@Christian Mano:
This stuff gets really really tricky.It depends on the county, city, your neighbors, tenants, you etc. A lot of issues can happen with the illegal rentals. For example: when you get an insurance, you have to make sure that they have accounted for this. God forbids, if somethign happens and there is a fire, they insurance company can deny all the claims if they find out that someone was living in the basement.
Other issue is with the county, there are local laws on this stuff, some counties are more forgiving than others but if the tenants complain or the neighbors complain because you had an issue with them, you can get into trouble.
Right way to do is by getting a C of O. However, it can get expensive as your taxes will potentially go up, you will have to pay for applying the CO and it may take a long time to get it done.
Having said all of this, is it done? All day and everyday. Especially in NY. I see so many people doing this.