In a move that could save the District from having a slow its building boom for lack of sewage capacity, Prince George's County said yesterday it is prepared to offer some help to the city.

The announcement eases the political feuding over competition for new business and other tax-producing development that flared up when Mayor Walter E. Washington announced a month ago that the city was unilaterally taking a bigger share of the big Blue Plains regional treatment plant.Washington took the action to forestall a possible sewer moratorium within a matter of months based on - enough to handleonly the equivalent of about 19,000 people - that the District has left at Blue Plains.

"My county is in the position to tell the other jurisdictions that we will submit population projections showing we have some [sewage treatment] capacity we can share," Prince George's County Attorney James C. Chapin said in a statement yesterday before U.S. District Judge John Lewis Smith Jr.

Chapin gave no details, but high level negotiations between Prince George's, the District and other regional users of Blue Plains will get under way after the holidays. Judge Smith, who has become the reluctant adjudicator of the feud, called for a status report on Jan. 11, just over a week after Mayor-elect Marion Barry takes office.

Preliminary population projections released last week snowed that Prince George's growth over the next two decades would be far slower than anticipated as recently as two years ago. By 1995, the county is expected to grow by 206,000 fewer people than forecast in 1976.

The cutback in growth adds up to about 20 million gallons worth of surplus sewage capacity daily-cr only a quarter of the amount the District says it needs to avoid declaring a sewer moratorium-in effect, slamming the brakes on its comeback after years of economic deterioration.

A recent analysis by the Metropolitan Washington Board of Trade showed that, in the downtown area alone, 34 major projects worth more than $1.3 billion are planned. The projects, which include new construction and renovation, would provide the potential for more than 23,000 permanent jobs, the board said.

Projects either under way, committee or in an advanced planning stage include a convention center, office and residential construction along the Georgetown waterfront, at Fort Lincoln and in the Shaw, 14th Street NW and H Street NE redevelopment corridor. Throughout the city, new houses are going up and often-vacant old ones are being renovated.

All this development would require sewer capacity. Earlier this year, it became apparent that the District was dangerously close to using up all its capacity-135 million gallons a day at the Blue Plains plant at the southern tip of the city.

On Nov. 17, Mayor Washington told the other users of Blue Plains that the District of Columbia would unilaterally take five million gallons a day of unused capacity at the plant. His justification was that suburban Maryland-the major user of Blue Plains-had not kept its promise to build another plant in Montgomery County, a move that could have meant more capacity for the city.

Although the city's move was clouded legally-Judge Smith said yesterday "the District was wrong in taking the unilateral action that it did"-the tactic had its intended effect. It forced the other users of Blue Plains to face up to Washington's predicament.

While the Washington Suburban Sanitary Commission-the sewer agency for Montgomery and Prince George's-went to court, Prince George's newly inaugurated County Executive, Lawrence Hogan, and his County Council sent Mayor Washington a conciliatory letter asking for high-level talks on the capacity issue.

Then yesterday Chapin announced that Prince George's had surplus capacity-the first official county confirmation of what the District of Columbia and Montgomery have maintained for months-and was willing to share it.

Counting actual flows and commitments, Prince George's has about 10 million gallons a day worth of surplus capacity at Blue Plains. It also will have considerable surplus at its own Western Branch and Piscataway plants, whose expanded capacities will be available early next year.

Chapin told Judge Smith that Prince George's cannot say at this time exactly how much surplus it could make available to Washington.

That can be done in 30 to 60 days, he said, when the population projections are broken down area by area.

Informed of the Prince George's offer, D.C. Planning Director Ben W. Gilbert said: "Well, what do you know? We have said there is enough capacity to meet all the region's needs. This is a confirmation of that position."

Gilbert would not say specifically how close the city is to using up its capacity at Blue Plains of 135 million gallons daily. But according to the Washington Suburban Sanitary Commission, the city, at the end of November, had only 1.9 million gallons a day left-or enough to handle the needs of about 19,000 people.

The 'Willard Hotel proposal selected yesterday by the Pennsylvania Avenue Development Corp. - with its 1,450 rooms and other facilities - would require a significant portion of that slender surplus.

The Prince George's gesture drew quick praise from R. Robert Linowes, president of the Board of Trade until he is succeeded this morning by area developer Oliver T. Carr.

"The fact that Prince George's has taken this kind of approach is a very encouraging thing for the city and its future development," said Linowes, who has been critical of Mayor Washington's unilateral action but who also has maintain there is extra capacity at Blue Plains for the District. "This sewer situation is a very, very serious situation. You can't play games with it."

In the meeting in U.S. District Court yesterday, suburban Maryland asked Judge Smith to order the city not to take the additional five million gallons daily while negotiations are under way. Smith instead decided to wait until early January after the Barry administration is in power and has a chance to decide what to do.