SACRAMENTO -- The Fresno County Assessor's Office might be undervaluing commercial construction, possibly resulting in lower tax revenues for the financially strapped county, according to an audit by a state tax collection agency made public Thursday.

The Board of Equalization found other issues and made 13 recommendations for improvement in its regular review, done every five years. But overall, the audit was mostly positive and noted the office was meeting state standards.

The tax board reviewed the 2007-08 tax year in which Fresno County's roll included 306,735 parcels worth $61.78 billion.

One issue dealt with how the assessor values new construction and building upgrades. The assessor uses information from a publication called the Marshall Valuation Guide. But the tax board criticized the assessor for not reconciling those estimates with the financial information reported by property owners.

For instance, the guide might estimate improvements as being worth $1 million, while the owner reports them as $3 million. If the correct figure is not used, the county collects less in property taxes than it deserves. The audit recommended the office improve documentation.

The audit did not put a dollar figure on the potential lost tax revenue because officials did not review every transaction. But "if we didn't think money was potentially being lost, we wouldn't have made the recommendation for a change," said board spokeswoman Anita Gore.

Assessor-Recorder Robert Werner concurred with the audit's finding on the construction valuation issues, saying in a letter that "chronic understaffing has required us to reduce the amount of time we spend on documenting work processes."