Sitting in its Pittsburgh session, the Pennsylvania Supreme Court will hear oral arguments next week in a case that is at the core of how Allegheny County -- and many others -- determines real estate property values.

In 2005, county Chief Executive Dan Onorato implemented a base-year property assessment plan, which sets assessments based on the value of property in 2002, including new construction. Common Pleas Judge R. Stanton Wettick Jr. threw out that system last year, declaring the state law that allows a base-year system is unconstitutional.

At a news conference yesterday, Mr. Onorato said the county will argue that Judge Wettick not only was wrong in his interpretation of Allegheny County's base-year plan but also overstepped his authority in finding unconstitutional a state law that is the basis for similar plans in counties across the state. The court will hear arguments on the case on Wednesday.

"We think [Judge Wettick] is wrong," Mr. Onorato said, adding that he will keep fighting for his base-year plan because he made a campaign promise to deal with "the reassessment problem" when he sought the chief executive's office in 2003.

The long-awaited assessment case -- Clifton et al, and Pierce et al v. Allegheny County -- hinges on a law that was created by the state Legislature and approved by then Gov. Dick Thornburgh in 1982, giving counties the authority to use a base year when setting property values.

In Allegheny County, the issue of assessments has been a lightning rod for many years, with property owners complaining of constant and excessive reassessments of their properties. Mr. Onorato said he sought to stabilize what amounted to a continuous "back-door property tax increase on homeowners" every time the county did a reassessment because values increase but school districts and municipalities never rolled back tax rates.

Furthermore, he argued, about 20 counties in Pennsylvania use a base-year market value system to establish property tax rates, causing chaos if Judge Wettick's ruling is upheld.

"The first thing I will do [if the Supreme Court upholds Judge Wettick's decision] is go to Harrisburg and ask them for a new law," he said. "Allegheny County is not going to do a reassessment any time soon."

He also will continue the practice of appealing to lower the property values of homeowners whose property value is raised by school districts and municipalities based on the sale price, he said.

In setting 2002 as a base year for property valuations in the county, Mr. Onorato set aside the results of a 2005 reassessment, which would have seen property value increases averaging nearly 20 percent for the county's 550,000 homeowners.

A group of homeowners challenged Mr. Onorato's base-year plan in Common Pleas Court, arguing that he had essentially implemented an assessment freeze, unfairly penalizing property owners with decreasing or stagnant assessment values while giving tax breaks to property owners with rising values.

More than a year after their initial challenge, Judge Wettick agreed with the homeowners and threw out Mr. Onorato's plan, ordering the county to complete a computer-assisted reassessment by March of this year and another reassessment by March of 2009.

So far, the county has done neither assessment because the case is still under appeal. And because Judge Wettick found a state law unconstitutional, the case went straight to the state Supreme Court, where it has been since last year, pending oral arguments.

At its core, the homeowners' case is based on the premise that the base-year plan violates the uniformity clause of the Pennsylvania Constitution and the equal protection clause of the 14th Amendment of the United States Constitution.

"It's a question of uniformity," said Ira Weiss, an attorney representing a group of homeowners in Franklin Park, Mt. Lebanon and Squirrel Hill. Another group of homeowners is represented by attorney Don Driscoll. Judge Wettick combined both cases, and both legal teams will essentially operate as one.

"The Pennsylvania Constitution requires that all taxes in the state should be structured uniformly. That is why we don't have a graduated tax," said Mr. Weiss. "What Allegheny County did [in implementing the base year] is so far from acceptable, let alone uniform. It is unconstitutional."

The inequities in property valuations that were created by the base-year plan, Mr. Weiss said, are far-reaching enough that "I'm very confident the court will affirm Judge Wettick."

And that could set the precedent for challenges of the other base-year plans across the state, said Mr. Weiss. "I think ours is a bellwether case. If [the challenge] stands, there is a chance all those other similar systems out there will fall."

But in his brief before the court, Allegheny County Solicitor Mike Wojcik argued that the state statutes that created the base-year plan "do not clearly, palpably, and plainly violate the Constitution."

"We have a strong argument to make because I think Judge Wettick exceeded his bounds. He wants the [base-year plan] to be a lot closer to perfection in uniformity [regarding how taxes are collected,] but I don't think the Pennsylvania Constitution requires that," Mr. Wojcik said.

The Supreme Court, which holds sessions in Harrisburg, Philadelphia and Pittsburgh every year, will meet in a courtroom on the eighth floor of the City-County Building next week.

Oral arguments in the assessment case are expected to start at 9:30 a.m. Wednesday.