In most of the United States it's not how much but rather how long, but that depends on each state.Once you owe 3 years of back taxes, the county will usually sell your property at a tax auction, although the specific procedures which jurisdictions will use vary and are usually specified by each state.For example, Jefferson County in state of Colorado places a tax lien on every property owing taxes on January 1st of each year. The county then holds an annual tax sale in October where it sells the liens to investors. The property owner has 3 years to redeem the tax lien before the investor can apply for treasurer's deed on the property (basically a tax foreclosure).Edit:I didn't even consider personal (income) taxes in my original answer and apparently IRS also has the power to place a tax lien or legally seize property for unpaid back taxes in the States, but again I don't believe there is a specific threshold.