There's nothing glamorous about Joseph Zenobi's property near 54th and Master Streets - a broken-down warehouse next to a vacant lot fenced in with razor wire against the hazards of a hard West Philadelphia neighborhood.

For a dozen years, Zenobi, 73, has used it only as a place to park trucks and machinery for his small paving company.

Suddenly, this humble slice of land looks like a gold mine to the city's Board of Revision of Taxes.

In its new project to overhaul property assessments, the BRT suggests the land's value has soared from $216,600 to $3.9 million - more, per square foot, than lots surrounding fashionable Rittenhouse Square.

"It's crazy," Zenobi said. "You want to buy it?"

Four years in the making, the so-called Actual Value Initiative is supposed to be the solution to the BRT's unfair and confusing assessments, long considered among the least accurate in the nation. The idea is simple: Every property's assessment would be the same as its value in the marketplace.

But on its first try to correctly assess the city's 25,000 commercial parcels - properties responsible for nearly 30 percent of Philadelphia's property-tax revenue - the BRT fell far short, an Inquirer review shows.

Scores of properties, such as Zenobi's yard, would be incredibly overvalued. Others are so low that even the owners say they're absurd.

The attempt to repair the values suffers from the same failings that have plagued the agency for decades: mistakes in property records, sloppiness in computer calculations, and assessors who used crude shortcuts instead of standard reappraisal methods, the review shows.

Among the findings:

Hundreds of the new commercial numbers were thrown off by mistakes littering the BRT's property records, including incorrectly sized lots and buildings that don't exist. At Seventh and Arch Streets, for example, the BRT calculated a new value of $5.2 million on what the agency thought was a huge, 200-space parking lot. But there is no such lot, just a narrow walkway next to the Federal Detention Center.

Instead of trying to figure out a property's real worth, the BRT's assessors slapped the same percentage increases on thousands of parcels across the city. More than 500 would get the same 40 percent increase - properties as different as a $6 million shopping center on Castor Avenue and a long-empty hoagie shop in North Philadelphia.

More than 6,000 commercial properties - a quarter of the total - are missing entirely, left undone as the BRT rushed to send the AVI numbers to impatient city officials last spring.

Apparent glitches in the BRT's computer models produced some bizarre results. Parking lots in a drug-ravaged section of Frankford, for example, were valued at a steep $140 per square foot - more pricey than many lots in Center City.

City Finance Director Rob Dubow said that if the BRT didn't produce accurate numbers, it risked blowing the credibility of the overhaul.

"This is our one time to get this right," Dubow said Friday. "So we have to make sure this is done correctly."

Mayor Nutter, through a spokeswoman, declined to comment.

Barry Mescolotto, the BRT's acting chief assessor, acknowledged some flaws but said the new numbers were only "a starting point" and bound to include some mistakes. The earliest they could go into effect would be the 2011 tax year.

"Are there some data errors in there? Yes," said Mescolotto, who said he had been supervising the process from home while on family medical leave. "We are in the process of correcting all of them."

But one BRT member, former Pennsylvania Supreme Court Justice Russell M. Nigro, said the new numbers never should have been sent to City Hall if they weren't ready.