A federal judge ruled yesterday that the city government cannot go ahead with plans to build 200 low- and moderate-income apartment units in the Southwest Washington urban renewal area until it obtains written consent from homeowners there who say the project would hurt their property.

The decision by U.S. District Court Judge Harold H. Greene could require the city to obtain approval from property owners in 11 other urban renewal aras in the city before any changes are made in existing urban renewal plans.

Lawyers for city agencies that oversee urban renewal projects had argued in court that a requirement to get written consent for properties "substantially and adversely affected" by a change in a renewal plan could provoke lawsuits and long delays in development.

Development on the land -- now a parking lot at 7th and g streets sw -- that was the focus of Greene's decision yesterday has been held up since 1974 while property owners in the area battled with the city government over the written consent requirement.

Greene ruled, however, that the D. C. Redevelopment Act of 1945, which set up the framework for urban renewal plans, sepcifically required that such consent be obtained before the city changed those plans.

The plan that Greene stopped yesterday would switch the parking lot parcel from chruch and parochial school use to apartment use. Forty-nine town house owners went to court, charging that the project would substantially affect their property values and increase population density, bringing more traffic. A proposed building height increase may also be detrimental to those owners "environmentally and aesthetically," Greene said, in ordering the city to obtain the written consents.

The owners of L'Enfant Plaza had also objected to the deveolpment, but Greene concluded that the effect was not so substantial to require their consent.

When Congress authorized renewal of the city's blighted areas, it knew that the redevelopment task would require massive cooperation from private sources, Greene said. In order to undertake bold changes in the then-slum areas of Southwest Washington, the city agencies needed huge amounts of private money, Greene said. And, he said, the city had to assure those investors that they would be protected from government action that might undermine the "ambitious" concept of a "new town in the city" for the Southwest area.

The consent provision in the redevelopment statute was designed to give those investors just that assurance -- that changes would not be made "against the will" of persons whose property would be substantially affected, Greene said.

The planning project "succeeded brilliantly," Greene said in his opinion yesterday. More than $265 million in private money had been invested there by 1973, compared to less than $230 million in government funds, he said. ". . .Assurances of stability" were a "major inducement" in attracting those private funds, Greene said.

The government "received what it bargained for -- the revitalization of a secton of the city that was a slum," Greene said. The property owners, Greene said, are similarly entitled to what "they bargained for -- a stable, reliable plan that may not be modified to their substantial detriment without their consent."

After the approval in 1956 of the Southwest Urban Renewal Project, covering 442 acres, the city's Redevelopment Land Agency did acquire consent from property owners who would be substantialy affected by changes in those plans, Greene said. But in 1968, the agency changed its policy and decided only to seek consent from property owners directly affected -- namely, the owner of the property to be developed.

City lawyers arggued in court that the earlier policy was carried out inconsistently and that the 1968 move reflected a careful analysis of the law's requirement. Greene, however, disagreed with the RLA's explanation for what he described as its "about-face" in policy.

Greene dismissed as "too pessimistic" the city's contention that the consent requirement would be abused by property owners and persons who lease in the area and give them the power of "veto" over plan changes.

Consent will only be required from those persons "substantially and adversely affected," Greene said, which will rule out the vast majority of landowners. In addition, Greene said, numerous jurisdictions outside the Washington area have successfully implemented similar consent agreements.

Greene said the city can ask Congress to amend the law to eliminate the consent provision as well as give the mayor or City Council authority to override any refusals. Lawyers said yesterday that the city government has twice gone to Capitol Hill unsuccessfully, seeking to eliminate the consent requirement.

The Redevelopment Land Agency is authorized to acquire parcels of land designated for renewal, which are then sold to redevelopers.

Greene suggested that the RLA set up regulations to determine which landowners or lessees are "substantially or adversely affected" by a plan change. The RLA could consider the economic impact of the change, traffic and parking impact and population density increases, Greene said.