new owners could rehabilitate the structure and apply for a condominium certificate after the current moratorium ends this month.

The fire that swept through the rambling, 2 1/2-story brick-and stucco house was laid to a woman resident who dropped lighted matches on a couch in the home's first-floor lounge.

From the couch, the blaze leaped up a wall and to an open stairwell in the east wing of the duplex. Within minutes smokes and flames filled the upper floors, where most of the residents were sleeping.

Ten of the 47 residents were killed by the blaze. Fire officials called it the worst in the city's history, surpassing the 1977 Cinema Follies theater fire that claimed nine lives.

The city's investigation of the fire was highly critical of the District's building inspecton procedures and licensing requirements for boarding homes.

Mayor Marion Barry said that the home should never have been licensed because it lacked a fire escape from upper floors, fire doors and other safety equipment.

Within days of the fire, the building inspector who had approved the house for occupancy was dismissed.

In the months following the fire, relatives of two of the residents who perished, Inman and Dorothy Cain, have filed lawsuits against the owners and operators of the home, alleging negligence in fire safety planning.

As for the other residents, a spokesman for St. Elizabeth Hospital said that more than half of them have been placed in other group homes. Those remaining are assigned to wards at the federal mental hospital. suffering from deteriorating physical conditions unrelated to the fire, or awaiting vacancies at other group homes.