A 1986 law limiting San Francisco’s office development has once again become a battlefield over growth and the city’s future.

Mayor London Breed is putting a measure on the March ballot that would increase the amount of office space the city can approve under Proposition M, by reallocating the square footage of offices that were demolished or converted. It would also prioritize approvals for office projects that subsidize community or retail space at 60% of market-rate rent. Breed filed the ballot measure on Tuesday for the March primary election.

The measure competes with a proposal by South of Market nonprofit Todco to restrict office development further if the city doesn’t meet its affordable housing goals, which is likely to occur because of inadequate funding for such housing projects. If it receives more votes, Breed’s measure has a provision that would negate Todco’s measure.

Proposition M was a reaction to the city’s changing skyline and fears of “Manhattanization,” or an abundance of tall buildings. The city can approve only 875,000 square feet of large offices — those 50,000 square feet or larger — a year. Offices smaller than 50,000 square feet have a separate pool, and unused space rolls over to the next year. (Breed’s measure also calls for the 50,000-square-foot threshold to double to 100,000 square feet, making midsized office projects easier to build because they won’t compete with larger ones.)

Prop. M hasn’t been a major obstacle for the city’s developers until the past year, after the city embraced taller buildings in the Transbay district and the Central SoMa Plan area. That has exhausted the amount of Prop. M approvals available, making the fate of future projects uncertain. Most of the new buildings have filled up with tech tenants, straining the city’s transit and housing and sparking a backlash. But the offices also pay millions of dollars in city fees and taxes, making them a boon for the city.

Breed’s ballot measure states that the city’s low office vacancy rate and high rents are hurting small businesses and nonprofits even more than well-funded large tenants.

“Without changes to Proposition M to allow for new office space, small businesses will continue to be priced out of San Francisco’s office market,” the measure reads.

“The mayor’s measure is a balance between the need for smart growth that creates opportunities for small businesses to access office space, and supports projects that fund badly needed affordable housing,” said Jeff Cretan, Breed’s spokesman.

Todco said in a statement that its measure was “a genuine development compromise that strongly prioritizes affordable housing for lower income, working, and middle income San Franciscans” while Breed’s “largely innocuous measure” was solely a “poison pill” that would negate Todco’s measure if it passes.

Cretain said the mayor is “very open to discussing a compromise on this issue so we can all work together on a policy that works best for San Francisco.”

But Todco showed no willingness to compromise.

“The voters of San Francisco will decide. We are confident they will vote for affordable housing development — not office development — first!” Todco said in a statement.

Last year, Supervisor Aaron Peskin and then-Mayor Mark Farrell had proposed a legislative change similar to Breed’s that would take conversions and demolitions of offices into account for the Prop. M limits.

Peskin said the measure didn’t move forward in part because the planning department was able to grant approvals for the first phases of Central SoMa projects, resolving the immediate need for more space.

Peskin said he was neutral on the two ballot measures and supports a compromise.

“It’s my hope that the proponents will work it out,” Peskin said.

Roland Li is a Chronicle staff writer. Email: roland.li@sfchronicle.com Twitter: @rolandlisf