San Francisco has no official definition of “family housing,” but Heather Hawkins knows what it isn’t.

It isn’t the little two-bedroom flat in Bernal Heights that she paid more than $4,000 a month to rent, where her baby slept in the closet of her sister’s room, and where space was so tight she knew the number of steps between every point. Seven steps from her bed to the toilet. Thirteen steps from her bed to the girls’ room.

Hawkins, her husband and girls, like so many other San Francisco families, have packed up to head for the hills — well, the mountains. Her family is renting an apartment in Truckee while they look for a house to buy. They’ll probably get twice as much space for half the price of anything they could find in San Francisco.

“It’s hard when your kid comes home and says, ‘But I love my little blue house!’ It’s this sinking feeling of, ‘This isn’t yours. This isn’t ours.’ That’s never going to happen for us in this city,” said Hawkins, a 42-year-old consultant in the health and outdoors industry whose husband works in tech. “I roll my eyes when people say it’s the techies. Nope! We’re leaving too.”

San Francisco notoriously has the smallest percentage of kids — 13.4 percent — of any city in the nation. But while San Francisco officials sweat and bicker over affordable housing, they rarely talk about family housing.

Supervisor Norman Yee, a father of two and grandfather of two, wants to change that. He commissioned a report from the Planning Department that outlines some startling statistics about San Francisco families and their housing options.

Hint: When it comes to those housing options, there aren’t many.

“Nobody has thought about it,” Yee said, noting the city needs to figure out how to keep families here for it to thrive. “The vibrancy of the city really depends on families and children. What families offer is stability — they’re the ones who will stay, who will invest their time and energy to make sure that neighborhoods are safe and productive.”

Yee said he’s tired of seeing plans for high-rises filled with studios and one-bedroom apartments and open spaces that are clearly designed for adults. Some of them even boast dog-washing stations, but playgrounds or stroller storage? Forget it.

“It’s great if you want to sit there and chat and drink Chardonnay, but very few of them have open space that would be useful for kids,” Yee said. “Where do kids climb? Where do kids play? The things kids want are not in there.”

The things parents want aren’t either. While the city requires that two-bedroom units make up at least 40 percent of projects in the Eastern Neighborhoods Plan and the Market-Octavia area, there is no incentive for developers to add three-bedroom units.

According to the Planning Department, there have been 23,202 housing units built in San Francisco since 2005, and the vast majority were studios or one-bedroom units. Fewer than 10 percent had three or more bedrooms.

Of all the homes across San Francisco with three or more bedrooms, just 30 percent of them are occupied by families with children. Many of them are inhabited by unrelated roommates, couples whose kids are grown or single people.

Conversely, single-room-occupancy hotels — intended for low-income individuals — are increasingly occupied by families because they’re so much cheaper than market-rate apartments. The city has an estimated 699 families packing into one room with no private kitchen or bathroom.

Sue Exline, a senior planner for the city and the mother of three kids, thinks about these issues all the time and has ideas for addressing them.

“We as a city have to play a role in doing what we can do to both shine some light on the issue and think about potential solutions,” she said, noting she’s taking the Yee-commissioned report on “a road show” to discuss it with city supervisors, school district officials, developers and nonprofits. She’ll discuss it at the Planning Commission meeting Thursday.

“We are hoping this launches a bigger effort in our department and the whole city,” she said.

Her colleague in the Planning Department and fellow mom, Sheila Nickolopoulos, agreed, noting, “If you’re building a city for kids, you’re building a city for everyone. The things you want your kids to have make it better for everyone.”

Ideas mentioned in the report include having a definition of family housing so city officials and developers can speak the same language. Seattle and Vancouver, British Columbia, have definitions, and Yee said he will introduce a definition soon. He said his conversations with developers make it clear one is needed.

“They said, ‘I don’t know why you think we don’t know how to build housing for families with children,’” Yee said. “I said, ‘Well, you’re not doing it.’”

Other ideas discussed in the report include:

•Encouraging developments with flexible units that can be used as two separate studios or turned into larger, family-friendly spaces by removing the wall in between.

•Offering the city’s help to seniors who want to downsize from large single-family homes after their children are grown to free up space for young families.

•Encouraging developments to set aside a guest apartment so nobody needs their own guest room but visiting relatives have a place to stay. Also, encourage common storage space for bicycles, strollers and other items.

•Encourage more development of the “missing middle” — stacked flats, apartments built around courtyards and other medium-size apartment buildings that are bigger than single-family homes but much smaller than high-rises. Popular in previous decades, they’re rarely developed today.

Whatever the city comes up with, it’ll be too late for Jacob Garcia. He and his wife moved out of their one-bedroom apartment in the Inner Sunset 18 months ago when she became pregnant with their second child. Sharing their bedroom with one kid was workable, he said, but not two.

They were paying $2,600 for the tiny apartment, and two-bedroom units were all going for $4,000 or more — so they bought a three-bedroom town house in Walnut Creek for $600,000. Garcia, 33, and his wife both have high-paying jobs — he works in software and she in grant funding. But affording a home in the city in addition to child care and the potential of private school tuition if they fared poorly in the public school lottery was unthinkable.

“It’s just one of those things where it’s hard to imagine,” Garcia said. “If we can’t do it, who the hell possibly could?”

Increasingly, it seems, almost nobody.

San Francisco Chronicle columnist Heather Knight appears Tuesday and Friday. Email: hknight@sfchronicle.com Twitter: @hknightsf