The language of this chapter shall be interpreted in accordance with the following rules of construction. Words used in the present tense shall include the future; the singular number shall include the plural, and the plural the singular; the word "building" shall include the word "structure"; and the word "used" shall include "arranged, designed, constructed, altered, converted, rented, leased" or "intended to be used." The word "may" is permissive, and the word "shall" is mandatory and not discretionary. All distances shall be measured horizontally, except height which shall be measured vertically.