A wall of cubbies filled with shoes greets visitors to Gusto’s new headquarters at San Francisco’s Pier 70. As the 275 workers pad around their ginormous space, they sport socks, slippers or bare feet; visitors are offered disposable paper slippers.

“We want our office to feel like a home, to be comfortable and authentic,” said Josh Reeves, CEO and co-founder of Gusto, which helps small businesses manage payrolls, benefits and human resources. “We started Gusto in a house in Palo Alto and had a no-shoe policy there, and we all grew up in shoeless houses.”

The home theme is carried out in the cavernous central space, which “gusties” refer to as a living room, albeit one appropriate for a giant. Groupings of couches, easy chairs and coffee tables alternate with rows of workbenches. Overhead loom the old gantry cranes complete with driver cabs, and a complex crisscross of pipes and beams, while huge arched windows bring in natural light.

“It’s a really big building,” said Marcus Hopper, design manager at architecture firm Gensler, which designed Gusto’s office. “We had to figure out how to break down that massive size to bring it down to a human scale.”

“Really big” is an understatement for a 55,000-square-foot space with a storied history as a former Union Iron Works machine shop, on a site where ships have been repaired since the Spanish-American War of 1898.

The challenge was “how to connect this building type to what Gusto does when the design is so different,” said Ian Young, Gensler design architect.

Still, with its exposed brick and eye-catching industrial remnants, the Pier 70 space lent itself to the type of quirky, ultra-creative office look favored by tech companies.

Gusto now has 60,000 clients, or 1 percent of all U.S. employers. The company says its client roster continues to expand; it also has several hundred workers in Denver. It had outgrown its South of Market offices and liked the history and openness of the Pier 70 space in Dogpatch, the city’s fastest-growing neighborhood, Reeves said. And the cost of $57 per square foot for a 10-year lease compared favorably to the high $70s it paid in the South of Market district.

Pier 70, which has multiple buildings, eventually will have still more as the site of a mixed-use development by Orton Development and Forest City, which together plan some 1,000 housing units and 2 million square feet of office space. Forest City is holding a groundbreaking for its portion of the project Thursday, while Orton has already moved some tenants in.

Gusto occupies only half of the building. About 300 employees in Uber’s Advanced Technologies Group, which works on its self-driving cars, moved into the other half earlier this year. The two sides flank an airy atrium.

Gusto and Gensler collaborated with an accelerated timeline, starting the design process in September, launching construction in December, and moving Gusto in on the last day of April. Orton, the landlord, had already handled a lot of infrastructure work, such as a seismic retrofit.

“Gusties” were heavily involved, participating in design charettes and visiting the unfinished space in the fall to experience concepts for its transformation via virtual reality goggles.

Employees can have assigned workbenches, and/or they can float among the many couches, counters and desks. Reeves himself doesn’t have an office or desk. “I change where I sit multiple times a day,” he said.

Even though Gusto’s workers all basically occupy one gargantuan room, it’s remarkably quiet, with the whir of exhaust fans far overhead providing soothing white noise, and thick noise-canceling panels in the meeting rooms.

An area called the garage doubles as the cafeteria and meeting room. Employees joined in to paint a mural on one wall depicting the many types of small businesses Gusto serves. And since every tech company has creative monikers for its meeting rooms, the “huddle rooms” inside the building have been named after clients: bakery, florist, cafe, taqueria, ramen shop, travel agency, hardware store, deli, library, farmers market.

“This is basically a new building inside an old building,” Hopper said.

Carolyn Said is a San Francisco Chronicle staff writer. Email: csaid@sfchronicle.com Twitter: @csaid