Recently, I've moved into a new, larger house. The previous owner had rented out two thirds of the house, so there are redundant functions. Of course, because of this, we wanted to do the same. However, the local town government told(MA, USA) told us that since it's registered as a single family, we cannot rent it out. How can we change what the house is registered as?
I’d start with the Zoning Board. If your area is zoned single family, the former owners were operating against current ordinances.
If multifamily homes are permitted, I’d go to the building department for guidance on how to get the building approved. If I am wrong, they’ll send you to the right department. I suggest them as they are responsible for having inspected the work done to create the 3 family house in the first place.