Section 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.