The size and slope of the building sewer shall be subject to the approval of the Loudoun County Sanitation Authority, but in no case shall the diameter be less than four inches for single-family dwelling units or less than six inches for multifamily dwelling units. The slope of the four-inch diameter pipe shall be not less than one-fourth of an inch per foot (two percent) and the slope of the six-inch diameter pipe shall be not less than one-eighth of an inch per foot (one percent).