basement rental legal?

[Runa Laila]

Hello, I found a house, whose asking price is 535000. rental income currently is 3100/month. even if manage to put down 20%down payment, the monthly mortgage comes 3787/month. the advantage is it has a basement which can bring rental income of at least 900-1000. The question is it legal to rent out the basement or if not how to legalize it? I am in north eastern zone. Thanks