@Pete0I would ask a few realtors you trust for their thoughts. Would a m-i-l suite be an asset? would you have any comps? to what extent will it change your property taxes? does the HOA (or city or both) have any input? would your current ABB guests be impacted by construction?
my listing is a detached structure: it was existing (kind of) and heavily rehabbed. It has separate entrance, separate parking, separate utilities, kitchenette (this is very adequate for my weekenders), shower only, outdoor space, semi-private bedroom, extra twin beds in living room. (make sure you have some lockable "owner only" space/storage.) We have concrete floors, wood walls, ikea, flea market, handme down furniture with trendy accents. It has a very distinct personality. We built the space for ourselves and could happily never rent it again if that's what we decided. We recouped the construction costs after about 15 months of rent.
If you have 100 to spend, and add value to your overall home, and then recoup thru rental some of your investment then that's a good plan. Unless that 100 could have been returning more elsewhere. ??
I would build the small space to the view of could you ever see yourself transferring over there and make more renting the "big house"
And I'd for sure make sure I was diversified to multiple platforms if I was counting on a space being full-time rented. Seattle may not ever kick you out, but ABB might....