ADUs have increasingly become the norm in Vancouver, too. The city says that 75% of new, single-family homes in the city – about 800 a year – come already built with either a laneway house, or some kind of secondary suite. For urban planners, this is a blindingly bright signal, and something cities should pounce on. We have “potential housing at our fingertips. It’s very low-hanging fruit,” Brown says. “And we should grab it.”

Helping homelessness?

Besides adding to the overall housing stock, could ADUs help fight homelessness? LA hopes so.

Los Angeles County saw a 12% increase in homelessness since last year, and the city proper saw a 16% climb. Close to 60,000 people in the county were sleeping rough during a January count, the Los Angeles Homeless Services Authority has revealed.

Last October, a large philanthropy firm donated $1 million to LA to help Angelenos turn their garages into dwelling units for that very purpose as part of a new pilot programme. (It hasn’t been a perfect rollout, though – hangups like the locations of power lines have denied building permits to some residents.)

“We thought, what a great way to add to the many strategies that we have to try to end homelessness in LA to try to match folks who are building these units with folks who are coming off the streets,” Los Angeles mayor Eric Garcetti said in a radio interview in October.

Press secretary Comisar says that “homeowners will be part of the solution to end homelessness, and will benefit from steady rental income and increased home equity.” LA’s goal by 2022 is to “house at least 200 housing-insecure people across nearly 150 ADUs.”

The California researchers also highlight additional benefits: more sets of eyes in the neighourhood could keep the area safer, and an additional stream of income for the renter: “I have friends [in LA] who moved into the garage, and rented out the house itself,” Brown says.

So, what did that old California law require? That each single-family home have two covered parking spaces off the street. But no more.

Donald Shoup, urban planning professor at the University of California, Los Angeles, who worked on the garage project with Anne Brown, estimates it costs around $80,000 to convert a garage into a flat. Before the laws were relaxed in California, many of those illegal conversions led to electrical fires, as the buildings weren’t necessarily up to city standards. Now, the relaxed permits allow residents to do this in a safer (and legal) way.

But would it work in other countries?

Shipping it elsewhere

Places like LA or Vancouver have idiosyncrasies that might make the plan difficult to transfer beyond the North American West Coast, like the car-centric nature of these cities. But for any big city that thrives on driving, garage ADUs could have a huge impact on urban planning in the future. It’s why places outside of North America, like New Zealand, have recently begun examining the potential of ADUs in their cities. (Although, in New Zealand’s case, living in garages has served as a worrying symptom of the housing crisis, rather than a solution.)

“There is this larger idea of, in planning – especially in a car-centric place – planning gets driven too much by the needs of how do we park our cars, what happens to them,” says Vinit Mukhija, professor of urban planning at UCLA, who collaborated with Shoup and Brown. “It takes over, increases housing costs, becomes the driving force – everything starts getting driven by this one factor. We’re hoping to change that. We don’t need to provide so much off-street parking.”

Instead, people can park their cars on their driveways, and turn their garages – houses for their cars – into houses for people. Especially those people who can’t typically afford any kind of house.

Shoup thinks that the more cities can identify ways to house people in already-existing structures (like garages), and make it easier to issue permits to convert the spaces and rent them out, the better off the cities will be.

This might take the form of basement flats in New York, Shoup says. For instance, one setup could be a “richer family and a lower income young tenant, especially near universities. This allows for more diversity in a single-family neighbourhood – people who otherwise couldn’t afford to live in a good neighbourhood.”

If the numbers are any indication, garage conversions are becoming more and more mainstream. It could change the way we plan cities – and help us house all the people in them.

“It’s a bright spot,” Shoup says. “To think we spent so many years building a city around cars – now we can make it more around people.”

Bryan Lufkin is BBC Worklife's feature writer. Follow him on Twitter @bryan_lufkin.