Zoning is probably the difference. In Seattle, ADU/DADU are built in Single Family House zone, you can tear down a single family house, build 1 main + 1 ADU +

Zoning is probably the difference. In Seattle, ADU/DADU are built in Single Family House zone, you can tear down a single family house, build 1 main + 1 ADU + 1 DADU which is total of 3 without zoning change, then you can sell all 3 of them as Condos, require buyers share something like water bill. Townhouse has to be build on the areas that are up zoned by the city.