The district boundaries are either lot lines, the center line of streets or alleys, or the extension thereof, and where the districts designated on the Official Zoning Map which is a part of this chapter are approximately bounded by lot lines, streets or alleys, the lot lines or center line of such streets or alleys or the extension thereof shall be considered to be the district boundaries. Where the district boundaries are not shown as being either lot lines, streets or alleys or the extension thereof, unless otherwise indicated, they shall be construed to be parallel to and 120 feet from the nearest street, provided that if a lot line or an alley shall hereafter be platted and recorded paralleling within 30 feet of such district boundary, such lot line or center line of such alley shall thereupon be construed as the district boundary. In such cases, where a district boundary line divides a lot in single ownership, and where the majority of such lot is in the district requiring the lesser restrictions, these restrictions shall prevail for the entire lot, provided the area of the land located in the district with the greater restrictions does not exceed the open area requirement of said district for the erection of the proposed building.