No matter how much one discloses, why provide what could be construed later as a misleading reporting? RE agents of course don't have this problem, but we do.
The best way to measure market reaction is with similar comps with sep guest houses or guest cottages.
House A has 2000 sf under one roof, House B has 1400 sf in main house and a 600 sf detached guest house . RE agents or assessors might treat both houses as having 2000 sf, however, buyers are not stupid. They can see perfectly well the differences between the two.
Are buyers paying, more, less, or equivalent for the functional and appeal differences between the two houses? The Sf if laid out different, so it's not just comparing sf $ 4$ to sf, and the two styles may appeal to different buyers.
Marketability is also an issue...are dwellings with detached guest houses selling in the same marketing time, or faster, or slower? In my region, there are areas where they are popular and areas where they are not , so it's not all about price but also if they have superior, inferior or equivalent market appeal in a specific area.