Section 27-2078. Rental of rooms to boarders

a. A family may rent one or
  more living rooms in an apartment to not more than two boarders, roomers
  or lodgers, if  every  living  room  in  such  apartment  has  free  and
  unobstructed  access  to  each  required  exit  from  such  apartment as
  provided in paragraphs (a), (b) and (c) of subdivision four  of  section
  two  hundred  forty-eight or paragraph (a) of subdivision one of section
  fifty-three of the multiple dwelling law,  and  if  each  such  boarder,
  roomer or lodger has access to, and the right to use, at least one water
  closet, bath or shower and one washbasin as may be required in or for an
  apartment in this code.
    b.  Where  a  tenant  rents  any  part  of  an apartment in a multiple
  dwelling to more than two boarders,  roomers  or  lodgers,  such  rental
  shall  constitute  a  use of the apartment for single room occupancy and
  such rental in an apartment of a converted dwelling shall constitute  an
  unlawful use as a rooming unit.
    c. A family may rent one or more living rooms in a private dwelling to
  not  more  than  two  boarders,  roomers or lodgers, except as otherwise
  prohibited under the zoning resolution of the city of New York.