You cannot simply use a mathematical formula as while one is simple, they will have to work with the engineers (civil, MEP) to coordinate exterior, entrances, grades etc.

We bought a 4 bedroom house with an unattached pool house in the back with a separate entrance through the back alley.

I was wound tighter than a violin walking into the courthouse; I'm surprised the officers at the entrance didn't frisk me!

All three units would probably still have stairs and landings for an entrance.

Don't look for properties, look to find the correct markets...and the properties will come like a Pezz dispenserRequirements for a Market to be good:1 - 4Sale Properties:  This is your Entrance/opportunity requirement. 

This comes to play when you want to add a second full kitchen in an area of your house that has a separate entrance, living space, and bathroom.

Is there already a separate entrance to the basement? 

We have an in law unit that’s 1 bedroom and 1 bath with a private entrance and its own garage and we listed in furnishedfinder for $1995 and got a ton of interest.

Details:2200 SF High Ranch(Mother Daughter)Top Floor: 3 Bed, 2 Bath, Living Room, Eat in Kitchen, Dining room (converted to 4th bedroom)Ground floor: 1 Bed, 1 Bath, Kitchen, Living RoomAdditional: Attached 1 1/2 car garage, .35 acre plot, 6 car driveway, separate entrances for each floor, gas heated, septic system, new roof, walking distance to major universityFinances:Purchased for: $420k (including $20k seller concession)Down payment: $14,700 - 3.5%Closing: $16,000 (after concession applied)Rehab: $3,000 initial, additional $5,000 planned for this year.Yearly taxes: $14,400Insurance: $140/moPMI: $280/moUtilities: ~$400Collecting $4,600 in rent each month, including all utilities.... while I’m living there!

My unit just needs to be one bed one bath with a private entrance hopefully.