No front yards or side yards are required in Special Mixed Use Districts. However, if any open area extending along a side lot line is provided at any level, such open area shall have a minimum width of eight feet; except, if the mixed use building contains no more than two dwelling units, the open area extending along a side lot line may be less than eight feet in width at the level of the dwelling unit. For a residential portion of a mixed use building, the required rear yard shall be provided at the floor level of the lowest story containing dwelling units where any window of such dwelling unit faces onto such rear yard.