The new apartment’s habitable rooms require minimum natural ventilation and daylight. Hopefully, the basement is only a partially in ground basement (the upper half of the walls are above grade with windows to the outside).
Also, the bedroom (sleeping quarters) is required to have emergency egress directly to the exterior. Hopefully, there is a suitable full size door which directly goes to the exterior and cannot be blocked by a cover over a stairway up to ground level. Another alternative would be a properly sized emergency egress window (21″ wide and height minimum with 5.7 sq ft minimum opening are the usual requirements for an emergency egress window) and the window is not more than 36″ high off the floor (obviously not practical for a basement window).
Also, there should be a smoke detector installed.
If this ‘apartment’ is to be used by someone other than the main house tenant as a mother-in-law suite (unrelated person paying rent), then the ceiling and walls between the new apartment and the main living space need to be fire rated.
Was it permitted?
Does your zoning allow for it?
This list could go on and on, but I think you may beginning to realize that putting in an ‘apartment’ is much more involved than just putting it in.
If you rent this out and someone is injured or dies in that ‘apartment’, you could (probably WILL be) held liable. You could loose your house and anything and everything else of value you have or will ever have.