Here is another version of the smaller house movement: changes to regulations in recent years have led to more “accessory dwelling units” in Portland.

And additional living spaces are springing up everywhere, providing affordable housing without changing the feeling or texture of established neighborhoods the way high-rise developments can…

Eric Engstrom, a principal city planner, has seen these small structures become increasingly popular during his 16 years working for the city. And as he put it, “Given the low vacancy rate, when they’re done, you can rent them out in about an hour.” Which means that adding an accessory dwelling unit, or A.D.U., increases the value of a piece of property.

Since the 1990s, Mr. Engstrom said, zoning laws in Portland have been slowly changing to accommodate the buildings. “There’s been a lot of pressure on us to allow them,” he said.

But it was in 2010 when the biggest changes took place. That was when the city relaxed the limitations on size and began offering the equivalent of a cash incentive by waiving the hefty fees usually levied on new development. Other cities in the Northwest have been moving in this direction, but Portland is the first to offer a significant financial benefit and one of the few that does not require owners to live on the site, provide additional off-street parking or secure the approval of their neighbors — all of which have proved to be obstacles elsewhere. Apart from Santa Cruz, Calif., and Austin, Tex., where secondary dwellings have long been allowed, Portland is alone in this country in its aggressive advocacy of the units.

Seems like this approach could be a reasonable solution in many communities: allow small dwellings that can be used for multigenerational family space, generate a little extra income, provide more affordable housing opportunities, and/or expand the inhabitable space for the household. Yet, the article says little about why this has moved forward in Portland and a few other places but hasn’t caught on elsewhere. Is it seen more favorably in cities with limited space and relatively high real estate prices? Does it require more progressive politics?