Alexandria officials are negotiating with the U.S. Marshal Service to obtain $3 million to add a floor to the city's planned new jail in return for reserving about 90 beds for federal prisoners awaiting trial or sentencing.

Alexandria Sheriff Michael Norris said that additional "per diem" payments the city would get for each federal prisoner housed could offset much of the city's cost of caring for its own prisoners after the jail's opening, scheduled for 1986.

Norris proposed the additional floor in response to statements from the U.S. marshals that they would be willing to provide $1 million toward building the jail in its current three-floor design if 45 beds were reserved for federal prisoners.

Norris is scheduled to discuss his plan with the marshals again on Tuesday. A spokesman for the marshals said it would get serious consideration.

The new jail will be part of a public-safety complex housing a magistrate's court and police and sheriff's headquarters. As now designed, it will house about 275 prisoners. Its construction, which will cost about $9.5 million, was ordered by a judge to ease overcrowding in the city's current jail.

Around the country, the U.S. Marshal Service is negotiating agreements to aid in jail construction and renovation to assure space for prisoners appearing at federal courts. In the Washington area, federal prisoners are now housed in D.C. or suburban jails, but at times as far away as Richmond and Petersburg, Va.

Norris said he did not expect civic opposition to the proposed expansion of the jail, which will be located in an industrial zone. If a jail is properly run, he said, the number of prisoners it houses has little impact on surrounding areas.