Engineers and developers at Zeta Communities in San Francisco (, are building prefabricated two and four-story townhouses and condominiums that feature extra-thick windows, denser insulation, and a host of other energy-saving features, as well as solar panels to generate electricity. They claim a single unit generates as much energy as its occupants will use, making it a “zero-energy” home. And building modular homes in a factory lets the company streamline construction, control quality, avoid weather delays, and reduce waste by 30%. But the cost, energy, and carbon footprint involved with shipping the prefab structures from the firm’s plant near San Francisco takes a bit of the green off the housing units. (A developer can buy a two-bedroom townhouse, including shipping, installation, mechanical systems, and appliances, for about $258,000, according to the company.) The homes meet California building codes and efficiency standards.

Edited by Stephen J. Mraz