Elected leaders proposed initiatives Wednesday aimed at reducing energy consumption and emissions of greenhouse gases in honor of Earth Day, but could face opposition from those concerned over its cost.

Mayor Michael Bloomberg and City Council Speaker Christine Quinn announced Wednesday morning the "Greener, Greater Building Plan," an extensive package of laws and policy initiatives.

According to officials, there are more than one-million buildings in the city, which make up nearly 80 percent of the city's carbon footprint.

The plan would require owners of large, older buildings to upgrade everything from light bulbs to boilers.

Owners of buildings that are 50,000 square feet or more would be required to have an energy audit once every decade, and make any energy-efficiency improvements that would pay for themselves in five years.

The legislation would also create the city's own energy code that buildings will have to meet whenever they make renovations.

The city estimates the upgrades would save property owners as much as $750 million a year in energy costs, while drastically reducing greenhouse gas emissions.

The carbon footprint would decline five percent.

"Implementing this strategy is just the first of its kind in the nation," said Bloomberg. "It would be equivalent to making a city the size of Oakland, California entirely carbon neutral. Just think about that. In fact, it's the world's most-comprehensive plan to reduce greenhouse gas emissions from existing buildings."

Officials say the plan would spur more than $3 billion worth of building improvement, and create 2,000 jobs in energy auditing and construction.

There are some concerns from the building industry, particularly surrounding the use of audits and the overall expense to building owners.

"The thing we like about this is that its' a terrific initiative, but the devil is always in the details," said Richard Anderson of the New York Building Congress. "How you go through the audit process? How much will it will cost. How do you evaluate it? These are questions to be worked out."

There are also concerns from tenant advocates that building owners would use the improvements as a way to increase their rents. Lawmakers say they are looking into that worry.

If the legislation is approved, the program is set to go into effect in 2013.