The "new downtown" of highrise office buildings won't be allowed to creep any further up Connecticut Avenue than Dupont Circle, if a decision tentatively agreed upon last week by the D.C. Zoning Commission sticks.

In a special session Thursday, the commission made tentative zoning changes that would generally decrease the size of future buildings not only on Connecticut Avenue between Dupont Circle and Florida Avenue but in a 66-acre area bounded by 15th Street, 23rd Street, M Street and Florida Avenue.

A formal vote on the zoning changes, which were proposed by the Dupont Coalition, an amalgam of five neighborhood groups, is expected March 29.

Under the plan, new buildings on Connecticut Avenue north of Dupont Circle, where 90-foot office buildings are now permitted, would be limited to a height of 60 feet.

At public hearings held over the past six months, developers testified that the proposed rezonings would lower property values and erode the tax base.

"The city could suffer as much as a $1.83 million reduction per year in real estate tax revenue," said Franklin Paulson, of the Washington Board of Realtors.

Proponents of the rezonings argued that the changes would, by preserving the area's low-rise ambiance, have a positive economic effect.

"Earning potential is not related to building size but to the general attractiveness of the area and the number of people interested in visiting it," said Advisory Neighborhood Commissioner George Wheeler.

The Dupont Coalition also testified that the rezonings would help stop demolitions of historic buildings, the depletion of housing stock in the area and the replacement of neighborhood shops and services by establishments catering to office workers in the "new downtown."

Under the plan, the commissioners tentatively agreed to reduce the size of structures that can be built along 16th Street between Scott Circle and Q Street from 90 feet to 65 feet. The National Wildlife Federation, the First Baptist Church and other institutions opposed this change, saying that in the future they might want to build highrises on their 16th Street properties.

The commissioners did not accede to the Dupont Coalition's request to rezone 16th Street north of Q Street, however. In declining to make the change that would have allowed apartment building of only 60 feet instead of 90 feet, Commissioner Walter Lewis said that the present zoning "would encourage more housing units."

The commissioners did agree to the coalition's proposal to rezone 17th Street from P Street to just north of R Street to allow more development -- from 90-foot structures. Community leaders said they wanted to encourage neighborhood commercial development there.

Other proposed zoning changes would:

Rezone Massachusetts Avenue between 20th Street and Florida Avenue. The new category would allow only 65-foot high apartment buildings -- instead of the current 90 feet -- and permit offices for professionals or nonprofit groups.

Downzone 20th Street between P and Q streets to a height limit of 65 feet; buildings now may be 90 feet high.

Neighborhood leaders said they hoped this last change would help save the century-old red brick mansion built on the corner of 20th and P by perennial presidential candidate James G. Blaine.

At hearings on the rezoning proposals, the owners of the Blaine mansion said they would like to demolish the building and put a highrise structure on the site.

Only three of the five zoning commissioners -- Walter Lewis, John Parsons and George White -- attended the public hearings on the rezoning proposals. Under zoning commission rules, only those three may vote on the changes and their decision must be unanimous.

The commissioners said they would not vote on the case until they had seen a map outlining the tentative changes. White also asked the Office of Planning and Development for an assessment of the economic effects of the proposed changes.

"We should know the fiscal impact of what we're doing," said White. "We're changing heights and uses. What does this mean in terms of property values and tax revenues to the city?"