Illinois Bonus Depreciation: 100% Add-Back, Recovered Ratably.
Illinois adds back the full federal §168(k) bonus depreciation deduction in the year taken, then recovers it ratably in later years by a formula keyed to the bonus percentage originally claimed. The same mechanism applies uniformly to corporations, trusts, estates, and partnerships.
Reviewed by Cost Seg Smart Editorial Team · Last verified against 35 ILCS 5/203 — Illinois Income Tax Act, base income modifications
The 30-second answer: Illinois has decoupled from federal §168(k) bonus depreciation. An individual claims the full federal Year-1 bonus on the federal return, but for Illinois purposes, 100% of that federal deduction is added back in the year taken.
This is a timing difference, not a lost deduction. The addition is recovered ratably in later years by a formula keyed to the bonus percentage originally claimed (35 ILCS 5/203(a)(2)(D-15) for the addition; 203(a)(2)(Z) for the recovery). Identical language applies to corporations, trusts, estates, and partnerships. For tax years 2026 and later, the addition also captures §168(n).
On a representative Illinois single-family rental (the single-family rental band we publish, 9–32% of depreciable basis reclassified, 16% representative), the reclassified components alone generate an estimated $18K–$165K of federal Year-1 depreciation — roughly $6,660–$61,570 of federal Year-1 tax savings at the 37% top bracket, with the full amount added back for Illinois in Year 1 and recovered ratably afterward.
Federal vs Illinois, Side by Side
For an individual investor's cost-segregation-reclassified components:
| Tax provision | Federal (IRC) | Illinois |
|---|---|---|
| Bonus depreciation under §168(k) | 100% may apply to qualified property acquired and placed in service after January 19, 2025, subject to §168(k) eligibility, related-party/prior-use, binding-contract, and election rules | 100% added back in the year taken (35 ILCS 5/203(a)(2)(D-15)) |
| Recovery of the added-back amount | Not applicable | Recovered ratably in later years by a bonus-percentage formula (203(a)(2)(Z)) — deferred, not lost |
| Who the add-back applies to | Not applicable | Individuals, corporations (203(b)(2)(E-10)), trusts/estates (203(c)(2)(G-10)), partnerships (203(d)(2)(D-5)) — identical language |
| 2026+ scope | §168(n) newly relevant alongside §168(k) | The addition also captures §168(n) for tax years 2026 and later |
| Federal usability of the deduction | Subject to passive activity, at-risk, basis, and business-interest limits | Same federal limits govern the federal deduction; Illinois's addition/recovery cycle applies independently |
Source: 35 ILCS 5/203 — Illinois Income Tax Act; IRC §168(k). Federal bonus figures reflect current law; eligibility depends on acquisition and placed-in-service dates and elections.
How Illinois's 100% add-back and ratable recovery work
The study itself does not change. The same engineering-based reclassification, using nationally-recognized construction cost data, MACRS classification per Rev. Proc. 87-56, and IRS Pub 5653 ATG-aligned documentation, produces the same component schedule whether the property sits in Illinois or anywhere else. What changes is what happens on the Illinois return: the full 100% of the federal §168(k) deduction is added back in the year it was taken, under 35 ILCS 5/203(a)(2)(D-15).
That is not the end of the story. Section 203(a)(2)(Z) brings the added-back amount back through a ratable subtraction in later years, computed by a formula keyed to the bonus percentage originally claimed — a different mechanical shape than Minnesota's flat one-fifth-per-year schedule, but the same underlying idea: the state recognizes the same total deduction over more years than the federal return does.
The identical addition-and-recovery language is repeated verbatim across entity types — corporations at 203(b)(2)(E-10), trusts and estates at (c)(2)(G-10), and partnerships at (d)(2)(D-5) — so there is no taxpayer-type split to worry about in Illinois the way there is in a state like Michigan.
Your CPA receives the same Cost Seg Smart engineered report, the same Form 4562-ready federal schedule, and the same Form 3115 §481(a) section if this is a lookback study. The Illinois addition and its ratable recovery are then computed from that same schedule.
Illustrative numbers: an Illinois single-family rental
Using the single-family residential (SFR) band we publish — 9–32% of depreciable basis reclassified into 5/7/15-year property, 16% representative — on a property priced between $250,000 and $650,000, with a 20% residential land allocation, and 100% federal bonus depreciation for eligible components under current law. The Illinois column is described qualitatively because the recovery formula depends on the bonus percentage originally claimed rather than a flat annual fraction:
| Line item | Federal | Illinois |
|---|---|---|
| Purchase price band | $250,000–$650,000 | Same property |
| Depreciable basis (20% land allocation) | $200,000–$520,000 | Same basis before the add-back |
| Reclassified to 5/7/15-yr (9–32% of basis) | $18K–$165K | 100% added back in Year 1 |
| Year-1 treatment | $18K–$165K deducted (100% bonus, if eligible) | Full amount added back to Illinois income |
| Later-year treatment | Not applicable (already deducted) | Recovered ratably per the (Z) formula, keyed to the bonus percentage claimed |
| Marginal tax rate | Up to 37% | Illinois individual income tax rate (verify current year with the Illinois Department of Revenue) |
| Illustrative Year-1 federal tax savings on reclassified components | ~$6,660–$61,570 (37% × $18K–$165K) | 100% deferred to later years via ratable recovery, not lost |
Figures are illustrative and use the site's published SFR reclassification band; your result depends on your basis, land allocation, bracket, component mix, and eligibility. Whether the full federal deduction is usable in the current year still depends on your passive-loss, at-risk, basis, and business-interest situation. The Illinois recovery formula should be modeled by your CPA against the specific bonus percentage claimed and the years the property is held.
See a sample cost segregation report
Look at exactly what your Illinois study delivers: the component-by-component 5/7/15-year schedule, the Form 4562-ready numbers, and the documentation your CPA files. Real reports are our best answer to "is this legit?"
Forms your CPA files for an Illinois property
For an Illinois investor, the workflow runs on both the federal return and the Illinois return, because Illinois's addition and ratable recovery mechanism has to be tracked over multiple years:
- Federal Form 4562: depreciation and amortization, including the §168(k) bonus deduction on eligible reclassified components. Flows to Schedule E (rental) or Schedule C (active business).
- Schedule E (or Schedule C): the accelerated federal deduction reduces federal taxable income in Year 1, subject to the federal passive-loss, at-risk, basis, and business-interest limits.
- Illinois income tax return: 100% of the federal §168(k) deduction is added back, per 35 ILCS 5/203(a)(2)(D-15) (or the parallel provision for your entity type).
- Ratable subtractions in later years: Illinois recovers the added-back amount per the (Z) formula, keyed to the bonus percentage claimed — this has to be tracked on the Illinois return until fully recovered.
- Form 3115 §481(a) section: included only if this is a federal lookback method change on a property placed in service in a prior year (see below).
The reclassified schedule is the same engineered output for both books; the difference is that federal claims the Year-1 bonus while Illinois adds back 100% in Year 1 and recovers it ratably afterward.
Form 3115 lookback on an Illinois property
If the property was placed in service in a prior tax year and depreciation was reported using a non-segregated method, a federal Form 3115 (Application for Change in Accounting Method) may allow a current-year §481(a) catch-up adjustment for the accelerated depreciation you missed, generally without amending prior returns. This is often the single biggest acceleration mechanism in cost segregation. Any catch-up dollars are illustrative and depend on placed-in-service date, prior depreciation claimed, bonus eligibility, land allocation, and component mix.
The federal §481(a) catch-up is the primary lever, and it remains subject to the federal passive-loss, at-risk, and basis limits. For Illinois, the catch-up year's bonus-eligible amount is added back in full the same way an ordinary Year-1 bonus would be, and recovered ratably in later years per 203(a)(2)(Z).
See our full Form 3115 cost segregation guide for federal mechanics, partnership and LLC pass-through treatment, and timing rules.
Should you skip cost segregation in Illinois? No.
Illinois's 100% add-back changes the timing of the state-side benefit, not the fundamental economics. The math still favors doing the study, for three reasons:
- The full federal Year-1 bonus is intact. Illinois does not reduce the federal deduction; the full §168(k) bonus is claimed on the federal return. How much you use in the current year still depends on your passive-loss, at-risk, basis, and business-interest situation.
- Illinois defers the benefit, it does not eliminate it. The full addition in Year 1 is followed by a ratable recovery in later years (35 ILCS 5/203(a)(2)(Z)), so the same reclassified deduction is fully recovered on the Illinois return over time.
- The Form 3115 lookback still captures missed federal years. If the property was placed in service in a prior year and depreciated without cost segregation, the federal §481(a) catch-up may be available, generally without amending prior returns.
The nuance to flag with your CPA is that Illinois's recovery formula is keyed to the bonus percentage originally claimed rather than a flat schedule, and starting with tax year 2026 the addition also reaches §168(n) — both need to be tracked on the Illinois return.
Frequently asked
Does Illinois allow bonus depreciation?
Not in Year 1. Illinois has decoupled from federal §168(k) bonus depreciation: an individual investor still claims the full federal Year-1 bonus on the federal return (100% may apply to qualified property acquired and placed in service after January 19, 2025, subject to §168(k) eligibility, related-party/prior-use, binding-contract, and election rules), but for Illinois purposes, the full 100% of that federal bonus deduction is added back in the year it was taken (35 ILCS 5/203(a)(2)(D-15)).
Do I get the added-back deduction back in Illinois, or is it lost?
It comes back — this is the reassuring part. The full addition is recovered ratably in later years through the subtraction at 35 ILCS 5/203(a)(2)(Z), by a formula keyed to the bonus percentage originally claimed. The deduction is deferred, not lost, and the practical shape matches Minnesota's add-back-then-recover pattern rather than Hawaii's permanent disallowance.
Does the Illinois add-back apply the same way to corporations?
Yes — identical language is repeated verbatim for corporations at 203(b)(2)(E-10), for trusts and estates at (c)(2)(G-10), and for partnerships at (d)(2)(D-5). There is no taxpayer-type split; the 100% addition and ratable recovery mechanism is the same across every entity type.
Has the Illinois add-back changed recently?
Yes, for tax years 2026 and later. The addition now also captures §168(n) alongside §168(k), so a taxpayer relying only on the §168(k) figure for a 2026-or-later tax year should confirm whether §168(n) property is also involved before computing the Illinois addition.
Is cost segregation still worth it in Illinois?
In nearly every case, yes. The federal Year-1 benefit is the dominant driver and Illinois does not reduce it: the full federal §168(k) bonus is claimed on the federal return. At the Illinois level, the 100% add-back defers the state-side benefit rather than eliminating it — the deduction comes back through the ratable (Z) subtraction as the asset's remaining schedule plays out. Whether the full federal deduction is usable in the current year still depends on federal passive-activity, at-risk, basis, and business-interest limits.
Can I use Form 3115 on an Illinois property I bought years ago?
Often, yes, at the federal level. If the property was placed in service in a prior tax year and depreciation was reported using a non-segregated method, a federal Form 3115 (Application for Change in Accounting Method) may allow a current-year §481(a) catch-up for the accelerated depreciation you missed, generally without amending prior returns. Any catch-up dollars are illustrative and depend on placed-in-service date, prior depreciation, bonus eligibility, land allocation, and component mix. The federal §481(a) mechanics are the primary lever; the Illinois effect follows the same 100% add-back and ratable-recovery mechanism described above, applied to the catch-up year.
Related guides
- Bonus depreciation by state: overview
- All 50 states: conformity reference table
- Minnesota bonus depreciation (decoupled, 80% add-back)
- Indiana bonus depreciation (decoupled, first-year add-back)
- Ohio bonus depreciation (decoupled, 5/6 add-back)
- Form 3115 cost segregation lookback: §481(a) mechanics
- What is cost segregation: the full primer
- Sample cost segregation reports