The District of Columbia City Council yesterday unanimously approved revised plans for the proposed Washington Civic Center despite continued reservations about the reliability of business commitments for spin-off developments.

The controversial project, estimated to cost $99 million, must win the approval of two key Senate and House subcommittees. The panels received the latest draft report yesterday detailing plans to build a scaled-down version of the facility.

Both subcommittees are expected to take final action on the proposal before Congress adjourns for the year Oct. 14.

Council approval of the center project came on a voice vote by 10 of its 13 members and contrasted with the 9-to-4 vote taken a year ago in favor of an earlier center proposal. Yesterday, none of the previous opponents chose to vote against the center, which is to be constructed on a three-block site southwest of Mount Vernon Square and bounded by New York Avenue and H, 9th and 11th streets NW.

Several council members expressed concern about different aspects of the project, however, and one city official told the council that estimates of spin-off development revenues may be erroneous because commitments for some projects by the business sector "vary in both firmness and detail."

The note of caution came from D.C. Auditor Matthew Watson, who nevertheless assured the council that "the convention center remains economically viable even if the actual spin-off represents only 25 percent of that projected in the report."

City planners spent the summer scaling down an earlier center proposal after the Senate Appropriations District subcommittee chaired by Sen. Patrick Leahy (D-Vt.) rejected the initial $110 million plan as too costly and directed city officials to find ways to have a part of the expense borne by the business community.

Councilman Marion Barry (D-At-Large) yesterday praised the new design and financing method for the center as "something that meets local needs as well as national needs."

But Barry, the apparent victor in last week's Democratic mayoral pri-the proposal after telling the council mary, said he was "bothered" that city planners had succeeded in improving the design and financing aspects of that the earlier plan "was all we could get."

Ben Gilbert, director of the D.C. Municipal Planning Office, had consulted again with potential users of the convention center "with some reluctance and trepidation" to see if they could "live" with a smaller facility.

He said the revised center would be about 50,000 square feet smaller than the originally planned 300,000-square foot facility but would be able to accommodate larger meetings and exhibit shows by converting parking areas into additional exhibition space.

In their directive to the city in May, the two congressional subcommittees had told officials that the center should generate enough new revenues from spin-off development to cover its annual net fixed mortgage and operating costs.

City Budget Director Comer Coppie said yesterday that although some of the subsequently announced development projects would probably "go forward" regardless of the civic center, "we concluded they were directly motivated by the city's decision to go ahead" with the center's construction. For this reason, he said, such projects are being included in the list of spin-off development.

It will now be up to the subcommittee to decide if the various development projects qualify as spin-off hotel, restaurant and retail store construction that can be related to the center.

Opponents of the center say it is a subsidy for the business sector and should be voted up or down in a referendum they hope to force the city to hold. But most council members supporting the project argued yesterday that building the center is "a calculated risk" that must be taken to attract new tourism and convention business to the city and help create new jobs for residents.