The tax assessors office placed a value of $4.6 million on John's 12,000-square-foot condo on Peachtree Street in Atlanta's trendy Buckhead neighborhood.
But that value is $900,000 too much, the 59-year-old singer's experts say.
Fulton County Superior Court Judge Wendy Shoob has ordered the two sides to try to resolve the dispute through mediation.
If he prevails, John would save $16,000 a year in taxes. If he loses, he can expect to pay $77,250 a year.
Craig Klayman, a tax consultant hired to represent John in his lawsuit, said money is not the issue.
"I just don't see how the county can justify that number," Klayman said. "It's just too high looking at what everybody else is valued at."
Fulton County residents have long complained that tax values bear little relationship to actual sales values. A critical audit released last year noted inconsistent values.
The audit eventually led to the resignations of three of the five assessors. The chief appraiser also stepped down.