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In an age of widening inequality, in a city with ever-more expensive rent, the "poor door" has become an outsized symbol.

Technically, it doesn't exist yet. But earlier this month, New York City approved plans for a high-rise apartment in Manhattan that will include it: a separate entrance for the property's subsidized tenants. The luxury condo will otherwise have more than 150 market-rate apartments. But its 55 affordable units, offered by the developer Extell through the city's inclusionary housing program, will be separated with different amenities, different views of the city and, yes, a different front door.

Affordable housing advocates have winced at the image — they see low-income tenants slinking in through a service entrance — as has the city's new mayor, the editorial board of the Boston Globe, and much of Twitter. But the idea of economically segregated housing under the same roof raises some legitimately thorny issues around how and where we design affordable housing that are not so easily condensed into slogans.

Take, for instance, Portner Place, a complex of garden-style Section 8 apartments near the popular intersection of U and 14th streets in Northwest Washington, D.C. The land is slated for redevelopment into a roughly 350-unit mixed-income property that will include two wings: one for market-rate professionals eager to live near the U Street scene, and the other for Portner Place's existing residents, plus another 48 units of affordable housing meant for households making less than 60 percent of the area median income. The wings will have separate entrances, off separate streets.

Portner Place's current tenants requested this.

"We were presented with several development ideas for the new project," Wanda Simms, the vice president of the Portner Place Tenants Association wrote in a letter to the city supporting the development. "One idea would have scattered us among market rate professionals with different needs and wants than our majority family and elderly community."

Existing Portner Place tenants — who will relocate during construction and then return to the property — opted instead for a design that keeps long-time neighbors and families together with a front door off of quieter V Street. The separation also entails different amenities: a computer room instead of yoga studio, a playground instead of a club room.

In New York, the "poor door" has been criticized as a mechanism to spare market-rate tenants the "terrible awkwardness" of encountering their lower-income neighbors. Much less consideration has been given to whether lower-income tenants — often families living in cities more interested in building studios and one-bedrooms — want to share walls, common space and similar amenities with young nocturnal professionals. (Simms declined to speak further about the community's preferences and referred The Post to her letter).

"There are many times in which residents feel that if they’ve lived together for a long time in a very tight community, that in an income-mixing circumstance, their tight bonds get diluted," says Jonathan Rose, one of the developers of the Portner Place property who has also worked on affordable housing projects outside of D.C. "This is not the only case where we’ve seen that."

Washington's zoning code generally prohibits economic segregation in mixed-income housing. But in a case like Portner Place, the Zoning Commission may recognize legitimate reasons for separation, says Ellen McCarthy, the director of D.C.'s Office of Planning.

Under typical inclusionary zoning policies that exist throughout the country — and that were first modeled in Montgomery County in the 1970s — developers can earn density bonuses to build more housing than would normally be allowed in exchange for the promise of setting aside some units for affordable housing for a period of time. In the District, developers who want additional flexibility from zoning regulations can also earn it by offering public benefits, including affordable housing on-site.

The District generally requires that those units be spread around a building and identified ahead of time on building plans. Affordable units don't have to be located on the top floor — the point isn't to create subsidized penthouses, or to wrest from developers their most profitable real estate. But they can't be clustered on the bottom floor, either.

In theory, this ensures that affordable units, for which the public has paid a price granting the developer some concessions, aren't just wedged next to the laundry room or the back door. But the hope is that less tangible benefits will emerge, too, in the interactions among residents of different backgrounds and income levels.

"The idea of mixing income within buildings and within communities has been proven to be successful all over the country and in many different circumstances," says Bill Whitman, a principal at Somerset Development Company, which acquired the Portner Place property last year in partnership with Jonathan Rose Companies and the Portner Place Tenant Association. "So I think the policy default is generally in that direction."

Extensive research has pointed to the value of mixed-income neighborhoods, as well as the harm to low-income families of living amid concentrated poverty. When low-income families are segregated together, they often have access to worse schools, fewer jobs, fewer full-service grocery stores, less social capital, less political power, lower social mobility, more crime and more environmental hazards.

Move a lower-income family into an affordable unit in a higher-income neighborhood, and those other variables change, too. So might a parents' chances of coming into contact with neighbors who know of job opportunities, or good dentists, or reliable repairmen.

This is why cities like D.C. and New York want developers to include affordable units within a development, rather than pay for or construct affordable housing across town. The tradeoff is not without costs: Money spent on some affordable units in a market-rate apartment building might pay for housing — likely of lower quality — for a larger number of families on cheaper land elsewhere.

(Financing is an under-appreciated variable here, too: Extell has said that it had to create a separate door for the affordable units, which are contained in a legally separate building. Whitman says the two wings on Portner Place will make the project easier to finance with low-income housing tax credits, and have also made it more feasible to replace Portner Place with twice as many affordable housing units as currently exist there. To mix all of the housing into one building, he says, would be "a difficult sell to a lot of tax credit investors.")

Amid all of the benefits of income integration, though, the question of scale is less clear. We know mixed-income neighborhoods are good, as are mixed-income developments. But mixed-income wings and floors?

"Now that there’s been some more experience nationwide with inclusionary zoning programs," McCarthy says, "we’ll be looking to see whether people have found that proximity — being on the same hall — has had important impacts, or whether it’s been more that you’ve given people access to low-poverty neighborhoods."

It's possible, in other words, that families living behind the "poor door" will still get many of the benefits of inclusionary housing — access to better neighborhood amenities, jobs, schools, transportation — even if they don't share a lobby with their market-rate neighbors.

With a project like Portner Place, where a low-income community is moving in already intact, it's also possible that separation entails its own benefits.

"How important is the network that exists in high-poverty communities that helps compensate for their lack of other benefits?" McCarthy asks. "You have a network of people who will watch your children, who will loan you something. That’s an interesting question that Portner Place raises."