The city's Office of Property Assessment has been taking a harder look at real-estate tax exemptions for nonprofits and religious groups, Philadelphia nonprofits said.

The University of Pennsylvania, the city's biggest private employer and among its largest property owners, said it was working with the OPA on "audits of exemption status at a number of Penn-owned properties. These audits appear to be a new initiative by OPA."

Across Philadelphia, nonprofit sports clubs, religious organizations, human services groups, and housing providers have been put on the defensive by the tougher tactics, with at least a dozen appeals pending in Common Pleas Court.

In one resolved case, Chestnut Hill Friends Meeting agreed to register for a city business tax after the city tried to force it to pay property tax on 6 percent of the value of its new meetinghouse. In another case, a tennis club in Germantown had to settle for a 50 percent exemption instead of a full exemption.

The OPA's strategy, if applied uniformly, could have serious financial ramifications in a city where eight of the top 10 employers are nonprofits and where universities, health systems, and thousands of other nonprofits own $13.6 billion worth of exempt property, nearly 10 percent of the total, plus about $500 million worth that is taxed.

Michael Piper, who heads the OPA, said the city is making "decisions based on a robust interpretation of the law."

He acknowledged the possibility of discrepancies with older rulings but said it makes no sense "to address what happened that was incorrect in the past by perpetuating incorrect policy."

That scrutiny has surprised lawyers.

"There should be some effort by the OPA or the city to alert nonprofits of the more rigorous application, especially with property values going up," said Carl Primavera, a lawyer with Klehr Harrison Harvey Branzburg LLC, who is representing the Tenth Presbyterian Church in a dispute over whether dorm rooms for seminarians in a building at 1716 Spruce St. should be exempt.

The current push to get nonprofits to pay more picks up on efforts dating at least to the 1990s, when Mayor Ed Rendell established a short-lived program to collect payments in lieu of taxes from nonprofits.

Two years ago, an OPA audit of nonprofit exemptions fizzled amid protests led by religious leaders.

The need to review historical exemptions has not gone away, Piper said. Asked whether there had been any discussions of such an effort, Piper said: "That's a touchy subject."

Qualifying as a charity under section 501(c)(3) of the federal tax code is not enough to receive a local property-tax exemption. The group must also pass a tougher, five-prong test established by a 1985 Pennsylvania Supreme Court decision.

The organization must advance a charitable purpose, donate or render gratuitously a substantial portion of its services, benefit a substantial and indefinite class of people who are legitimate subjects of charity, relieve the government of some of its burden, and operate entirely free from the private profit motive.

It is not enough for the charity to own the property. It must also use the property for its charitable purpose.

A strict application of the five-part test could have a deleterious effect on the city's vast nonprofit sector. "There are not many places that are going to meet it, but that doesn't mean they are not engaged in a nonprofit activity," said Robert L. Archie Jr., of Duane Morris LLP.

But the vagueness of the criteria has given courts and taxing authorities a huge amount of leeway in evaluating nonprofits.

Piper estimated that 75 percent of the exemption applications sail through, but the OPA did not provide a record of how many of the exemption applications it received and denied in recent years. Property owners can appeal to the Board of Revision of Taxes. Appeals of BRT decisions, by the city or the property owner, go to court.

Although in some cases, the city has argued that nonprofits failed all five parts of the test, in others it has focused on one issue, such as whether the organization provided enough free services, relieved the government of a burden, or was using a portion of the building for a non-exempt purpose.

In the case of Penn, which says it pays more than $2.2 million in property taxes on space leased to businesses, it's not clear what the OPA is looking at. "We do not expect any significant changes in the tax status of these properties as a result of the audits," Penn said.

Some small nonprofits, many of which scrape by, might not be so lucky.

The city rejected the bid out of hand, saying in a court filing that the club "fails to meet any of the five prongs ... yet alone meet all of them."

The judge sent the case back to the BRT "to evaluate the club's argument that all or a portion of the club's mission is to serve to encourage low-income children to engage in the sport of tennis and mentoring of those children."

Last fall, the BRT granted a 50 percent exemption and the city did not challenge that, after the club went to another BRT hearing and showed photographs of its youth programs, club president George Guy said. The club's parcels are valued at $323,700 on the OPA website.

• InterCommunity Action, a Roxborough nonprofit founded in 1969, has historically received property-tax exemptions at its community homes for people with intellectual disabilities.

But in 2015, OPA denied the exemption for two new sites, saying they did not meet the requirements because they do not relieve the government of a burden -- even though the group has 18 other facilities with the same state license and the same funding that do have the exemption.

After losing its appeal to the BRT, InterCommunity Action appealed to Common Pleas Court. The case is in the early stages. InterCommunity Action officials declined to comment, citing the litigation.

• At issue for Oxford Village, a student housing complex built in 2003-04 by Beech InterPlex Inc. to help Temple University with a housing shortage at the time: Does it provide enough of its services for free? The property had a construction abatement until 2014.

At a BRT hearing on April 4, Beech's chief executive told the panel that about a quarter of the roughly 400 students living there receive a need-based discount, averaging about $100.

"Temple doesn't pay taxes on its dorms. Why should Beech?" asked Archie, the Duane Morris lawyer, who represented the nonprofit at the hearing.

OPA attorney Drew Aldinger argued that Oxford Village charges market rates for rent and should not get the exemption. The BRT decision is pending.

• Housing for seminarians is the main point of contention at 1716 Spruce St., a property owned by Tenth Presbyterian Church, at 17th and Spruce Streets.

Last month, the BRT, convinced that the seminarian housing did not qualify, granted a 30 percent exemption on the property valued at $1.8 million. Tenth Presbyterian appealed to court.

The seminarians stay there while they engage in missionary work on local college campuses, said Primavera, Tenth Presbyterian's lawyer. "They need to have this exposure to get ordained. If they don't get ordained, then, ultimately, the church runs out of ministers," he said.

More broadly, Primavera said it was disconcerting that a church, where there is no doubt that it is a legitimate nonprofit, would get such a rigorous look. "The OPA sent an inspector out, and it really was like the Spanish Inquisition. It was very draconian," Primavera said.

The inspector went deep into details at 1716 Spruce St., even asking about dust on a floor because it might indicate that the space is not in use and therefore taxable.

City officials declined to comment on the nonprofit tax disputes in court.

Primavera supposed that one of the cases in the city will go on appeal to Commonwealth Court, which could issue guidance that would help nonprofits avoid surprises.

There has to be an equal standard, he said.