One person's infill is another person's environmental disaster.

The Saltworks 50-50 project in Redwood City is a prime example. In Sunday op-ed pieces for the San Francisco Chronicle, architect Peter Calthorpe called Saltworks "one of the most compelling smart growth proposals." On the other hand, Save the Bay Executive Director David Lewis called the very same project "enormous sprawl into the bay." The differing perspectives from two thoughtful experts demonstrate just how difficult implementation of Greenbelt Alliance's all-infill plan for the Bay Area will be.

As I wrote last week, Greenbelt recently rolled out "Grow Smart Bay Area," which says the region can accommodate all projected growth through 2035 in existing cities by developing, redeveloping and better using 40,000 "opportunity sites."

Advocacy groups frequently publish glossy brochures and launch new websites. Grow Smart Bay Area is extraordinary in that it has three years of solid research as a foundation, and because it has an air officialdom. San Mateo County Supervisor and Association of Bay Area Governments President Rose Jacobs Gibson endorsed Grow Smart Bay Area as complementary to ABAG's own planning. California Air Resources Board Chair Mary Nichols called the Greenbelt plan a model for the rest of the state.

Still, developing another 800,000 housing units in the Bay Area is an enormous challenge, which brings me back to the Saltworks project. Developer DMB and property owner Cargill propose reusing 1,400 acres between the Bayshore Freeway and the bay itself for 12,000 housing units. Cargill and prior owners have used the site for salt production for many decades. With half the site set aside as open space and restored marsh, the development would be quite dense and, according to proponents, walkable and rich with transit.

Calthorpe � who helped DMB and Cargill design Saltworks � endorses the project because it would provide housing in an area with a great deal of employment while opening up the waterfront to the public. Lewis says it's crazy to build new houses at sea level in an area at risk of liquefaction during an earthquake, and the salt ponds should be restored with the others lining the bay.

Even though the project would qualify as industrial land conversion within existing city boundaries, I don't think this is the sort of "redevelopment" Greenbelt envisioned. Rather, the Grow Smart Bay Area website talks about traditional redevelopment of downtown Redwood City with additional housing.

There is little doubt downtown certainly could accommodate more residences, as could sections of El Camino Real corridor and some outdated commercial strips in town. But it's difficult to see how all of those projects would add up to 12,000 new units � and there is no doubt the mid-Peninsula can use the housing.

So, does the Saltworks project qualify as infill? If it does, is it a wise project environmentally? Keeping in mind that some locals are still fighting downtown redevelopment that planners consider first-rate, is the Saltworks project even close to politically feasible in Redwood City?

The answers that been provided to questions like these � as well the inability to provide any answers � has forced most of the Bay Area's new housing into the Central Valley since the 1990s. Greenbelt's all-infill plan will succeed only if it accompanies a sea change in attitudes about Bay Area land use.

- Paul Shigley