# Illinois — State conformity to federal Section 179 and bonus depreciation rules

For Illinois, section 179 conformity is conforms; state section 179 cap is federal limit applies; state section 179 phase-out is federal phase-out applies; bonus depreciation conformity is decouples — requires add-back; add-back rule is for taxable years beginning after December 31, 2021, the state decouples from the 100% depreciation deduction, recorded from its source on 2026-08-15.

- **State:** Illinois _(verified: appears in the quote below)_
- **Section 179 conformity:** conforms
- **State Section 179 cap:** federal limit applies
- **State Section 179 phase-out:** federal phase-out applies
- **Bonus depreciation conformity:** decouples — requires add-back
- **Add-back rule:** for taxable years beginning after December 31, 2021, the state decouples from the 100% depreciation deduction _(verified: appears in the quote below)_
- **Has income tax:** yes _(verified: appears in the quote below)_

## What the source says

> State Follows Bonus Depreciation-TCJA of 2017 : No For taxable years beginning after December 31, 2021, the state decouples from the 100% depreciation deduction. For tax years beginning before December 31, 2021, Illinois conformed to the Tax Cuts and Jobs Act provision that provides a 100% first-year deduction for the adjusted basis of qualified property acquired and placed in service after September 27, 2017, and before January 1, 2023. State Follows IRC § 179-TCJA of 2017 : Yes Illinois does conform to the Tax Cuts and Jobs Act provision that increases the maximum amount a taxpayer may expense under IRC § 179 to $1 million, increases the phase-out threshold amount to $2.5 million, and provides for indexing for inflation.

## Sources disagree

More than one authority states this, and they do not state the same
thing. Both are reproduced with the source each came from.

### Bonus depreciation conformity

accountants.intuit.com says bonus depreciation conformity is **decouples — requires add-back**, as of 2026-08-15.

> State Follows Bonus Depreciation-TCJA of 2017 : No For taxable years beginning after December 31, 2021, the state decouples from the 100% depreciation deduction. For tax years beginning before December 31, 2021, Illinois conformed to the Tax Cuts and Jobs Act provision that provides a 100% first-year deduction for the adjusted basis of qualified property acquired and placed in service after September 27, 2017, and before January 1, 2023. State Follows IRC § 179-TCJA of 2017 : Yes Illinois does conform to the Tax Cuts and Jobs Act provision that increases the maximum amount a taxpayer may expense under IRC § 179 to $1 million, increases the phase-out threshold amount to $2.5 million, and provides for indexing for inflation.

Source: https://accountants.intuit.com/support/en-us/help-article/asset-depreciation/state-conformity-special-depreciation-allowance/L27kRyetJ_US_en_US

crosslinktax.com says bonus depreciation conformity is **conforms**, as of 2026-08-15.

> Illinois | Yes | Yes |

Source: https://www.crosslinktax.com/customer-resources/tax-resource-center/tax-updates/state-compliance-with-federal-bonus-depreciation-and-section-179-expensing-2025/


### Add-back rule

accountants.intuit.com says add-back rule is **for taxable years beginning after December 31, 2021, the state decouples from the 100% depreciation deduction**, as of 2026-08-15.

> State Follows Bonus Depreciation-TCJA of 2017 : No For taxable years beginning after December 31, 2021, the state decouples from the 100% depreciation deduction. For tax years beginning before December 31, 2021, Illinois conformed to the Tax Cuts and Jobs Act provision that provides a 100% first-year deduction for the adjusted basis of qualified property acquired and placed in service after September 27, 2017, and before January 1, 2023. State Follows IRC § 179-TCJA of 2017 : Yes Illinois does conform to the Tax Cuts and Jobs Act provision that increases the maximum amount a taxpayer may expense under IRC § 179 to $1 million, increases the phase-out threshold amount to $2.5 million, and provides for indexing for inflation.

Source: https://accountants.intuit.com/support/en-us/help-article/asset-depreciation/state-conformity-special-depreciation-allowance/L27kRyetJ_US_en_US

crosslinktax.com says add-back rule is **N/A**, as of 2026-08-15.

> Illinois | Yes | Yes |

Source: https://www.crosslinktax.com/customer-resources/tax-resource-center/tax-updates/state-compliance-with-federal-bonus-depreciation-and-section-179-expensing-2025/


## Source

- https://accountants.intuit.com/support/en-us/help-article/asset-depreciation/state-conformity-special-depreciation-allowance/L27kRyetJ_US_en_US
- https://www.crosslinktax.com/customer-resources/tax-resource-center/tax-updates/state-compliance-with-federal-bonus-depreciation-and-section-179-expensing-2025/

Last verified: 2026-08-15. Review by: 2027-08-15.
Part of [State conformity to federal Section 179 and bonus depreciation rules](https://referencesource.org/state-section-179-bonus-depreciation-conformity/).