The Montgomery County Council adopted yesterday a new formula for calculating the amount of sewage generated by new single family dwellings, a move expected to allow construction of about 700 units of low and moderate-income housing.

Using the figures approved at a council work session the county would gain, without building new sewage treatment facilities, 310,000 gallons a day of sewage capacity that could be allotted to new development, one council member said.

Council member Mike Gudis said the council voted to allocate two-thirds of the new capacity to low- and moderate-income housing and one-third to economic development projects such as new commercial or industrial facilities.

The move would appear to win for the county a small measure of relief from a sewer moratorium which was imposed on the county last month affter reports indicated Montgomery was exceeding its sewage allocation.

Even so, county environmental planner Dave Sobers said, the new sewage surplus, "really won't dent the needs of the county," which has long faced moratoriums on development because of limits on sewage capacity.

The 310,000 gallons of new capacity represents about 5 percent of the 7.28 million gallons of capacity required by applicants already on the county's waiting list for development, Sobers said.

Montgomery is currently sending about 71 million gallons a day of sewage into the treatment system and has issued commitments to developers for about 12 million more. According to Gudis, the council was told that the county before the new calculations was generating about 340,000 gallons of sewage more than permitted.

Sewage figures are computed by adding measurements of existing flows to estimates of flows that would be generated by projects which have been approved but remain uncompleted.

The council action, which must be endorsed at a formal session, affects only the estimates for new houses.

Previously the Washington Suburban Sanitary Commission, the bi-county water and sewer agency in the Maryland suburbs, had estimated flows generated by single family houses at 400 gallons a day and by townhouses at about 375 gallons a day. In recognition of plumbing improvements and householld water conservation the agency recommended reduction of both the single family and twonhouse figures to 350 gallons.