Montgomery and Prince George's counties, concerned about future development, moved yesterday to settle their own sewer feud in preparation for what they say may be a new larger war between suburban Maryland and the District over the use of the Blue Plains sewage treatment plant.

Council members of the two suburban jurisdictions informally endorsed a compromise yesterday that would resolve a two-month-old dispute over the rates homeowners and developers should be charged to connect to the area water and sewer system.

And they were briefed on preliminary plans to file a lawsuit against the District over Blue Plains. D.C. Mayor Walter E. Washington announced last week that the city is unilaterally taking a bigger share of the capacity of the regional sewage treatment plant, thereby making it possible for the city to have sewage treatment capacity available to handle the equivalent of 16,500 new homes.

Montgomery and Prince George's County officials reacted angrily to Washington's announcement because they, too, are seeking new businesses and other tax-producing development.

The Washington Suburban Sanitary Commission, which provides water and sewer service to both counties, will meet today to discuss legal action to block Mayor Washington's plan. WSSC Chairman Johanna Norris and other officials said the commission will almost certainly direct its staff to prepare a lawsuit against Washington's unilateral takeover.

The mayor informed the other users of the Blue Plains plant - Montgomery, Prince George's and Fairfax counties - that the District is increasing its present allocation of 135 million gallons of treatment capacity at the plant to 140 million gallons a day.

Currently, Prince George's has an allocation of 67 million gallons, Montgomery has 78 million gallons, and the city of Rockville has 9 million gallons. WSSC general manager Robert S. McGarry said yesterday that the District cannot changed those figures without first calling a "summit" of the various jurisdictions, under previous court agreements.

"In light of what D.C. has done, we've got to pull ourselves together," said Norris, a Prince George's commissioner. "Maryland has got to stand together on this."

"The election is over, and that will help us with a lot of our problems," said WSSC Vice Chairman David R. Scotton of Montgomery. "The catalyst has been the D.C. government trying to abrogate Blue Plains. I think now we will be able to work out our differences in a . . . quieter way."

In this atmosphere, the two counties moved to resolve one of their long standing differences on sewers in an effort to present a united front against the District.

The two counties reached a compromise Friday on the rates that will be charged homeowners and developers to hook up to the bicounty water and sewer system.

The WSSC at its special session today is expected to approve a compromise rate structure for house connections, which has been delayed more than two months. Montgomery members of the WSSC charged that Prince George's representatives were holding up the necessary rate increases because they had been instructed by their county executive not to raise any taxes while he was running for relection.

Montgomery members said the WSSC lost $650,000 because of the delay.

Commission members from both Montgomery and Prince George's pledged that the next major issue facting the WSSC, Montgomery's proposed sewage treatment plant in Potomac, will be resolved more easily.

The Montgomery County Council voted yesterday to give formal approval to a site in Potomac for a 20-million-gallon-a-day plant that will permit the county to grow by as many as 200,000 people in the next 15 years.

The problem is money. Montgomery County believes that the WSSC should pay for the entire project, but Prince George's officials have said that the commission should seek federal funding for the project first - even though the Environmental Protection Agency has announced it will not pay for any more new sewage treatment facilities in the area because they are saving their money to pay for sewage plants needed to clean up the environments.

Despite the current disagreement, Norris said that she expects the two counties to compromise on development of the Potomac plant - and on the related issue of Montgomery's request for temporary or permanent use of part of the capacity of the Piscataway sewage plant in Prince George's - "within 60 days."

"The situation is flexible because new people are coming on the line," Norris said, referring to the turnover in executives in both counties. "And Prince George's is going to be reasonable."

Last Friday, the two counties agreed on a rate structure proposed by McGarry that will raise the current base rate of $1,095 to $1,100 for connections to houses in areas were sewer lines do not presently exist. For connections to homes in already developed areas, the charge will be $3,100.

McGarry said that the WSSC, which he estimates lost $40,000 every week the new rates were not enacted, will continue to lose money under the new structure because of the reduction in the higher rate. During the rest of this year, McGarry said, the WSSC will lost $300,000 on connections to developed areas.

Those costs may result in higher rates next year for WSSC taxpayers, McGarry said. "The county governments are responsible for setting up the budgets and rates for the commission, so they are going to have to live with this solution," he said.