San Francisco Mayor Gavin Newsom on Monday signed a law that analysts consider to be the nation’s toughest green-building standards.

The standards set out to improve energy efficiency in new and existing buildings to help achieve the city’s goal of reducing greenhouse-gas emissions. Greentech Media in July wrote that Newsom had expected to sign the standards in the first week of August (see San Francisco to Boost Small Wind).

The new ordinance modifies the city’s building code, requiring applicants for residential and commercial building permits to follow a city-approved checklist and rating system, such as one created by the nonprofit Build It Green or the ratings system created by Leadership in Energy and Environmental Design (also known as LEED). Companies would also be required to obtain certification from the U.S. Green Building Council.

The new ordinance requires applicants to include separate and adequate storage and collection for recyclables, and to reduce the use of potable water for landscaping by 50 percent. The ordinance also governs storm-water management, construction-material recycling and the use of renewable energy.

The standards apply to new residential buildings and new commercial buildings that are 5,000 square feet or larger. The rules apply to projects for new and renovated interior commercial space of 25,000 square feet or more, and to work that significantly changes the structural, electrical and mechanical systems of a commercial building that is 25,000 square feet or larger.

The ordinance’s provisions, which set increasingly strict rules for new permit seekers through 2012, will take effect in 90 days.

The ordinance grew out of a city task-force report last year that recommended a comprehensive plan requiring energy-efficient technologies in construction and renovation projects.

Newsom had asked the task force to come up with a plan to help the city’s previously established goal of reducing greenhouse gas emissions to 20 percent below the 1990 levels by 2012.

A 2004 city report found that buildings in San Francisco contribute to about 50 percent of the city’s greenhouse-gas emissions. The city used about 5,000 gigawatt-hours of electricity and 300 million therms of natural gas in 1990 and emitted about 4.5 million tons of carbon dioxide, according to the report.

The city expects the ordinance to reduce energy and water use over the next four years. According to the city, the new law will reduce San Francisco’s carbon-dioxide emissions by 60,000 tons and save 220,000 megawatt hours of power by 2012, while saving 100 million gallons of drinking water and removing 90 million gallons of wastewater and storm water.

The city has created other programs in recent years for improving the energy efficiency of buildings, including reducing the cost of getting permits to install solar power systems and requiring the recycling of construction and demolition wastes.