The plan for a mixed-use development in San Francisco’s Jackson Square that would include a new, privately funded $25.5 million firehouse just got bigger.

Related California, the developer selected to construct a fire station at 530 Sansome St., is proposing to combine that property with an adjacent parcel just to the east at 425-439 Washington St.

Taken together the combined development would include a new firehouse, 35 deluxe condos, a 200-room fitness-oriented Equinox Hotel, and a 36,000-square-foot athletic club.

While the condos, gym and hotel will be high-end, the project will also provide $12.1 million in fees to help fund an affordable housing development nearby at 772 Pacific Ave. in Chinatown.

John Updike, a former city director of real estate who is consulting on the project, said he thinks the firehouse costs will be closer to $24 million, meaning that another $1.5 million could be funneled to the housing, for a total of $13.6 million.

Related was one of four developers who bid on the proposal, but the company was the only one to secure an option to buy the adjacent parcel, where a different developer had been hoping to build a hotel.

“It’s a pretty neat deal,” said Supervisor Aaron Peskin, who started working to redevelop the firehouse property 15 years ago during this first stint on the board. “It’s very creative and, if it all comes to pass, it will fulfill a dream I’ve had for a decade and a half.”

Updike said Related has agreed to pay the “full cost of delivery” of the firehouse, “soup to nuts.” The cost analysis of the firehouse project was extremely conservative and detailed, he said.

“It was just short of noting the thread count of the linens on the firehouse beds,” he said. “What we are getting is a privately developed public asset.”

The new firehouse would replace a 1965 station that has become antiquated, according to incoming San Francisco Fire Chief Jeanine Nicholson, who said the department “supports this project 100 percent.”

“That firehouse needs to be replaced, and this is a great opportunity to see that come to fruition,” Assistant Deputy Fire Chief Tony Rivera said.

The property is zoned for 200 feet so no height variance would be sought for the project.

The affordable development at 772 Pacific would still need another $15 million to $17 million, according to Dan Adams, deputy director of the Mayor’s Office of Housing and Community Development.

The city purchased the property in 2017 for $5.5 million, at the urging of late Chinatown power broker Rose Pak. It is the site of the New Asia Chinese restaurant, and a new version of the restaurant is to be included in the development.

Equinox Hotels — its motto is “it’s not fitness, it’s life” — is a new concept owned by Related, a prolific developer also building projects in the Transbay district, Sunnydale, Potrero Hill and Civic Center.

The first Equinox Hotel is set to open in June at New York City’s Hudson Yards. The company has plans for additional hotels in Houston, Los Angeles, Chicago and Seattle. It is a spinoff of the high-end Equinox Fitness, which operates 100 gyms around the country, including three in San Francisco.

The land deal between the city and Related was approved by the Board of Supervisors Land Use and Transportation Committee on Monday and will go to the full board for its vote at a later date.

The environmental study for the firehouse-hotel project and the affordable housing development will be completed simultaneously, said Peskin, who called the project a rare example of a “controversy-free” development in San Francisco.

J.K. Dineen is a San Francisco Chronicle staff writer. Email: jdineen@sfchronicle.com Twitter: @sfjkdineen