Taxpayers can itemize deductions using Schedule A on Form 1040, according to the Internal Revenue Service. Common itemized deductions include mortgage interest, state and local property taxes, charitable contributions and medical and dental expenses. Itemized deductions can only be taken instead of, not in addition to, the standard deduction.
Taxpayers with deductible expenses that exceed the amount of the standard deduction pay lower taxes by itemizing those deductions, according to the IRS. High-income taxpayers are subject to deduction phase-outs over income levels defined in Schedule A. In 2014, filers with adjusted gross incomes above $305,050 for married couples, $254,200 for single filers, $152,525 for married people filing separately and $279,650 for head of household are subject to limits on their itemized deductions.
In some cases, taxpayers must itemize deductions, according to the IRS. Married people who file separate tax returns must use the same deduction method on both of their returns, so a married person may not use the standard deduction if his or her spouse itemizes. Nonresident aliens and dual-status aliens must typically itemize deductions, and they can only use the standard deduction if they are married to U.S. citizens or residents and they choose to be treated as residents.