Check with the county and make sure it was permitted.

There is a provision in all my contracts that permit me to assign the contract to a newly-formed entity that I control.

Insurance and LicensesThe Subcontractor agrees to maintain:General liability insurance of at least $[Insert Amount].Worker’s compensation insurance (if applicable).All necessary licenses and permits required by law.6.

Had an issue with trying to self-permit a build a few years back that was an HPR but had an "A" and "B" designation.

Because this clause wasn't in the lease agreement, they closed the case and told me to take it up civilly because they couldn't prove, beyond a reasonable doubt, that he wasn't permitted to take everything in the house.  

Duplex have different zoning that may permit additional units.

No lending institution or bank would be permitted to operate with those ratios.

My second house was under contract before it was even listed, but the combination of my GC disappearing and having to restart the permitting process for the carport in March right at the start of Corona lock downs dragged that out and eventually feel out of contract.

You cannot even apply for an STR permit on a single family home until after you've owned it for five years (and there is no guarantee you will get the permit).

Just make sure the town is ok with the land being zoned for multi-units, and look into construction costs, financing options, and permits ahead of time.