Marin is growing more green and has the highest percentage of land protected from development in the Bay Area, but still faces pressure to build housing, according to a new report by Greenbelt Alliance, a San Francisco-based open space advocacy group.

Marin County has 58 percent of its 333,000 acres protected from development. That is a 2 percent increase since the Greenbelt Alliance’s last report in 2012.

The county has 11,600 total acres that could be developed. Of that 2,400 acres are likely to be built on in the next 10 years, according to the report, “At Risk: The Bay Area Greenbelt,” released today.

The largest area that could be developed is the Silveira-St.Vincent’s property, the 1,200-acre greenway bounded by San Pablo Bay and Highway 101 between San Rafael and Novato.

Marin’s Countywide Plan allows for more than 200 units on the site, the report states. While earlier proposals planned to cluster homes and prioritize open space, new discussions include spreading out homes over the site.

“That will not happen immediately, but there is a medium-term risk there,” said Teri Shore, the alliance’s North Bay regional director. “There is the potential for affordable housing there.”

The 2007 Countywide Plan allows for 221 dwelling units for the combined St. Vincent’s and Silveira sites, and 100 of those must be priced for low- or very-low-income households. However, the plan does allow for a slightly higher number of units if they are for senior housing that doesn’t increase traffic in the area.

The report acknowledges the county needs more affordable housing.

“Marin County’s greatest environmental challenge is building homes for workers who cannot afford to live there,” the report reads. “The lack of affordable homes close to jobs puts sprawl pressure on outlying areas, and as of 2014, 68,000 people or 62 percent of the workforce, drove in every day.”

Those commuters create more than 284,000 tons of carbon dioxide pollution per year. More affordable homes are needed close to jobs and transportation, such as Golden Gate Transit and the new SMART train. But any new housing should not go on open space, the report asserts.

“Though Marin very much needs affordable homes, they would be more appropriate in already developed areas where residents can be close to services, schools and jobs,” it says.

Shore said that means building where housing already exists.

“We need development in the right places and we believe there is zoning there to do that,” she said. “We need to work together to make that happen. Once a greenbelt is built on it’s lost forever.”

A ridge in Tiburon on the Easton Point land and property northwest of Novato near Olompali State Historical Park are among the other areas in Marin that could see development, Shore said. The alliance is pushing for a voter-approved renewal of an Urban Growth Boundary passed by Novato voters in 1997. It expires this year.

Bay Area-wide, the report finds 293,100 acres of the Bay Area’s open spaces could be developed within the next 30 years. That equals 458 square miles, almost 10 times the size of San Francisco.

Of those lands, 63,500 acres, or 99 square miles, could see development within the next 10 years.

“We’ve done a comprehensive review of what every city and county in the Bay Area is planning, and where land speculators are proposing new developments,” said Jeremy Madsen, chief executive officer of Greenbelt Alliance. “These development threats are often invisible. By the time you see the bulldozers in the orchard, it’s too late.”

Rick Wells, chief executive officer of the Marin Builders Association, said his organization supports a diverse housing stock in the county.

He cited his group’s policy on a variety of housing: “It helps to create a well-balanced community that can house its own workforce. A locally housed workforce puts less strain on the limited natural resources and less pressure on an overburdened infrastructure.”