Prince George's County Executive Winfield M. Kelly Jr. announced the lifting of a sewer moratorium for the northwestern portion of the county yesterday that will allow the start of the first residential development there in the past seven years.

With what Kelly called "an easing of building restrictions in the lower Anacostia basin," the county will permit the Washington Suburban Sanitary Commission to allocate an additional sewage treatment capacity of 400,000 gallons per day for commercial, industrial, public and residential users.

The plan could bring up to 600 singer-family homes and townhouses to the area, "upgrade the housing stock in Prince George's," according to Kelly, and increase employment in the sluggish construction industry.

Kelly hesitated to say the move would create a housing boom but added that the new townhouse developments produced by the allocations would offer "young professionals and starter-housing seekers a new opportunity" to buy in Prince George's.

The allocation specifically denies permits for apartment and multi-family dwellings of any kind as part of Kelly's continuing policy to reduce the number of apartments and "low income families" in the county.

One builder, Albert Turner of MCD Enterprises, who originally applied for a sewer allocation for 140 garden apartments in the Calverton area, has switched his design because of the restrictions on multifamily houses and plans to build 126 townhouses on the site.

Russell Shipley, attorney for Turner, said the "moratorium hurt us in the sense of requiring the owners to carry their ground, pay taxes, without being able to do anything to the property. But marketabilitywise, I don't think it has hurt them. (The market) is now better for townhouses than garden apartments."

The moratorium began in 1970 when the state health department imposed restrictions on new sewer hook-ups in the Anacostia and Beaverdam drainage basin bcause of overloading at the Blue Plains Sewage Treatment Plant. At the same time it ordered the county to provide an interim sewage treatment plant with a capacity of a million gallons per day to accommodate new development.

The county, however, opted to build a plant with double that capacity, adding another million gallons of capacity for overflow relief. The state recently amended the moratorium and is permitting the county to issue sewer permits for new development after WSSC provided assurance that "things looked good and new construction couldn't cause an overflow," according to county sources.

Under a resolution that Kelly said he would send to the County Council today, the allocation plan provides 70,000 gallons of treatment capacity per day for public use and 100,000 for commercial and industrial uses, with the remaining 230,000 to be divided up between single family and townhouse development.

Individuals and corporations who had filed applications with the WSSC for sewer allocations from 1970 to last May are now eligible for a go-ahead with a restriction that no single application exceed 50,000 gallons per day.

A Kelly spokesman said there were so many applications during the seven-year period that all the housing capacity was filled by the back requests.

A special restriction in the plan reserves 20 per cent of the housing allocaion for builders who construct 10 or few single-family detached houses each year. Kelly said this was done "to bring the small builder back to the county. He was forced out of the market by the moratorium."