"Thanks but no thanks" is the response of local social service agencies to a Reagan administration offer to permit Army reserve facilities in Montgomery and Prince George's counties to be used to shelter the homeless.

Representatives of several private and public agencies in the two counties called the federal offer ill-conceived and unsuitable. The offer came after reports that suburban agencies lack funds to care for a growing number of wandering "street people" released from prisons and mental hospitals. The suburbs send many homeless people to the District to seek shelter. The administration's offer of the Army reserve facilities was an attempt handle this problem.

Hal Silvers, directer of Prince George's Office of Emergency Preparedness, said the county was offered the use of two indoor drill fields, at Camp Springs and at Riverdale, which could accommodate a total of 56 people.

The Army said the space could be used only between 5 p.m and 7 a.m., at a combined cost of $180 a day. It also would be up to the agencies to provide bedding and food, and there was some question as to whether the Army would allow the use of toilet facilites.

Silvers, who chaired a county committee of more than a dozen agencies to discuss the plan's feasibility, said he was incensed by the offer, which would allow his group to run little more than "a Bowery-type flophouse."

In a letter last Friday to Prince George's Executive Parris Glendening, Silvers said: "It is the unanimous consensus of opinion of the committee that these facilities are not suitable. The daily costs are prohibitive. . . . The social service organizations representing the public and private sectors determined that a warehouse design facility did not lend itself to the dignity and self-respect for innocent victims of the existing depression who, through no fault of their own, are now rendered helpless and homeless."

"Those hours are ridiculous. What do we do with people after 7 a.m.?" asked Curtis Gleason, social services director for the American Rescue Workers in Forestville. He said the open-space facility posed problems of privacy for men, women and families.

The Prince George's committee has formed two subcommittees to look for more appropriate shelter sites and ways to finance them.

Montgomery County government spokesman Charles Maier said social service officials in that county also have turned thumbs down on similar plans to use the Maus-Warfield Center in Rockville.

That facility, which Maier said is "off the beaten track," could house up to 40 people but not on weekends. Maier said that, instead, the county is looking into using an old state-owned building near the Silver Spring Metrorail stop, as well as the Rainbow Hotel on Rockville Pike.