Looking for a house hack project and found a "duplex" that used to be a fourplex. There are 2 - 2bd 2ba units upstairs mirroring one another. Downstairs these 2/2 each have their own 2 car garage. But in the back there used to be 2 1bd 1ba apartments that haven't been lived in for a long time and need gutted and put back together. The reason it is not a fourplex to my knowledge is the septic system it is on is only big enough to support the 2-2/2 upstairs. So, the questions is... If the only thing stopping you from this was all the risk of the unknown what would you do next? My list - get a septic upgrade quote, get a renovation quote, negotiate with seller, find the money... what am I missing?
I'd start by calculating the value of renting it as a four unit vs a duplex. Keep in mind that being a 4 unit may diminish the rents in the existing units.
Then you need to guestimate how much it would cost to upgrade the septic and rebuild the other units (probably similar cost to new construction for the square footage)
Once you have some numbers, you can decide if you want to pursue it further.