The trial pitting two groups of homeowners against Allegheny County's base-year assessment system begins today before Common Pleas Judge R. Stanton Wettick Jr., and has the potential to change the way millions of parcels of property are assessed statewide.

The plaintiffs, homeowners from Pittsburgh, Franklin Park, Mt. Lebanon, Braddock and other Allegheny County municipalities, are mounting a "full frontal challenge" of Allegheny County's 2002 base-year property assessment system, arguing the taxation method violates the Pennsylvania Constitution.

They want to see the county's system -- which relies on property values frozen indefinitely four years ago during the last full reassessment -- declared unconstitutional, and therefore invalid. The state constitution calls for uniform taxation of property.

Tax attorney and former county solicitor Ira Weiss, who is representing one of the groups of homeowners and leading the combined challenge to the base-year assessment, said the current system creates glaring inequalities in every corner of Allegheny County.

"We think the evidence is pretty compelling that the system has created significant disparities," he said.

Last year, Allegheny County Chief Executive Dan Onorato put a stop to the regular assessment method and, with County Council's approval, decided to make 2002 the base year for assessments.

The state Legislature created the base-year system in 1982, and some Pennsylvania counties have not done new assessments in decades.

By not adjusting for market change over time, said Mr. Weiss, the system penalizes owners of property with dropping or stagnant values.

"The other part of it, which is really not our focus, is that it [the base-year system] also tends to be regressive; the tax load is shifted to lower-valued properties," he said.

When higher-valued properties don't reflect true market value, Mr. Weiss explained, the result is that lower-valued properties carry more of the tax burden, paying more than their fair share.

Mr. Weiss plans to call two witnesses, a real estate appraiser and an expert on the methodology of property assessments, who will present a market study of Allegheny County demonstrating the heightened tax inequalities among county property owners.

According to their planned testimony, the county's more expensive homes are more likely to be underassessed, while cheaper homes are more likely to be over-assessed.

It is also unclear, said Mr. Weiss, how the 2002 base-year system is used to value new construction.

The homeowners represented by Mr. Weiss are being joined in their challenge by a second set of plaintiffs under the Community Justice Program. Upper St. Clair was to be the third prong of the plaintiffs' case, arguing that the 2002 base-year system forces it to violate the constitution, applying its millage rate to market values that are not uniform. But township attorneys pulled out of the case late Friday, said Mr. Weiss.

Mr. Weiss, who has represented several taxing bodies in the past, including the Pittsburgh Public Schools, said the far-reaching case has implications for school districts.

"The school districts face unique challenges, because now they have limits on the millage increases they can implement without voter approval, but their costs are not limited," he said.

Potential tax revenue of millions of dollars is at stake in the case.

If the challenge is successful and upheld on appeal, the decision could affect tens of thousands of property owners around the county alone, said Mr. Weiss.

"I'm sure the trial will be interesting," he said. "It'll give the county much to think about."