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A quick note about language here. One reason people get so confused, and perhaps heated, about accessory dwellings is that language about them is confused. The different names conjure up different images; who wouldn’t rather live in a “carriage house” than in a “ancillary unit”? The first one suggests the property has a heritage of horses; the second one a bureaucrat with a thesaurus. “Basement apartment” conjures up some creepy place, whereas “apartment over the garage” conjures up the Fonz!
There are at least a dozen names in circulation that are used for accessory dwelling unit. Here they are. I will add some links to usage examples as I have the time.
This Babel-in-a-teacup might be okay if all these terms referred to exactly the same thing. But unfortunately they don’t. The usage of some real estate agents and appraisers suggests a difference between in-law suites and accessory dwellings, with in-law suites being either internal to the building or less independent than standard accessory dwellings. And HUD makes a distinction between “accessory units” and “secondary units” that can have an effect on appraisals.
I recommend people use “accessory dwelling” or “accessory dwelling unit,” despite their somewhat bureaucratic sound. For better or worse, these are the terms used most by planners and inspectors and governments, who you may want to work with or influence. And these terms are general enough they incorporate all the architectural forms of this kind of dwelling, whereas something like “granny cottage” does not.
But please, whatever you’re doing in the field–building ADUs or protesting them–for the sake of sanity please don’t add another synonym. Thanks!