[mhays]

Stacked flats and alleys make the three-unit concept way easier. But Seattle (not sure about Minneapolis) doesn't allow three-flats in SFR areas, and some neighborhoods don't have alleys.

The concept is really about adding a cottage or two in the back yard, aided by a typical width that's often more like 50' but varies quite a bit. If there's flexibility to build parking or not, more people will dive in and new units will be much cheaper.

Minneapolis appears to have alleys in most areas but they're not universal.

BTW, a glance at some Chicago examples shows a lot of those 25'-wides have two parking spaces and a walkway.