Cost segregation and the QBI deduction pull in opposite directions, and most investors come out ahead taking the depreciation anyway. Qualified business income under IRC section 199A is computed after depreciation, so a large bonus deduction from a cost segregation study reduces QBI dollar for dollar. For rentals that rise to the level of a trade or business, including those using the Rev. Proc. 2019-38 safe harbor, that means a smaller or vanished 20% deduction in the year the study lands.
The mechanics
- Dollar-for-dollar reduction. Every dollar of allowed depreciation reduces QBI by a dollar before the 20% is computed.
- Negative QBI carries forward. Under IRC 199A(c)(2), a net negative QBI year is not lost; it carries forward and offsets positive QBI in future years, trimming future 199A deductions.
- Suspended losses do not count yet. Under Reg 1.199A-3(b)(1)(iv), losses disallowed by the passive activity rules reduce QBI only in the year they are finally allowed. If your cost segregation loss suspends under IRC 469, your QBI deduction is untouched this year.
- UBIA is unaffected. The wage-and-property limitation for higher incomes uses unadjusted basis immediately after acquisition, measured before depreciation. Bonus depreciation does not shrink the 2.5% UBIA limb, a point that matters for fund investors above the 199A income thresholds.
Worked example
An investor's rental portfolio produces $90,000 of QBI, supporting an $18,000 deduction under 199A, worth $5,760 at a 32% marginal rate. She buys a $1,000,000 property in 2026, and a cost segregation study generates a $210,000 bonus deduction under IRC 168(k) that is non-passive because she has real estate professional status. Portfolio QBI swings to negative $120,000. The current 199A deduction disappears, costing $5,760, and the negative $120,000 carries forward against future QBI. Meanwhile the $210,000 deduction saves $67,200 at 32%. She finishes the year roughly $61,400 ahead, with a known haircut to future 199A deductions as the carryforward burns off. The trade wins; it is just smaller than the headline number suggests.
When to look twice
- OBBBA made the 199A deduction permanent, so this interplay is now a lasting feature of rental tax planning, not a sunset issue.
- If your losses are passive and will suspend anyway, there is no current QBI cost at all, which quietly improves the case for the study.
- Electing out of bonus depreciation under IRC 168(k)(7) for selected classes can smooth QBI across years when preserving a steady 199A deduction matters, for instance ahead of a planned income spike.
Amadae's tax preparation runs the 199A computation alongside the depreciation schedules, and its Tax Strategy Projector shows the net of both effects, the depreciation savings and the QBI give-back, before you commission an engineering study.