Converting a 2 family into a 3-4 family in Yonkers

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Looking for guidance or to be pointed in the right direction to figure out how hard of a task is it to convert a 2 family legally into a 3-4 family in Yonkers, NY.

So I found a property that is legally a two family home. And the stairs lead up to the attic which is not finished but has windows and a toilet right in the middle of the attic (lol) and a sink near one of the walls. I am not aware of the laws and I’m really starting in my investment journey. So I don’t know where I would begin to ask these questions. Maybe someone can refer me to a reputable contractor or someone knows whether this is a trap cause it’s not doable but here is my thought process:

2 family house with a finished basement, accessible from the outside (don't know why this isn't considered a separate apartment) and from the looks of it each apartment is accessible through stairs which is a common area, which leads to the attic. Adding a whole other unit should raise the value of this house tremendously if I've been understanding what I been reading on the forums and from the books I have read. So attempting to make this my first investment in my BRRRR Strategy and the fact that I can apply for a 301k loan if what I mentioned is accurate then this might be a great opportunity for me. Thoughts and opinions...?