The rise of "granny flats"

• "Accessory dwelling units," or ADUs, are small and typically built in or near existing homes, so they don't face as much resistance from neighbors — and may get people more comfortable with adding density.

"A lot of homeowners can see themselves wanting to build an ADU, whether it's for a family member to stay in or for guests or a short-term rental, or they may see it as a tool to make the mortgage more affordable," said Emily Hamilton, research fellow and director of the Urbanity Project at George Mason University's Mercatus Center.

What's happening: Seattle, Portland, Minneapolis, Los Angeles, Austin, San Francisco and Washington, D.C., have made it easier to get permits for granny flats.

• While California and Oregon have OK'd upzoning, other statewide proposals face steeper climbs. A Virginia bill to allow one ADU per single-family dwelling was tabled last month and won't be considered again until next year.

The other side: Critics say these units allow development too close to property lines and reduce street parking.

• It's also expensive to convert a basement or garage attic into space that meets building code requirements, or squeeze a detached structure in the back yard.

What they're saying: Jenny Schuetz, a fellow with Brookings' Metropolitan Policy Program, said, "gentle upzoning" — allowing for ADUs, duplexes and triplexes in neighborhoods currently zoned only for single-family housing — is only part of the solution.

"ADUs are like the scooters of housing policy — they're flashy and cute and everyone loves them. Scooters are good at getting people thinking about other ways to get around, but inherently they don't scale. Even if we put an ADU on every single-family lot the U.S. — which of course won't happen — we wouldn't solve the problem."