Converting single family to multi family dwelling

When changing the occupancy of a home, you typically need to start by (1) checking any Certificate of Occupancy on file for your building (which will more than likely need to be changed, or if there isn't one existing because of the age of the home, you'll need to file one), and (2) you need to check the permissible uses of the zoning district where your property is located. Then once you get past those two points, you need to consider any building requirements you have.

You'll have to investigate your zoning/building rules and regulations for any differences between boarding houses and homes for rent, but just by their nature, you can see that their differences most likely merit different rules. A boarding house is transient in nature, with different people staying for different limited amounts of time who don't typically set up residence, while a rental is usually for a term of years where the tenant establishes residence. There is also (at least in NYC where I live) a building type called Single-Room Occupancies (SROs), which are residences that include either a kitchen or a bathroom, but never both, and are occupied either on a temporary or permanent basis.

In any event, if you find that the zoning allows the use you want, there may be different building requirements you would then need to meet.

As you can tell, there's a lot to consider. The smartest thing you can do is talk to an engineer with knowledge of your area's zoning rules and building code, as well as consulting an attorney in your jurisdiction.

Forgetting about the possibility of a negative reaction from your neighbors, there's a lot of technical informaiton you need to know before you can make your decision. Good luck!