City Council members are so skeptical about new property assessments across Philadelphia that they are looking to hire a real estate appraising firm to audit the city office that made the calculations.

A request for proposals was issued Thursday for a vendor to examine the process used in determining the 2019 property values, which skyrocketed in some neighborhoods, as well as the accuracy of those assessments. Rising values will lead to tax hikes for many homeowners even before the mayor's proposed 4.1 percent increase in the property-tax rate is considered.

"From our cursory review there are significant inconsistencies in the assessments, literally block to block," Council President Darrell Clarke said in an interview. "Normally it may not raise so much concern, but when there is a request for a tax increase on top of inconsistent and dramatic swings of value, we want to make sure there is a process … that people can feel that was done in a way that is consistent."

The median market value of single-family homes increased 10.5 percent from 2018 assessments, from $112,800 to $124,600, according to an Inquirer and Daily News analysis of the city's assessment data. The median tax bill would increase $165, excluding any homestead exemption or the proposed rate increase. But homeowners in neighborhoods such as South Philadelphia, Fishtown, and Chestnut Hill are seeing higher increases in value and taxes.

Overall, market values for all types of properties increased nearly 8 percent to $162 billion, according to city data.

City spokesman Mike Dunn said Thursday that the city is "confident" in its assessments and welcomes the audit.

"The Office of Property Assessment has greatly improved the accuracy of assessments, using nationally recognized methodologies, since the implementation of the Actual Value Initiative in 2014," he said, adding that the administration believes an audit will "alleviate the concerns of Council members and residents."

During the assessment office's budget hearing this month, council members grilled City Chief Assessment Officer Michael Piper over how property values could jump so much in one year. The idea of introducing AVI was assessments would be done annually so people would only see 2 to 3 percent increases each year and be able to adjust more easily to higher tax bills.

Councilwoman Helen Gym asked that the algorithm used for the 2019 assessments be made public.

On Thursday, Gym said: "We need our assessments to be regular, consistent, efficient, and predictable, and when we don't do that we risk losing the public trust."

For his part, Piper has said that the city housing market was hot last year, and the assessments reflect recent sales.

If all goes according to plan, the audit would be done by early September. That timeline, however, could delay a vote on the mayor's proposed tax-rate increase for schools; a vote typically would take place in June, when Council considers the budget.

Councilwoman Maria Quiñones-Sánchez said Thursday that she was not prepared to vote on a tax increase until she had a full grasp of the new property values. Before we ask people for more money in the increase that's requested by the mayor, we have a responsibility to assure them that our numbers are accurate," she said.