Expected use is the key, if you  have 5 acres or even 10 and that area is generally single family dwellings on similar parcels, it can then be  considered for residential development. 

I have really no experience in real estate, but I have envisioned myself getting into smaller buy and holds (single families or small dwellings). 

House hacking financials get significantly stronger as you increase the bed count above 4, or add a ADU or separate dwelling unit.

Depending on the Company for the current Homeowners policy you probably will have to rewrite the coverage to a Dwelling/Fire (aka Landlord) policy. 

We weren't registered with hpd (private dwelling). 

Your policy may say that any coverage is excluded for and dwelling or addition that does not meet code requirements.

Don't spend too much time dwelling on the "unknown" and prepare for the what-if's.  

Besides this option, are you also open for a house hack (finish of a basement, Accessory dwell unit, mother-in-law, .....)If you are open for the two options, you will have more houses to choose from.Always open to more conversation over phone or coffee if that's helpful!

One of them is a multi family dwelling where by I have 5 tenants on one property.

It is a legitimate dwelling as its metered separately and has its own address.