How do you determine an ARV with unique properties?

[Justin James]

Hey Biggerpockets! I've been a member for a while but this is my first post.

I'm currently looking at properties with mother-in-law apartments and I'm running into trouble getting an accurate ARV. A 2/1 with a 1/1 in-law does not seem the same as a 3/2 single family. I could see looking at comps for 3/2 single families if the in-law were attached, but what about detached suites?

Do you tend to find that a property with an in-law is worth less or more than a single family home with the same number of beds and baths? 

Aside from in-law suites, what are some strategies you use to comp other unique properties? Such as single families with workshops, larger than normal garages, bonus rooms, and other features that may not be included in any available comps?

Thanks in advance for any input!