San Francisco is all but certain to get another multimillion-dollar windfall this year — and one supervisor wants at least half of it to go toward alleviating the city’s housing crisis.

Supervisor Sandra Lee Fewer said the city expects about $140 million to fall into the city’s general fund for fiscal year 2019-20. If the city receives the money, Mayor London Breed and the Board of Supervisors will have free rein on how to spend it.

But Fewer plans to announce a proposal Tuesday that would advise the board to allocate 50% of that windfall to a fund for creating and preserving affordable housing.

Fewer’s ordinance would create the Affordable Housing Production and Preservation Fund, which would collect money to build new affordable housing units or buy existing market-rate housing and make it affordable. But the ordinance has little power to divert funds on its own, as it only states the board’s “intent” to allocate the money toward affordable housing. That means the board can choose to spend the money elsewhere if it wants to.

Forcing the board to divert the money in a certain way would require a charter amendment, which can be done only through a ballot measure.

“We just need a steady funding stream,” Fewer said. “We want to build affordable housing ... this is a commitment to really put our money where our mouth is.”

The extra cash comes from the Educational Revenue Augmentation Fund, or ERAF, a state program that shifts a portion of local property taxes to public school systems in each county. When county auditors determine the fund has enough money to meet the minimum state funding requirements for its public schools and community colleges, the remaining funds are returned to the local governments with few restrictions on how it can be spent.

Part of the total windfall must go to existing uses such as budget reserves, the Municipal Transportation Agency, public libraries, tree maintenance, public schools, and child care and youth services, according to the City Charter. The rest falls into the city’s general fund, which is controlled by the mayor and board.

Since excess ERAF funds are typically indicative of high property-tax values and low student enrollment — two factors San Francisco has had for years — the city likely will get similar windfalls in the coming years unless state law is changed. City Controller Ben Rosenfield said the city also will hear in the “coming weeks or months” if it will receive a total of $130 million in excess ERAF from Fiscal Year 2016-17.

While it’s likely San Francisco will receive the money for the upcoming fiscal year, Rosenfield cautioned that state legislators could still propose changes to the fund that would impact whether the money is disbursed back to the city.

Despite Rosenfield’s wariness, the supervisors are still preparing for it: Fewer’s ordinance is co-sponsored by Supervisors Vallie Brown, Aaron Peskin, Shamann Walton, Matt Haney, Gordon Mar and Rafael Mandelman.

San Francisco received its first ERAF windfall last year, which prompted a contentious and drawn-out fight over how to spend the available $185 million.

At issue were two competing plans: one from the mayor, who wanted to spend all the money on homelessness and housing, and a second from the supervisors, who wanted to divvy up the money among homelessness, housing, teacher pay raises and municipal energy independence.

After months of political wrangling, a compromise was reached during a tense Budget and Finance Committee meeting in February. The mayor — who was not entirely pleased with the final result — signed the ordinance a few weeks later.

Fewer said she hopes that suggesting how half of the money is spent will alleviate the fights over how to spend it.

Building more affordable housing “is a huge commitment, and we’re putting money behind it,” she said. “There are all these ongoing needs in the city, but the other 50% of the windfall can still be decided upon by the mayor and Board of Supervisors.”

Funding for affordable housing currently comes from sources such as fees developers must pay if they choose not to include below-market-rate units in their projects.

But those fees have dropped dramatically in recent years — from the $111 million high in fiscal year 2015-16 to $35 million in the current fiscal year — because of a softening market, a slowdown in the construction of market-rate housing and rising building costs.

Fewer hopes the fund, which would be administered by the Mayor’s Office of Housing and Community Development, will provide a consistent stream of money for affordable housing, regardless of changes in the economy.

The fund isn’t the only source of money city officials plan to create for more affordable housing. The mayor is also working to place a $500 million affordable housing bond on the November ballot. It needs the approval of at least eight supervisors before it could be placed on the Nov. 5 ballot.

The bond would require a two-thirds majority from San Francisco voters to pass.

Trisha Thadani is a San Francisco Chronicle staff writer. Email: tthadani@sfchronicle.com Twitter: @TrishaThadani