The most logical was if the owner rents out both units and one tenant locks out the other from a common bathroom it would be trouble.

Some dont include bathrooms.

As far as the successful layout of the property, no more than 3 people to a bathroom.

Look for homes with more than usual bathrooms.

It is a 2000sqft property with 3 bedrooms, 4 bathrooms.

I can’t tell you the exact price per sf, but generally, I’d pay about 10-20k for just drywall, carpet, doors etc in an unfinished basement, if you have any minor  foundation issues/leak that’s probably another 5-10k adding a bathroom is probably in the 8-10k range maybe a little more if you have no drain stack in the basement, digging down can easily be 50-100k 

Can adding a bedroom and bathroom without increasing the square footage of the home still increase its value?

New bathrooms- Bath tub tile surround.

New electrical, plumbing, drywall, flooring, bathrooms, kitchen siding, etc... 70% of that place was redone.

Tangibly, this could be a detached or attached space converted, or maybe you find a master down floorplan with other bedrooms upstairs (and that master has a door to the outside and is large enough to add a kitchenette tapping into water and drain from bathroom or kitchen).