Montgomery County would have to renege on promises made to the developers of 1,300 homes and "declare a moratorium on all building" if the District of Columbia were successful in its partial take-over of the regional Blue Plains sewage treatment plant, the Washington Suburban Sanitory Commission was told yesterday.

Calling Mayor Walter E. Washington's unilateral announcement that D.C. will use a larger portion of the capacity of the plant "intolerable" and "outrageous," the six-member commission representing Montgomery and Prince George's counties voted unanimously to fight the District of Columbia in court.

"This dance of the lame ducks is an affront to the newly elected officials (in the region), said Montgomery Commissioner David R. Scotton in a reference to Mayor Washington. Washington leaves office Jan. 2.

WSSC general manager Robert S. McGarry acknowledged, however, that despite the threat of a moratorium, "this doesn't mean carpenters are about to lay down their hammers."

When McGarry talks about a moratorium, he is referring to "allocations" of future sewage treatment resources. These are promises that developers will get access to sewage treatment facilities when they are ready to build their projects.

"Allocations,"however, are not commitments, which are legally binding, according to the WSSC. Montgomery has made commitments of sewage treatment capacity to developers preparing the equivalent of 20,000 houses.

While allocations are a step behind commitments, they are important to developers because one they are promised sewage treatment, they usually begin engineering studies and start preparing site plans - steps that involve major expenditures of money.

If the District of Columbia increases its share at Blue Plains at Montgomery's expense, McGarry said, the WSSC would have to remove the equivalent of about 10 percent of the projects on the allocation list. At present, the lists has enough capacity for the equivalent of 13,000 houses.

"Developers go like gangbusters when they get on the allocation list," David Sobers, Montgomery's environmental planning director said.

McGarry's forecast was based on the assumption that Montgomery and Prince George's counties would each lost 2 1/2 million gallons a day at Blue Plains if the District of Columbia unilaterlally took 5 million gallons a day.

However, Prince George's could agree to absorb a bigger share of the loss. Even if it absorbed the entire loss, Prince George's would still have far more unused capacity at Blue Plains than Montgomery - enough, in fact, to allow construction of the equivalent of about 16,000 houses.

McGarry did not spell out any of these implications in urging the WSSC commissioners "to take every and all legal action to stop this (the District's move)."

Although they have been quarreling among themselves recently over sewer issues, the Montgomery and Prince George's commissioners united in assailing the D.C. move.

For Montgomery's Scrotton, it was a "usurption." For chairman Johanna S. Norris of Prince George's, it was "outrageous" - a description she suggested be inserted in the agency's news release, a draft of which most of the commissioners took a hand in attempting to rewrite during their meeting.

Unlike the public affairs department the WSSC's legal office received an entirely free hand to fight the District of Columbia. The WSSC's first legal move is likely to be in U.S. District Court, which has been involved in regional sewer issues through most of this decade.

In justification of the District of Columbia's decision, Mayor Washington said the city's allocation of 135 million gallons daily was only temporary. He said suburban Maryland promised to build a second regional plant, but this was not done thus giving D.C. no alternative but to take a bigger share at Blue Plains.