In-Laws aparement rentals legal?

[Matthew Mello]

Currently throwing in offers on a house south of Boston. The agent informed me that the town assessors have it as a single but the building department has it listed as a 2 family so I just called to verify before putting in the offer. They were wrong both have it as a single. 

So my main question is can I legally rent out the in-law, I'm hearing mixed things?

Now my next thought is just gone and make it a legal two-family by going for the variance. Everything seems to be to code, utilities are even separate, just shared septic (which is what scares me) I believe every unit should have its own in order to make it a legal 2.

Any and all input is welcome. Thanks in advance.