Here's the problem w/ancillary dwellings: Airbnb, for one. The other? Longer term renters/students (in my neighborhood)@ the U of O. A person purchases a nice home with a nice garden in which they invest lots of sweat equity and greenery. Nothing sweeter than to sit in that backyard, enjoy the birdsong. Then, your neighbors, who also had peaceful spaces, decided to build a "mother-in-law cottage" that never was meant to house the same. If both your neighbors do that, your formerly peaceful garden now has many more opportunities for disruptive noise, including the parties and music choices of the younger adults. 
I'd suggest if a person needs the income from a rental to be able to afford a home, let them not purchase it in the first place. Ruining garden after garden using the excuse of planning is bogus and destructive. There are plenty of areas where multiple housing is allowed and where common green spaces provide needed quiet. Continuing to destroy neighborhoods is crazy.