§ 172. Yards and courts. 1. Except as provided in subdivision two, there shall be a yard thirteen feet or more in depth across the entire lot in the rear of every converted dwelling, or an outer court three feet or more in width which alone or with any open space at the rear of the dwelling equals the required area of such a yard.

2. On a corner lot there shall be a yard ten feet or more in depth across the entire lot. Such yard shall be thirteen feet or more in depth for any part of the yard which is more than sixty feet from any side street bounding such lot. The provisions of this subdivision need not apply to a dwelling situated on a corner lot if (a) the required windows of all living rooms in such a dwelling open directly upon a street (b) the height and bulk of the dwelling are not increased and (c) the bulk of the dwelling above the first story is not increased.

3. The bottom of a yard may be as high as but not above the second tier of beams, provided the first story is used exclusively for business purposes and there is no living room in or below the first story; and provided also that whenever any means of egress from the upper stories opens into such yard, a portion of the yard ten feet or more in depth and open from the ground up shall be maintained at the rear of any extension of the dwelling.

4. In the rear of every converted dwelling on a corner lot an extension of the dwelling over the yard may be built up to the second tier of beams, provided such extension does not obstruct any required means of egress from the upper stories.

5. Except as provided in subdivisions three and four, the required yard or court and unoccupied space shall be open and unobstructed from the ground up.

6. The restrictions of this section and those of subdivisions one and two of section one hundred seventy-one in regard to bulk and volume shall not apply to extensions erected before April thirteenth, nineteen hundred forty, in conformity with the provision of a local zoning resolution which restricted bulk and volume.

7. Nothing in this section shall be deemed to permit extensions in the yards or courts of any converted dwelling, or to permit business or commercial uses therein, which may be contrary to any local zoning law, ordinance or regulation.