3-Family Common Area

[John Messier]

Hi, I'm in process of purchasing a house with 3-tenants, each on a separate floor.  Each tenant has access to the basement through a common interior stair case.  The unfinished part of the basement includes a washer/dryer and storage shelves for each tenant.   The basement also includes a fairly large finished area - 2 rooms and a bathroom.  Currently, this area isn't locked.  The finshed area cannot be made into a separate apartment because it doesn't have the required windows / egress for a bedroom I was told. 

So I'm hoping for ideas as to what to do with the basement.  It's accessible from outide the home through a first-floor door (which has a lock available to all tenants) to the common stairway.

Some questions.   I assume allowing tenants access to the common basement is OK and standard practice?   Also, should I consider renting the finished basement as an office to a non-resident of the house and add a lock to the door?   What are the requirements for an office?  [I couldn't find any issues with one window, one door, and two means of egress from the basement.)   Should I look at renting the finished area to a resident tenant for additional space (as an office or exercise room).   What would you do?   THANK YOU