St. Urbain is the secondary frontage, and since this street has more vehicular traffic, it might be a good place for vehicular access.  The prominence of this side may, however, make an underground parking entrance on this façade inappropriate.  Perhaps a passenger drop-off point could be incorporated on the St. Urbain side. 

Clark Street would be the ‘back’ street for the building, which may be the right place for the entrance to parking, but may be problematic because residents on the street would probably object to added traffic near their houses.  The location of a parking lot entrance will be tricky.  

Service access should probably be handled using the existing dead-end alley on the south side of the side.  Large delivery trucks would not be able to use Clark Street, making St. Urbain their best point of access.