How much is underpinning for a new foundation? HahahahaHaha. You mean a third storey patio door to nowhere is a bad idea? No provision left for a deck. You could bolt up juliet balcony railings or more likely permanently pin the doors so they only open less than 4".
This is a huge stinker. I expect major issues were buried. Imo, it's worth much less than the base house they started with as it will cost more to get this caught up than to start from scratch.
How much to rip off the new floor and resurrect the original design? Ouch. then it's only a three-plex so only 75% of the future income.
R&R drywall and hopefully acceptable work underneath. They have a good record of following the rules so far...NOT.
The "Hey, I got away with it" attitude sometimes gets you when it's the most expensive.
Lot value??