The new sliding water and sewer rates proposed by the Washington Suburban Sanitary Commission would not necessarily mean larger families, WSSC general manager Robert S. McGarry told the Montgomery County Council this week.

The proposed rate changes would save the majority of home owners using less than 350 gallons a day an estimated $6 to $35 a year, according to WSSC. The new rate scale would charge heavy consumers more per 1,000 gallons than light consumers.

The result of the WSSC survey requested by the council showed that among 18 sample families with low to moderate incomes, the bills for three large families - with six or more members - would increase while for two large families they would decrease. One family of 11 that was surveyed would end up with a decreased bill. Of 12 families with fewer than six members, eight families would get lower bills, two would receive higher ones and two would receive the same bills.

In a mail survey of 10,000 low to moderate-income families, WSSC found that the size of the family had nothing to with whether or not the members were using more or less than the average 100 gallons of water per day per person.

Council members had worried that the sliding water and sewer rates, intended to encourage conservation, would adversely affect large families who have low incomes.

The council voted 6-1 to endorse WSSC's proposed rate structure, which may go into effect in early 1978.

"Where there are social impacts of using pricing policies, we just have to deal with that," council president John L. Menke said.

Council member Jane Ann Moore, who cast the dissenting vote, disagreed, "I really do not think our public policy should be one that jeopardizes large families," she said.

"If one family has used 15,000 gallons of water and a larger one is using 30,000, the larger family is going to pay $15 more for that quarter billing period," council member Norman Christelod," council member Norman Christeller remarked. "That's not a major penalty. Let's not exaggerate it."