The dog jumped up on the toilet and bit into the window blinds, jumped on the toilet paper holder breaking it off the wall, and scratched up the paint on the door and frame trying to get out. 

Minimum approach: Put a kitchen in the basement and do some minor doors/walls shuffling to make it a duplex (con: utilities would have to be included as I can't separate them easily).

Nothing too crazy, I estimate $800 per wall.

Things to look out for are old windows, old wood siding, peeling paint on exteriors and interiors, and damage to window sills, door frames, and walls.  

I was skeptical of the issue because it was an older building and there was no indication of cracking in the walls or doors that were out of alignment and would not close.

I've got a few properties that would really benefit from updated heating/cooling systems. 1 duplex I have has electric baseboard heat / window AC (Old 1880s property, no crawlspace or basement, no existing ductwork), another I'd like to add central air too, and a 3rd that has old gas wall furnaces / window AC that I would replace with mini splits (again no ductwork). 

Also, I'd check A/C unit -central vs in wall - central will be more expensive to fix/replace vs in wall unit that can be swapped out easily as DYI.

Quote from @Jaron Walling: @Lyle M Smith Big win man!

Because laws here prevent us from collecting for some wall damage/scuffs after a certain length. 

Every investor I know has hit that wall.