PORTLAND, Ore. — The city of Portland may burnish its green reputation with a carbon tax on new housing that merely meets the building code, and payments to builders of houses that are extra efficient with energy.

The plan would charge builders whose houses meet Oregon's requirements for energy efficiency. Builders whose houses are 30 percent more efficient than the code requires would escape the fee. Builders could get cash back if their houses are at least 45 percent more efficient.

The proposal would also require sellers of existing houses and commercial buildings to give buyers the results of energy audits.

"This is obviously an ambitious and potentially controversial undertaking, but with the new urgency and call to action on issues around global warming, this is the type of policy that Portland needs to be a leader," City Council member Dan Saltzman told The Oregonian newspaper at a Chicago conference on green building.

No figures on the tax and payments were released. On the city's Web page, the city's Office of Sustainable Development said details would be released in mid-November, and the proposal is expected to go before the City Council next year.

Mayor Tom Potter and a leader of one major Portland developer, Gerding Edlen, endorsed the idea, but leaders in the home-building business criticized it.

"There is no way the home builders will ever support a mandated program," said Jim McCauley, vice president of government affairs for the Homebuilders Association of Metropolitan Portland.

Randy Sebastian, president of Renaissance Homes of Lake Oswego, said a fee-based system seemed "heavy handed" when his company already incorporates green methods in many homes and offers options such as solar panels and tankless water heaters.

"There's more green building going on in Portland, Oregon, than anywhere, and there's not a mandate now," he said. "What's broken?"