The D.C. City Council's tentative approval yesterday of far-reaching changes in the city's rent control laws represented a major victory for Washington's real estate industry and a surprising setback for Council Chairman David A. Clarke.

For weeks, landlords and real estate brokers, who contend that 10 years of rent control have destroyed incentives to develop new rental housing, lobbied furiously to derail Clarke's bill, which would have extended the existing law for another four years.

In the final day or two of private meetings involving key City Council members and Mayor Marion Barry, opponents of Clarke's measure fashioned a bare seven vote majority to brush aside Clarke's plan and adopt amendments favored by landlords.

The legislation would mean tenants could face increases of 12 percent on vacant apartments, instead of the current 10 percent ceiling, and could face unlimited increases on rented single-family homes, which would be exempt from all controls once they became vacant.

The bill passed yesterday also contains a controversial "vacancy decontrol" provision specifying that four years from now, assuming certain conditions, apartments would become exempt from all controls as they become vacant.

Representatives of the Washington Board of Realtors, the Apartment and Office Building Association and the Greater Washington Board of Trade, who waged a hard-fought campaign against rent control, sat serenely in the council chambers as the debate roared on.

Council member John A. Wilson (D-Ward 2), who favored Clarke's measure, criticized some of his colleagues for going along with the real estate industry. At one point, he bitterly snapped that this was "still the best government money can buy."

After the meeting, industry spokesmen gave only grudging approval to the measure adopted.

"I think the bill that was passed was an improvement over the Clarke bill," said Tom Borger, the past president of the Board of Realtors. "We obviously still have rent control and problems that still have to be addressed."

Donald R. Slatton, executive vice president of the Apartment and Office Building Association, said the vote "is not a victory at all."

While some council members concede that the well-financed real estate industry lobby was effective in getting across its points, the final agreement came down to intense horse-swapping of favored amendments by council members. Clarke hurt his cause by not being more accommodating in considering some of the proposed amendments, according to other council members, while Barry gave assurance in private meetings that he would not object to most of the amendments.

"The executive branch was aware of what was going on every step of the way," said a member of the winning majority.

Clarke criticized the Board of Trade and other industry representatives for refusing to compromise with him and instead seeking agreement with willing council members "away from the center."

He also said that the mayor could have helped to defeat the rent decontrol provision if he had spoken out during the last day or two. "We asked the mayor to reiterate his opposition to vacancy decontrol but he didn't do it," Clarke said.

The architects of the 7-to-6 victory were council members John Ray (D-At Large), the original author of a bill to phase out rent controls gradually, and Charlene Drew Jarvis (D-Ward 4), who served as the point person during yesterday's often heated debate. Ray, who had angered many tenant groups with his original plan, stayed quietly in the background for much of the debate.

Clarke's bill was voted out of Ray's committee March 22, but it was widely known that it had only six votes and needed a seventh.

The crucial vote was cast by freshman at-large council member Carol Schwartz, the lone Republican on the 13-member council, who indicated last February that she would oppose efforts to phase out rent control but who went along with the majority yesterday.

Council member Polly Shackleton (D-Ward 3) angrily reminded Schwartz that she had told a gathering of Ward 3 residents that she would oppose decontrol. Schwartz contends that she only promised to oppose decontrol as provided in Ray's original bill.

"I said I would probably vote for Dave Clarke's bill, but fortunately I had an alternative . . . that adds in some stimulation for housing development ," Schwartz said.