Once upon a time it was a stately Philadelphia townhouse with a high mansard roof, stained glass and a dramatic bay window. Today, the 19th-century building is a weed-choked wreck with bricks popping out of the facade. Upper windows hang slack-jawed, like a drunk who just passed out. Graffiti dances across a side wall. A family of possums has colonized the interior.

It has been like this, neighbors say, for a good 15 years, perhaps longer. They've called building inspectors, signed petitions and corresponded with city officials - with little results. Neighbors have even offered to buy the house from its owner, who currently owes $31,772 in back taxes.

Such tales of neglect and lax enforcement could be told about any number of blighted, vacant houses that litter the hardscrabble corners of Philadelphia. What distinguishes this one is that the property is located two blocks off Rittenhouse Square, at 18th and Delancey Streets, one of the city's toniest neighborhoods.

It's hard to imagine a crossroads more densely populated by people with clout. Arthur Makadon, a partner at the powerful law firm of Ballard Spahr and good friend of former Gov. Rendell, spent years averting his gaze from the ruin. Former Comcast CEO Stephen B. Burke lived up the street before being named head of NBC Universal - and putting his house on the market for $5.6 million. Philadelphia Orchestra President Allison Vulgamore is a few doors away.

As annoying as this crumbling house is, I doubt that its existence will generate sympathy for these wealthy folks. They're probably not losing sleep worrying about declining property values.

Rather, it is the rest of us who should be concerned that a Delancey Street mansion has been allowed to rot in full view for the better part of two decades. If Philadelphia's most influential people can't get City Hall to lay down the law about a blighted property in the best part of the city, who can?

I've passed the 18th Street house dozens of times over the last decade, always marveling that an unsafe shell propped up by wood planks could exist amid so much wealth. After speculator Richard C. Basciano's Market Street Hoagie City building collapsed last week during a botched demolition, killing six and injuring 13, the city's weak enforcement took on greater public meaning.

It's no secret that there are some 12,000 vacant buildings strewn around the city, a legacy of Philadelphia's huge postwar population loss. Even though the city is growing again, many will never again be inhabited. But how to explain the dozens, if not hundreds, of empty structures on vibrant blocks bubbling with people and new businesses?

Take the antebellum mercantile building at Second and Bank Streets, two doors from Jose Garces' Amada restaurant. It has been boarded up with plywood since 2008 and its elegant series of stone arches is steadily being reduced to dust. Or, the graffiti-scarred, art deco department store on Chestnut Street's 1100 block, flanked at one corner by the popular MilkBoy music venue, at the other by a fancy cupcake shop.

The inclination is to blame the Department of Licenses and Inspections for doing nothing. That is not the case at 325 S. 18th Street, which sits within the Rittenhouse-Fitler Historic District.

Since 2007, when the Nutter Administration began posting records online, L&I has dogged owner Teresa Isabella (aka Theresa Kennedy) and hit her with 21 code violations. City lawyers have taken her to court, for both safety violations and unpaid taxes.

City Hall has clearly not been idle. But for much of the last 15 years, it has been ineffective.

No city official would speak about the case. But it appears that each time Isabella is close to losing the property, she does just enough work to appease the authorities. In the last few weeks, the pace of work has picked up again, as a result of a court order issued last July.

The shredded black tarpaper that covered the bay window for years has now been replaced with a red plastic material, and the sidewall has been partially rebuilt. Historical Commission Director Jon Farnham said in an e-mail that the work complies with a year-old city-approved reconstruction plan.

Laura McMunigal, who has lived nearby for 30 years, has seen the pattern before. "She does a little work and then gets a two-year grace," she says. The project has proceeded in such fits and starts that a local teenager, who spent her entire life in view of the wreck, told neighbors she was writing her college essay about the experience.

In a telephone interview, Isabella, now 66, denied that she has neglected the house, which she has owned since 1973. "I've been redoing it," she told me, but has encountered a series of setbacks. Five years ago, she discovered a scam artist had transferred the deed to his name and had to go to court to reverse the action. Before that, she says she had to be treated for breast cancer.

Her lawyer, Nino Tinari, a prominent defense attorney, says Isabella now hopes to get a bank loan to convert the house - newly assessed at $841,000 - into four apartments.

Many Delancey Street neighbors say Isabella has already been given too many chances. The city should have foreclosed years ago, repaired the building and sold it to the highest bidder, argues Marty Rosenblum, an architect who specializes in preservation. "This has been like punching air. You just can't hit it," he says.

As for the rest of the 12,000 vacant buildings around the city, L&I spokesperson Maura Kennedy says the city has been working its way through inspections over the last 18 months. More than 51 percent of the owners have already made some improvements, says Kennedy. The city has collected $1 million in fines and 13 properties are in foreclosure.

Well, that's a start.

Contact Inga Saffron at 215-854-2213, ingasaffron@gmail.com or on Twitter @ingasaffron.