In any commercial district the requirements for lot area, lot width, side yards and rear yards, as specified for dwellings in Schedule A, shall apply to any commercial building in which one or more dwelling units are also located. In lieu of the requirement for such buildings, there may be provided on the lot at ground level free, accessible and usable open space of at least 400 square feet for each dwelling. Said open space shall not be used for storage, automobile parking, accessory buildings or other uses, but shall be available and usable for outdoor recreational use and for household activities which are normally carried on outdoors.