Good point.
So back to OP - it seems like a H&BU analysis would be in order for the Fern. The first two tests are interchangeable - namely the 'legally permissible' nature of possible uses and the 'physically possible' nature of possible uses. As regards the legal portion - again, I think CA has relaxed the laws about not being able to rent ADU's, in which case, both units could be rented (if, in fact, there are two units). So then, according to the legal test - SF with ADU AND duplex would be possible 'legal' uses. As to the physically possible test - obviously it's physically possible if two units are already on the site. So, then, based on the first two tests (for CA - and based on my limited knowledge of the CA ADU laws) both duplex and SF would be legally permissible and physically possible. The next step (for me) would be to ascertain what the market participants are doing with such properties. If the market recognizes SF owner occupied with guest quarters as a more desirable use, then I'd lean that direction. If the market recognizes both units as rentable units, then I'd lean that direction. The obvious question is: how do you know what the market prefers? How are similar properties marketed and sold in MLS? What do discussions with investors elicit?