In a move that curbs affordable housing development potential in Strawberry but not at the Silveira-St. Vincent’s tract in north San Rafael, county supervisors Tuesday approved 20-unit-per-acre zoning for three affordable housing sites, replacing a 30-unit designation.

The action covering a total of six acres won’t change overall development potential of Silveira-St. Vincent’s or a Marin City parcel, but allows cutting 20 units earmarked for a parcel at the former Golden Gate Baptist Theological Seminary in Strawberry because the county failed to list it in a report to the state.

The Board of Supervisors unanimously overturned the Planning Commission, which had refused to make the change. Commissioners said the “affordable housing combining district” calling for “a density of 30 units” should remain intact — rather than cut to “20 units” as allowed by a new state law.

The supervisors, who lobbied hard to get AB 1537 passed, enabling Marin’s designation as a “suburban” county rather than an “urban” region subject to 30-unit affordable zoning, noted Assemblyman Marc Levine pushed the legislation through at the county’s request.

The 20-unit designation does not change how many units can be developed at sites in Marin City or Silveira-St. Vincent’s, because acreage available for development there will be expanded to accommodate development allowed in the county’s general plan. At Silveira-St. Vincent’s, for example, 100 low-income units previously allowed on 3.5 acres as part of the 30-unit zoning will instead be spread over five acres under the 20-unit zoning, in order to be consistent with general plan designations.

But on a two-acre parcel at the former seminary in Strawberry, affordable development will remain on two acres for a total of 40 units, rather than spread out on three acres to make up for the 60 units allowed previously. That’s because the parcel inexplicably was not mentioned on the official development “site inventory” provided the state. The list that neglected to include Strawberry indicated the county had adequate sites zoned for affordable housing.

Supervisor Damon Connolly, who wanted to cut the number of affordable units allowed at Silveira-St. Vincent’s, cast the lone dissent in voting against the sprawl of the bigger five-acre footprint. Connolly wondered whether the combining district and related moves “were the correct mechanisms to address our affordable housing goals,” and added he was troubled that planning commissioners “were confused about what they were doing.”

A parade of speakers, many of whom had appeared at previous housing hearings, recited familiar arguments pro and con, with housing density foe Stephen Nestel of Marinwood noting that three board members face re-election next year.

“Why did we go through all the trouble of getting the legislation passed?” a Los Ranchitos woman wondered, if the county was not going to move ahead with a 20-unit program.

“Every tool we (have) for affordable housing is needed,” said housing advocate Wendi Kallins of Forest Knolls, citing rents as high as $2,800 a month for a one-bedroom dwelling. She noted that allowing 30 units an acre lowers costs and increases resource efficiency. All the 20-unit program does, she added, “is spread the homes out to cover more property.”

But supervisors were determined to embrace the 20-unit suburban zoning they instructed Levine to get.

Supervisor Kate Sears, reading a statement, said the 20-unit zoning allows Marin “to be treated like the suburban county it is,” and brings county “policy in line with reality.” Supervisor Judy Arnold said a troubling anti-newcomer attitude has emerged in recent years, and noted the county’s conservation history included 60-acre zoning that preserved West Marin farmland while centering development along the freeway.

Supervisor Steve Kinsey called the day a session of “important political theater,” adding that “none of the things we are talking about are going to have a meaningful” impact on housing. Housing solutions are elusive, and even his own second-unit project in Forest Knolls has proved an expensive proposition, with costs including a $10,000 payment for a water hookup.

The biggest threat to the county, Kinsey added, is not overdevelopment, but lack of diversity. Though “we aren’t solving that problem,” he said the board owed it to Assemblyman Levine to embrace the 20-unit designation the county told him it wanted.