Regulations allowing San Francisco property owners to convert common spaces into accessory dwelling units have brought forth a flood of applications to carve new apartments out of everything from garages and basements to old boiler rooms.

In the first nine months of the year, property owners applied to create 593 accessory dwelling units, known as ADUs. That is more than double the 242 ADUs that were applied for during the first nine months of 2016. There are now 1,046 ADUs in the pipeline, with building permits approved for 531 of them.

San Francisco’s big jump in ADUs — often known as granny flats or in-law units — started in 2014, when the Board of Supervisors passed an ordinance by former member Scott Wiener allowing ADUs to be added to buildings undergoing mandatory and voluntary seismic retrofitting. In 2016 legislation by Supervisors Aaron Peskin and Mark Farrell allowed buildings with five units or more to create an unlimited number of ADUs.

And many more are likely to come, said John Pollard of the SF Garage Co., a contractor whose clients are either building or waiting for permits for more than 300 ADUs.

“We are getting a ton in the Sunset, the Richmond, the Castro, Noe Valley, Haight-Ashbury, Russian Hill, Telegraph Hill,” Pollard said. “Pretty much every multiunit building with crappy old storage rooms is taking a look at this. You’ve got all these property owners that realize they are sitting on dead equity.”

So far 317 property owners going through soft-story retrofits, which enhance earthquake safety by strengthening wood-frame buildings that have poor reinforcement at the ground level, have applied to add 660 units, about 63 percent of the total ADUs in the pipeline.

“When we first started the program, there was just a trickle of applications, and some people pounced and said the program wouldn’t work,” said Wiener, now a state senator. “As with anything new, it’s going to be a slow start. People have to evaluate their buildings and finances and talk to an architect. Now we are seeing a great acceleration of applications. I’m very happy to see it.”

At 735 Taylor St., a 62-unit building on lower Nob Hill, Veritas Investments, one of the biggest landlords in San Francisco, is adding seven units in a ground-floor space previously used as a dining hall and common kitchen.

The seven units are small — between 220 and 381 square feet — and will rent for $2,400 to $2,800 per month. Veritas President Yat-Pang Au said the ADUs cost $300,000 to $400,000 a unit to construct — less than the $600,000 it typically costs to develop a unit in San Francisco, but still costly. He said the economics don’t work as well if there’s only enough space for one or two ADUs.

“It’s about making the economics work — one unit can cost $1 million,” he said.

Au said Veritas is looking at adding 200 to 400 ADUs in 100 buildings over the next five years.

While ADUs are commonly added to single-family homes in many cities, in San Francisco most of the action has been in larger apartment buildings. Owners of 39 buildings have added five units or more since the legislation passed.

“We are very happy to see the large apartment buildings are taking advantage of this,” said Kimia Haddadan, policy and legislative planner with the San Francisco Planning Department.

Kristy Wang, policy director at the urban think tank SPUR, said San Francisco is the only California city to create an ADU program for multifamily buildings, rather than just single-family homes.

“Owners of apartment buildings are already landlords and are accustomed to rent control and other rental regulations and have more experience managing construction projects,” she said. “It's a soft way to increase density in a dispersed fashion without changing the physical landscape very much.”

Pollard said city residents’ changing transportation preferences are allowing landlords to get creative with garage space. A property owner on the 1700 block of Mason Street, for example, is putting three ADUs into a garage and a vacant boiler room.

“They took out the boiler 25 years ago and can take out the parking because the Millennials don’t use it — they all ride bikes and take public transportation,” he said.

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