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SAN FRANCISCO - More than 400,000 open breathing space acres near growing Bay Area cities could be swallowed up by housing and other growth within the next three decades if local governments don't ratify stricter ordinances containing sprawl, a new study found.

The Greenbelt Alliance review released Thursday found improving open space protections all through the nine-county region, with more than 1 million acres now enduringly secluded from development, up 27 percent since 2000.

But 125,300 open space acres are at "high risk" of development within the next 10 years, and 276,300 more are at "medium risk" within the next 30 years, the report found.

"The region is doing better than it was in 2001, but 400,000 acres is still an enormous amount of land at risk," said Tom Steinbach, the group's executive director.

Developers criticized the findings, in conflict that more land is needed to meet demand for a ballooning inhabitants and that growth restrictions have limited supply and driven housing prices further than what many buyers can pay.

"I have a fundamental difficulty with urban limit lines," said Joseph Perkins, president of the Homebuilders Association of Northern California. "They artificially restrict land use, which makes it more difficult to build affordable housing."