In the course of cutting the budget last month, Congress watered down one of the few devices the federal government has to achieve residential desegregation.

Because of the changes in the so-called Housing Assistance Plans, it will be easier for localities to get community development block grants without having to build low-income housing.

Towns like Parma, Ohio, and Warren, Mich., will be pleased with the change. The towns, virtually all-white suburbs near cities with large black populations, for years have turned down some federal grants rather than build low-income housing that could draw poor blacks from nearby Cleveland and Detroit.

The Housing and Community Development Act of 1974, which enjoyed bipartisan support, was designed to ensure that cities spent federal funds for low-income housing efficiently.

Under the act, communities that wanted to receive block grants had to develop a Housing Assistance Plan (HAP). The plans were to provide a review of the local housing situation, including the availability of low-income housing, and recommendations for addressing housing problems.

But the regulation writers at the Housing and Urban Development Department put more teeth into the HAPs when they defined the law's "expected to reside" provision. Congress had said the plans were to be based on the needs of the community's current population and of those "expected to reside" there. HUD defined the latter as people who worked in the community but lived outside it and people who might move there because of new job openings.

That could include maids who worked in the suburbs but lived in the inner cities. It also could include people from the cities who wanted to get jobs that opened in the suburbs. While the plan was not an ironclad commitment, the locality generally had to follow it to continue receiving federal money.

Many people saw the regulations as the Carter administration's way of trying to distribute low-income families more evenly within metropolitan areas. Others criticized the plans as being complicated to prepare and often based on unreliable data.

Sen. Richard G. Lugar (R-Ind.), who pushed for the latest changes in the plans, charged that under HUD Secretary Patricia Roberts Harris, federal officials actually wrote some towns' plans, rather than just reviewing them. In essence, he said, HUD was telling the town how it should plan its housing.

But Eugene T. Lowe, research director of the National Committee Against Discrimination in Housing, said Harris was just doing what was necessary to open suburbs to everyone who wanted to move there. Low- and middle-income people should be able to choose where they want to live, and only subsidized housing gives them that choice in the affluent suburbs, he said.

About a dozen cities rebelled a few years ago, deciding not to apply for community development block grants if it meant they had to come up with a plan and live by it.

Congress made three major changes in the housing plan requirements in the budget reconciliation bill:

Towns with fewer than 50,000 people no longer must design a plan when they apply for federal grants.

Cities can consider a decline in population, as well as an increase, in determining housing needs.

Cities can consider vacancies in neighboring towns in designing their plans, so that suburbs don't necessarily have to build public housing if there is a housing surplus in a nearby community. This was not incorporated in the statute but was included in the language of the conference report.

"It's really been gutted," said Raymond J. Struyk, director of the Urban Institute's center for housing and community development research.

But Lugar said the changes simply corrected the excesses of the Carter administration and restored the plans to what Congress originally intended. HUD spent so much time arguing over the minutiae of the plans that the goal of bolstering low-income housing suffered, Lugar said.

Stephen J. Bollinger, the new assistant secretary of HUD for community planning and development, said the Reagan administration welcomed the changes in the plans because they eased the administrative burden on communities.

But Bollinger said, "I don't want any city to think that any relaxation of the HAPs means we are not committed to the cities housing low-income people. They are not going to be able to hide behind any conferees' report."