The Washington Suburban Sanitary Commission has proposed building a 100-million-gallon underground reservoir in the Norbeck area of Montgomery County that would cost $32 million and could be covered by tennis courts and other recreation facilities.

The reservoir for drinkable water would be a 35-foot-deep underground box of concrete measuring 800 by 500 feet. It would increase the agency's capacity for storing water in tanks and reservoirs to 260 million gallons.

Its construction would be financed by sale of bonds if the Montgomery and Prince George's county councils agree to the project. A WSSC spokesman said its user rates would rise if the project is approved, but could not say how much.

The WSSC provides water and sewer service to the two counties, and the new reservoir would hold water for distribution to both, although it would be especially useful in serving the higher elevations in Montgomery County around Damascus, Olney and Gaithersburg.

WSC spokesman Arthur P. Brigham said the reservoir, which would receive filtered and treated water from the egency's Potomac River treatment plant, would provide extra water for times of peak usage and could be helpful in an emergency such as the one in early July when a fire put the treatment plant temporarily out of operation. During the July water crisis some taps ran dry in the higher elevations of Montgomery County because the pumps that served them were not working.

The $32 million estimated cost of the reservoir project would provide water mains ranging from 30 to 60 inches in diameter and a pumping station, as well as the reservoir itself.

The reservoir would be built next to the Leisure World development in Norbeck, a planned community reserved for persons aged 50 and over.

If the project is approved by the county councils, public hearings will be held on the appearance of the reservoir.