Powelton Village has every reason to top the list of Philadelphia's most desirable neighborhoods. Let's start with location. As the first residential area west of Center City, it is a brisk 15-minute walk from downtown. It boasts some of the best transit connections in town, a rich stock of Italianate villas and Victorian twins, and postcard views of the skyline. Geographically, it occupies the same urban niche as Georgetown and Cambridge.

Yet no one would ever utter Powelton in the same breath as those tony enclaves. Powelton is not a desirable place today because it lacks one ingredient crucial to successful neighborhoods: homeowners.

For decades, Powelton stalwarts could blame Drexel University for turning the neighborhood's gracious blocks into a student ghetto. But Drexel has done a complete about-face in recent years. It has committed itself to building enough on-campus dorms to kill the market for rentals, and is providing subsidies for employees to purchase neighborhood homes. The only thing holding Powelton back now, it turns out, is City Hall.

While Drexel and a core of committed homeowners have formed an alliance to revive Powelton's 19th-century elegance, Philadelphia's Zoning Board of Adjustment seems bent on undermining their efforts. The board, whose members are appointed by the mayor, routinely allows Powelton landlords to convert their villas to rooming houses. Now it has gone a step further by approving the construction of a new, 18-bed student rental on Hamilton Street, smack in the middle of Powelton's best-kept and most historic block.

It's hard to fathom the legal logic that informed the Dec. 14 decision granting the developer, University Realty, a stunning package of seven variances. Single-family homes bookend the site, an empty lot at 3506 Hamilton. Since the current zoning permits twins on the block, the developer could have easily built two houses there without resorting to variances.

Looking at the drawings that University Realty submitted to the zoning board, you might indeed mistake the project for a pair of three-story rowhouses. The facade resembles a twin, but actually camouflages six three-bedroom apartments. University Realty is effectively building a small dorm for 18 people. Based on average rental rates, the company could gross $172,800 a year from the property.

Just the fact that it took seven variances to make the residence fit suggests the building is too big for the site. To squeeze the faux twin onto a 4,000-square-foot lot, University Realty persuaded the zoning board to let it occupy virtually every inch of land. Unlike the single-family houses and twins on Hamilton Street, University Realty's residence won't have to be set back behind a small front garden. The side walls of the interloper will be so close to the property lines that the neighbors will be able to touch it.

It wasn't only the neighbors who were dismayed by the zoning board's decision. Drexel submitted a letter urging the board to reject the project. As university President John Fry told me in an interview, "We're fighting to preserve these kinds of streets, not destroy them." He's concerned that the oversize building will "break up the rhythm" of Hamilton Street.

In neighborhoods such as Powelton, where the buildings are designed to conform to a code of behavior, a lone standout can have a devastating effect. The worst aspect of this faux twin, however, will be its use. Instead of being inhabited by owners, it will be filled with transient students.

It's not that students or renters are inherently bad for a neighborhood. Far from it. The problem is when their numbers swell to the point where the neighborhood's balance tips in favor of transients. That's happened in Powelton with the full complicity of the zoning board, and it's getting worse. Between 2000 and 2010, the neighborhood's percentage of owner-occupied homes dropped from 16 to 12 percent.

Once the zoning board approves a rooming house, all maintenance seems to cease. You can see the legacy all over Powelton - in the sagging front porches on once-stately homes, in the concrete slabs that have replaced genteel gardens tumbling with roses, in the rows of trash cans where hedges used to be. The blight is exacerbated by the inability of city inspectors to strictly enforce property-maintenance codes.

Two seemingly contradictory things occur: The bad maintenance drives out the good owners, and the huge rental profits drive up land values. The properties become too expensive for normal homeowners, and the neighborhood spirals down, becoming a student ghetto.

Powelton is hardly the only Philadelphia neighborhood that has been subjected to this sequence. The same thing is happening now near Temple and St. Joseph's Universities. The gorgeous neighborhood of Spruce Hill, near the University of Pennsylvania, was nearly ruined before the school adopted policies a decade ago to reverse the decline. Fry, incidentally, was Penn's vice president when that turnaround began.

What makes the ZBA's recent actions in Powelton especially disturbing is that they run counter to the extensive planning work there. The Powelton Village Civic Association just completed a master plan, overseen by Brown & Keener, that lays out strategies for increasing the percentage of owner-occupied housing. Drexel is about to release its own master plan recommending the same goal.

These serious, professional plans provide guidance for neighborhood development. But one has to ask, what policies and logic guide the zoning board?

One of Mayor Nutter's pledges after his first election was to professionalize the ZBA to ensure that its rulings are rooted in law. Many applicants assume they have a right to variances from the zoning code. They do not. The law demands that they show hardship to alter the legal zoning. Yet there appears to be no evidence that University Realty proved it had any such burden in Powelton.

It's not too late for the zoning board to save Powelton from further deterioration. After neighbors said they were denied the right to speak at the November hearing, the board agreed to reopen the case and accept written testimony. That's not exactly standard procedure, and it's not clear whether the variances approved Dec. 14 are still valid.

It's one thing for justice to be blind. But it's quite another for it to be divorced from public policy and the public good.

Contact architecture critic Inga Saffron at 215-854-2213, isaffron@phillynews.com, or @ingasaffron on Twitter.