Under the ACA, Internal Revenue Service Code Section 1411 imposes a 3.8 percent tax on a taxpayer’s net investment income. Investment income includes both passive sources like dividends, capital gains, interest, royalties and rents as well as passive business income.
The NIIT will apply to a taxpayer only if their modified adjusted gross income exceeds $250,000 for married taxpayers filing jointly and surviving spouses, $125,000 for married taxpayers filing separately, and $200,000 for unmarried taxpayers and heads of household. The amount subject to the tax is the lesser of a taxpayer’s net investment income or the amount by which modified adjusted gross income exceeds the threshold amount that applies. Thus, a taxpayer will not be subject to the additional tax on net investment income unless their modified adjusted gross income exceeds the respective threshold amount that applies to them.
Take, for example, a married couple with a modified adjusted gross income of $270,000 for 2018, of which $100,000 is net investment income. They would pay the additional tax on only the $20,000 amount by which their modified adjusted gross income exceeds their threshold amount of $250,000. That is because the $20,000 excess is less than their net investment income of $100,000. Thus, the couple’s net investment income tax would be $760 ($20,000 × 3.8%).
When it comes to the taxability of business income under the NIIT, because the law only captures passive business income, most owners of pass-through entities must pay the NIIT; however, active owners of S-corporations are exempt. Likewise, if someone qualifies as a real estate professional, their income is considered active and so their rental income is also exempt.
There are several types of income that are exempt from the net investment income tax. Items excluded from income for regular income tax purposes are generally also excluded from net investment income. This would include such items as tax-exempt interest, gains subject to the like-kind exchange rules and the excluded gain from the sale of your principal residence. Additional items of income not included in the definition of net investment income are distributions from qualified retirement plans such as individual retirement accounts and Roth IRAs, and wages and self-employment income.