You run a cost segregation study, take 100% bonus depreciation, and watch a big deduction land on your federal return. Then your CPA mentions the state return looks different — and the number you were expecting isn’t there. That’s not a mistake. It’s state conformity, and it’s the part of the bonus-depreciation story that almost nobody explains up front.
Bonus depreciation is a federal rule. Your state gets to decide whether it plays along. Plenty don’t. Here’s the plain version of what that means for a residential study.
What does “state conformity” actually mean?
Every state with an income tax starts from the federal Internal Revenue Code and then decides how closely to follow it. States that adopt the Code as it currently reads — “rolling conformity” — generally accept federal bonus depreciation automatically. States that adopt the Code as of a fixed date — “static conformity” — only follow the version they’ve pinned to, which may or may not include the current bonus rules.
Then there’s a third group that conforms to most of the Code but specifically decouples from bonus depreciation under IRC §168(k). These states say, in effect: compute your depreciation our way, without the bonus. California is the long-standing example — it has never allowed bonus depreciation — and it is far from alone. The result is that the same study can produce two very different year-one deductions depending on which return you’re looking at.
Why do some states add your bonus depreciation back?
Money and timing. A 100% write-off is a huge hit to state revenue in the year it’s claimed, so decoupling states require you to add the bonus back to state income, then depreciate the property on the regular schedule for state purposes. The important word there is timing. You are not losing the deduction. You’re collecting the state portion over the asset’s recovery period instead of all at once, and your property carries a different basis for federal and state purposes until the two catch up.
That distinction matters when you’re weighing whether a study is worth it. The federal lump sum is real and immediate. The state benefit, in a decoupling state, arrives on the installment plan — smaller now, but not gone.
A worked example: a $500K rental in a decoupling state.
Say you buy a single-family rental with $500K of depreciable basis (land already excluded). A typical residential study reclassifies roughly 22% of basis into 5-, 7-, and 15-year buckets — about $110K of short-life property. Because the property was acquired after January 19, 2025, that $110K is eligible for 100% bonus depreciation on the federal return: a full write-off in year one. At a 32% marginal federal rate, that’s roughly $35,000 in federal year-one tax savings.
Now the state. In a full-conformity state, that same $110K also deducts in year one for state purposes — at, say, a 6% state rate, about $6,600 more in year-one savings stacked on top of the federal number. In a decoupling state, you add the $110K of bonus back and instead depreciate it over its 5-, 7-, and 15-year lives. You might see roughly $20K–$25K of that deduction in year one for state purposes, with the remainder spread across the following years. At 6%, that’s about $1,300 of state savings in year one rather than $6,600 — but the other ~$85K of deductions still comes back to you over the recovery period. You collect the same state benefit; you just collect it slower.
Does cost segregation still help if my state decouples?
Yes — and this is the part that gets lost. Even in a state that adds back every dollar of bonus, a study still reclassifies your property into shorter recovery periods. Instead of the whole $500K crawling along a 27.5-year straight line for state purposes, that $110K of short-life property depreciates over 5, 7, and 15 years. That acceleration happens regardless of bonus. So the study still pulls state deductions forward compared with doing nothing — it just doesn’t hand you the entire year-one write-off that the federal side allows. The reclassification is the durable part of the value; bonus is the accelerator that some states switch off. It’s the same 5/7/15 split we walk through in what a cost segregation study actually reclassifies.
What if my state has no income tax?
Then this whole question is moot for your rental income, and the federal benefit is the entire story. If you own in a no-income-tax state — Texas, Florida, Tennessee, Washington and the handful of others — there’s no state return adding anything back, so a study’s full year-one punch lands and stays. Investors who hold across several states are the ones who have to think hardest here, because a single portfolio can touch a conformity state, a decoupling state, and a no-tax state all at once.
So how do you plan around it?
Start by knowing which bucket your state falls in before you assume the federal number is the whole benefit. Conformity rules also change from year to year as legislatures act, so this is genuinely a “check the current year” item — it belongs with your CPA, who files the state return and tracks the federal-state basis difference. What a study gives you is a clean, audit-ready allocation that supports the deduction on both returns, whichever way your state treats bonus. If you want a candid read on how the numbers pencil for your property and your state, our savings estimate is a fast first cut, and we’re happy to talk through the state wrinkle before you commit.