Andrew Berman, Executive Director of the Greenwich Village Society for Historical Preservation, has posted a wonderful piece about back tenements on the GVSHP blog. There are a lot of these back houses throughout the LES, from the EV all the way to Chinatown. You can view a couple on the corner of 13th Street & Avenue B, where the corner building was cleared and replaced with a community garden, so you can look into the interior of the block, unobstructed. Right there in front of you you'll see a row of back tenements. It was common practice to build a front tenement about 50' deep (the depth of a row house or townhouse of the day) leaving about 50' of unused back yard. To maximize the rental space, the owner would build a second structure behind the front tenement.
Why the original owners didn't build one deeper structure, rather than two shallower ones, is, as Mr. Berman points out, the great mystery. If the back tenements were built later, there would be no mystery: the owner built the front structure on the current model of row house coverage, then as the market for housing grew, simply built a second structure rather than the more immediately expensive effort of demolish-and-build-larger. But, as Andrew also points out, many of these back tenements seem to have been built simultaneously with the front tenement. Why build two structures when one structure would have saved one stairwell and maximized the rental space? After all, tenements were built solely for rental profit.
Absent a memoir of an 1850 property owner, one can only speculate. Two considerations over the years have occurred to me: the shallow row house allows front-to-rear window ventilation; deeper structures would have required new and challenging design. As we know, when interior subdivisions without windows appeared, the city had to respond with the 1867 Tenement House Act specifically including a window in every room.
So it's my guess that owners built on the rational, shallow model that allowed rational window inclusion and ventilation. It was only when the housing market stepped up after the Civil War that owners viewed the extra stairwell as a serious economic liability, especially for single-lot owners who were stuck with only one space to maximize. Money is the father of invention: owners figured out a design to eliminate the second stairwell and the second entry and maximized the lot coverage for maximal rent.
So I view the back house as a holdover from row house design. That design made sense; it worked well enough; why change it -- why sacrifice rational ventilation just to save a little stairwell space? Only when the market upped the profit potential did the two-structure begin to look inadequate and the old methods were rethought. Until then, the back house had been a marriage of rationality and complacence.
But that's just one guess to file in the drawer of idle historical speculations on NYC architecture, social history and its interaction with the real estate economy and development practice. Mr. Berman speculates in his post, "Perhaps the
conventional expectation that these still-relatively rare (and generally
looked down upon) structures would at least look like a house pushed
builders to use this two-building form...". I suspect there's a lot of truth to that speculation as well. It's difficult to break social conventions of acceptability, whether in development or fashion or human behavior.