So, for instance, you could have someone sleep on the sofa in the living room, of a one-bedroom apartment, but you could say only 2 max in a studio (with no separate bedroom).  

Options for 18" are too limited and they are the same price if not more expensive.I agree it may be overkill for a small studio like this, but it also seems expected (or at least an advantage) for the type of tenant I'm going for.Thanks for the input on STR Dan, I will look more into this for San Diego and my neighborhood.

Her intention is to live in half of it and run a tattoo and piercing studio out of the storefront.  

And more granular, small studio and one-bedroom Class C properties drop the most in price.

I rented mostly really tiny studios, so there were few couples, but the ones I had apply after the above example all had decent credit or no credit - I had a lot of immigrant techies apply and I didn't worry if they had zero credit.

I managed a 26 unit building with mostly small studios and it was shaped like a "U" with apartments on the outside and inside of the "U". 

don't mess with me or else I'll go medieval on you & turn your studio cell into a 2 br. 

Even though I was very friendly, I was also very firm on the building policies, which was really important in a 26 unit building shaped like a "U" made up of mainly tiny studios, where many of them could hear everything that was going on in the other units. 

Put it wall to wall throughout the entire unit, except maybe the bathroom.I used to manage an old apartment building that was bare bones - a good price in a good neighborhood, clean, well-maintained (great manager ha ha), but mostly really tiny studios, no washer/dryers even in the building at all, showers only, no disposal - you get the picture. 

Then for Covid-19 isolation fun my wife & I just finished building a studio apartment from scratch & the 5x9 bathroom was <$1000 total (4x3 shower stall base, no tub).