At first town houses were refitted for multiple families, often expanding the fourth floor servants' quarters attic into a full floor for two apartments. As space became dearer, the houses were built higher with an additional backhouse behind the one facing the street. Why build backhouses rather than building even higher or building deeper into the lot? Either of those options would have saved the space of a second stairwell and a second backyard space.
The design of a town house allows natural light into the front room and the back room, each about twenty feet deep. If the building is built deeper to accommodate more rooms for more families, the interior room would have no light at all. Who would rent such a space, when there are other town house rooms elsewhere with natural light? A dark interior room would have no market, so the only benefit to a deeper structure would be deeper rooms, which would give more space to a renting family, but not more families to rent.
As long as the demand for naturally lit rooms did not exceed the supply, there was no reason to build deeper. It made more sense to build a back house with naturally lit rooms. That would allow more families to rent, more rents per lot.
When the labor population increased demand for local residences, the civility of natural light could be disregarded. It became marketable to build one structure deep into the lot, with only one stairwell and only one small backyard and with dark interior rooms for poorer families or borders.
|The white section represents the building footprint, the blue is the backyard space|
The ghetto law of a captive market:In the ghetto, as rents go up, amenities like space, light, fresh air and privacy go down -- the opposite of the free market where money chases amenities upward. This desperate exploitation of a captive market -- immigrants not only had to live near the workplace, but as immigrants were not welcomed in "respectable" neighborhoods -- eventually led to successive housing reforms, though not until after the four-day immigrant labor uprising known as the 1863 Draft Riots. Along with a political revolution led by the populist mayor Fernando Wood, Tammany Hall, relying on Catholic labor for its constituency, recognized that housing was the essential problem to be solved for New Yorkers. Plus ça change...
S/D=S/P=A (as supply decreases over demand and price, amenities lower).
The politics of housing requires its own post.