The new D.C. Jail, opened only five years ago, has 18 percent more inmates than it was built to handle and corrections officials are using day rooms, gymnasiums and open spaces to provide sleeping quarters for 250 excess inmates, city officials said yesterday.

The jail's population, which was 1,610 as of yesterday, has risen unabated since last November when the average daily number of inmates first exceeded its capacity of 1,356, according to assistant director of corrections George Holland.

Holland, who runs the facility located at 19th and D streets SE, attributed the increase to the city government's firmer anti-crime stance, an enlarged police force and more arrests.

City corrections officials plan to remedy the situation through a $1.5 million renovation at the alcoholic rehabilitation center on the grounds of the Lorton Reformatory, but that work will not be completed until February 1982.

Unused portions of the old minimum security complex at the Lorton Reformatory, which now houses 382 residents of the District's Rehabilitation Center for Alcoholics, will be renovated with towers and fences to hold 398 persons now serving misdemeanor sentences in the jail. That reconstruction is scheduled to be completed in February 1982.

In the meantime, the jail's swelling population has spilled into eating and recreation areas, as well as an empty area once designed for additional cells but uncompleted because of budget reductions. In that area, there are no toilets or showers.

The city has no additional guards to ensure security in the makeshift facilities and regularly is paying overtime to members of the 448-person guard force to do the job, Holland said.

"It's nothing new. There has been a gradual increase," said director of corrections Delbert C. Jackson. "We are daily making adjustments in the population" by sorting out less troublesome inmates, he said. "A large number of our clientele have been through the system before and we know many of them who are not likely to pose a problem."

Bernard Demczuk, a lobbyist for the American Federation of Government Employees who led last year's wildcat strike by guards protesting staff reductions at the jail, said the situation at the jail could be troublesome.

"I think all we have to do is look at Michigan, New York and Graterford, Pa., and I think the potential is there in Washington, D.C.," Demczuk said. "The potential for inmate violence on inmates and on staff is on the increase."

Holland said there have been no incidents indicating unrest, but "anytime you are in a dormitory as in the makeshift detention areas there is less security."

Guards walk among the prisoners in the cell portions of the jail as well as the temporary sections, he noted. "We get very paranoid in this business, but the mood seems all right," Holland said.

The normal use of day rooms in each cell block for games, letter writing, television and other activities, has been restricted, he said, as has activity in the gymnasiums, but outdoor exercise facilities are available.

A 1975 U.S. District Court ruling in a class action lawsuit set minimum square foot spacing for jail inmates that precludes doubling up in cells, and that solution has not been used, according to the papers filed by the city last month as part of the monitoring of that decision.

Jackson said he believes the District's need for additional jail cells will continue to grow. He said the jail, completed in 1976 at a cost of $30 million to replace the decaying, 100-year-old, red-stone facility a few hundred yards away, should have been larger.

The Supreme Court ruled last June that double-celling of prison inmates is not unconstitutional. But cell size as outlined in the 1975 ruling prohibits that practice in the city's jail.