SAN FRANCISCO, Calif. — The city’s ambitious green building codes would be the most stringent in the country but not without a short-term price tag, according to a new report from San Francisco’s Office of Econonic Analysis.

Citing higher construction costs and uncertainty about green building, the report said other approaches to reducing greenhouse gas emissions are available and less expensive, such as a carbon tax.

It predicts that San Francisco’s proposed building codes, which would force new large residentail and commerial buildings to meet LEED-Silver and LEED-Gold certification, respectively, would negatively impact the city’s economy and job market in the short term but would reduce greenhouse gas emissions by more than 1 million tons during the next 20 years.

The report found that over a 20-year time period, the codes would cost San Francisco’s economy between $30 million and $700 million of Gross City Product, as well as reduce jobs by between 70 and 2,300. The report expects per capita income to drop by $10 to $90.

But beyond the 20-year time period, the negative costs enter the positive column because of savings in resources, such as energy and water.

The report’s author argued that a market-based approach, such a carbon tax, would be a more effective strategy than a mandate. San Francisco Mayor Gavin Newsom disagreed, citing the potential for green collar job creation.

"There's a lot of mythmaking regarding the up-front costs, and we have argued that it is not as significant as others suggest," Newsom told the San Francisco Chronicle. "I don't accept the premise that there will be a hit to the (city) economy."