2011 is an auspicious year for urban planning in New York City. This year is the 200th anniversary of the 1811 Commisioners' Plan, a bold vision that combined orthogonal regularity with architectural indeterminacy in the form of the Manhattan street grid. It is also the fiftieth anniversary of the 1961 Zoning Resolution, a comprehensive series of regulations and requirements that continues to guide the evolution of New York's built environment. These two documents differ in their external objects—the layout of streets in the former, the bulk of buildings in the latter—yet share the responsibility for determining the shape of New York. Even the most deterministic urban plans, however, can only enhance or inhibit what people do with space. They can't fully determine it.

Last week, New York's Mayor Bloomberg announced a new housing initiative as part of a revised PlaNYC, the City's strategic plan "to prepare the city for one million more residents, strengthen our economy, combat climate change, and enhance the quality of life for all New Yorkers." This initiative takes a frank look at the changing demographics of the city's population—specifically an increase in the numbers of elderly and single-person households—in order to explore "regulatory changes affecting existing housing stock that would enable the addition of a legal apartment to one- and two family homes, where appropriate." In effect, to meet the changing and expanding housing needs, the City is looking into how to get more housing out of its existing supply of buildings, which may mean smaller units to correspond with smaller or non-conventional households.

Whether you want to live green or you're just plain broke, the new environmental and economic imperative is to make do with less. In real estate, this usually means make do with less space. Sure, there are other ways to save money on property: expect fewer amenities or live in a less desirable neighborhood. But the amount of space still drives price, and still determines what we consider, in legal terms, to be habitable. With fewer and fewer people able to find homes conform to the legal standard of habitability, more and more are forced to live outside of it. In New York, the Tenement Law of 1901 and the Multiple Dwelling Law of 1929 set standards for space, light and air in an effort to protect the vulnerable against the squalid conditions typified by Jacob Riis' photographic crusade to document (and reform) New York's tenement housing in How the Other Half Lives (1890). If Jacob Riis were to photograph "the other half" today, the poverty he'd seek would be a lot less clustered, and probably found in unexpected places. He might find a group of impoverished immigrants in an illegal basement conversion on a nice tree-lined street in suburban Queens. He might find the new sweatshop conditions in space intended to house the back-office operations of a multinational company. In other words, he'd have a tougher time identifying and illuminating any uniformity, much less spatial agglomeration, to the conditions of extreme poverty. Poverty will always concentrate in certain areas. But that doesn't mean that illuminating the living conditions in areas of concentrated poverty is telling the whole story.

Real estate, according to Black's Law Dictionary is "land and the property permanently attached to it, such as buildings, houses, fences and trees." Of course, there are plenty of forms of property that you can take with you when you go. And if it's not real estate, it's called chattel. For those (like me) unfamiliar with the term, Black's is kind enough to give us a few examples of chattel: "Cars, bank accounts, wages, securities, a small business, furniture, insurance policies, jewelry, patents, pets and season baseball tickets are all examples of personal property."

Yet while the "real" in real estate refers to permanence and fixity, living in the contemporary metropolis is anything but sedentary. It's a migratory practice that often requires new forms of mobility and new ways of decoupling practices—like working or sleeping or cooking—from the spaces in which they are traditionally located—the office, apartment, or private kitchen. These new modes are often hidden from view. But a tide of entrepreneurial innovations, from ZipCar's ridesharing to Lulu's print-on-demand publishing to the variety of experiments in co-working, base their business models on our economic and societal shift from goods to services, from stocked inventories to nimble production lines, from owning to sharing.

The Internet, according to ZipCar's co-founder Robin Chase, is the ideal mechanism to share a scarce resource among many, dispersed people. Affordable housing in New York City is certainly scarce. But when we think of sharing in the context of the residential real estate market, the image conjured is more likely to be of a timeshare in Bermuda than a basement in Queens. Nonetheless, contemporary practices among itinerant, impoverished and often undocumented newcomers to New York have more in common than we might expect with the networked sharing platforms popular with people on the opposite side of the digital divide.

I don't own any real estate, but I have a fair bit of chattel. Book, clothes, a few pieces of furniture and some art: enough stuff that when I moved into my current apartment I decided to rent a van and could have used some help. But all of my friends were mysteriously unavailable that weekend. Enter a young Ecuadorean who lurks—when he has a day off from his back-of-house restaurant job in Manhattan—among a crowd of young men in front of a U-haul rental facility in Brooklyn, looking for a couple hours of moving work for people like me. We became fast friends, and like so many conversations in New York, our talk soon turned to real estate. When he learned how much I was to pay for my new apartment (which I thought was a pretty good deal, by the way) he thought I was getting ripped off. I asked him his strategy for finding a cheap place to live. "Simple," he said, "share." I soon found out, though, that he wasn't talking about roommates or a live-in lover. He was talking about paying for the use of the room he sleeps in for precisely the number of hours when he's there. An acquaintance of his who works the dayshift uses the room the other half of the day. They both seem to find this arrangement preferable to dividing up the room, bunking their beds or coming up with some other spatial solution to a problem they deemed solvable through scheduling and logistics. After all, "in this city," my new friend told me with a shrug, "work is 24 hours a day." But he did add that if both of them were in the space at the same time, their landlord wouldn't like it. These guys had opted into this arrangement. But it's not hard to imagine an unscrupulous landlord (not to mention a people-smuggler) introducing various forms of exploitation or neglecting basic safety in a system such as this.

On submarines, they call this "hot-bunking": assigning more than one soldier to the same bunk, to be used in rotation. The term has slipped into some analyses of undocumented immigrants and urban poverty, but its use is still not very widespread. The practice, however, may be on the rise. So too is the more visible practice of illegal subdivisions. According to a community development corporation that works on the housing needs of new immigrants in Queens, almost 40% of the new housing units created in New York between 1990 and 2005 were illegal. Some of these units are illegal because they are unsafe, others because they don't conform to zoning regulations (such as when two families live in a single family home), and others because three or more unrelated adults are sharing the same unit. I wonder how these statistics would change if we had a more accurate picture of the full range of ways people are sharing space, if the facts counted the numbers of people forced into illegal housing situations rather than the numbers of illegal units. When it comes to understanding the contemporary city, should we talking about space? Or about what people do with it?

Cassim Shepard is the editor of Urban Omnibus, an online publication of The Architectural League of New York, and a producer of non-fiction media about the design, planning and experience of cities, buildings and places. He has lectured at New York University; Parsons, the New School for Design; the National Institute of Design in Ahmedabad, India; and the Cities Programme at the London School of Economics. He has exhibited work at the Musee de la civilisation, Quebec; the Cineteca di Bologna; the Salone del Mobile, Milan; and the Venice Architecture Biennale 2006. He is currently an adjunct assistant professor of architecture at Columbia's Graduate School of Architecture, Planning and Preservation and a Poiesis Fellow at the Institute for Public Knowledge at New York University.