Current zoning laws in many cities don't allow for accessory dwelling units or A.D.U.s. Portland is a rarity as it grappled with an affordable housing and homeless crisis that forced it to look at more options for housing in minimally existing urban space.
Portland began allowing ADUs in 2010 on 5,000-square-foot residential lots and cut development costs by $15,000 to encourage more homeowners to build. The number of ADUs increased from about 35 in 2010 to 250 last year.
Now, much of the Bay Area, exlcuding Palo Alto, and British Colombia have adopted ADU permits. Some argue it's a great way to solve a housing crisis in a pinch, offering rental income to homeowners to help make ends meet and providing more housing. Others argue it's inefficient.
From The New York Times:
Richard Potestio, an architect in Portland who is interested in multi-unit housing, said he believed that larger, more efficient and less expensive accessory dwelling units could be incorporated into new multi-unit buildings.
If a condominium, like a single-family home, could have an accessory unit, he said, a 10-unit condominium building could have 20 dwellings. And while multi-unit buildings are allowed in commercial zones, Mr. Potestio said that similar rules for residential lots would help the environment. “When A.D.U.s are incorporated with duplexes that share common walls, less open permeable space is lost,” he said.