My husband wants his parents to live in our backyard.

Well, not in the yard itself, which is presently in an appalling state of post-winter disarray featuring an embarrassing quantity of dog poop. His actual designs are for the space occupied by our two-car garage, which, in the style of most historic homes here in the Twin Cities, is located at the back of the lot for alley access. His idea — which I support — is to use that space to build upwards, replacing the garage with a two-story unit with parking on one level and a granny flat on the other. Even if his parents never moved in, the structure would be a safe long-term investment we could rent via Airbnb or, more likely, to less transitory tenants.

Luckily for us, this plan is legal where we live. That has not always been the case, nor is it the case in lots of American cities. Accessory dwelling units (ADUs), as they're called in urban planning parlance, are prohibited in many localities. The legal tide is starting to shift, but it should be shifting faster. ADUs are an obvious answer to America's increasingly urgent housing needs, and they ought to be an easy sell across the political spectrum.

In my own neighborhood, ADUs were legalized less than a year ago. A very limited pilot program permitted construction of granny flats, carriage houses, mother-in-law apartments, and the like in a few areas of St. Paul starting in 2016 (some older properties were previously grandfathered in), but legalization only went citywide this past October. "I've heard from many of my constituents who are really excited about the opportunity ... for affordable housing, to create additional density, and providing housing for seniors to age in place," said one city council member who supported new ordinance.

Her comment summarizes the case for legal ADUs for urbanists and YIMBYs ("yes in my backyard") on the left: It's a grassroots option for creating affordable housing that increases density without destroying the historic character and community of established neighborhoods. ADUs can help mitigate homelessness and accommodate urban population growth almost invisibly, especially when new units are added by adapting existing structures instead of constructing from scratch.

Though ADUs can be expensive — we estimate our garage apartment project would cost slightly more than what we paid for our house, though less than its present value — for homeowners who can afford to build, it's the ultimate in buying local, keeping investment dollars very close to home. ADUs also increase property values and thus property taxes, boosting local school budgets, and this sort of smaller structure is an environmentally friendly development option compared to larger new builds. Added density can prevent displacement in newly popular neighborhoods, and ADUs can make more expensive neighborhoods accessible to young adults, the elderly, and other populations who can't swing (and don't need) an entire single-family home in those areas.

For Americans who lean right, the case for ADUs is even simpler: If you want to build a granny flat on your own land, the government should not be able to stop you. For all the communal benefits ADUs have to offer, their legality is also a matter of individual liberty and the basic privileges that ought to come with property rights in America.

Moreover, the rising interest in ADUs around the country is the market in action: Americans have noticed there's a housing need to be filled, and they're eager to fill it — profitably — if only local bureaucrats will let them. ADUs are a "bottom-up, decentralized, incremental, scalable, and adaptable" niche in the housing market, and the state should stop erecting obstacles to their construction and onerous limits on how they can be used.

I'm sold on both cases. I want to be able to use my property as I like and to seize what I think could be a worthwhile investment opportunity in a city where housing prices seem to climb daily. But I also want to help slow that climb, to make a quite literally small contribution to affordable housing in our neighborhood.

The added density ADUs encourage is also a big part of their appeal for me: I want more neighbors. They'll enrich our community culture, supporting local businesses and encouraging walkability. Their presence alone provides what urbanist Jane Jacobs called "eyes on the street," which is the idea that cities are safer and more peaceful when there are lots of neighbors around. Empty city neighborhoods can be a magnet for crime; bustling ones usually aren’t.

Though I am a committed St. Paul partisan — as all Twin Cities residents should be — I must concede Minneapolis is strides ahead of us where this sort of housing policy is concerned. Still, last year's ADU legalization was a major point of progress, and the sort of progress cities across the U.S. ought to imitate. Whether justified on grounds of affordable housing, personal freedom, or both, legalizing granny flats is a cause that should unite America.