Splitting a room into two

[Mina G.]

Hello, I currently have an occupied apartment that is a 2 bedroom with a huge living room. on the 2nd floor where I live it's the same setup but instead of the huge living room it's a regular living room and a bedroom about 8.5" by 12.5".

Thinking ahead... When/if the tenant move out can I put up a wall to split the living room making a 3rd bedroom? there is already a window in place and heating radiator (same setup as the apartment above just no wall)!

Is this legal? and will I be able to rent it out as a 3br?