Cost Segregation Study in Illinois for Airbnb and Rental Property Investors

Illinois imposes a flat 4.95% individual income tax, and unlike most states, it does not simply mirror the federal bonus depreciation calendar. Under 35 ILCS 5/203 and the related instructions to Schedule M and Form IL-4562 (both on tax.illinois.gov), Illinois requires taxpayers to add back the full amount of any federal bonus depreciation deduction claimed under IRC Section 168(k) in the year it's taken on the federal return. In exchange, Illinois allows the taxpayer to recover that addback through offsetting subtractions in later years, spreading the deduction out on something closer to a standard depreciation schedule instead of letting it land in year one. Public Act 104-0453 recently extended this same addback-and-recover mechanism to the new federal bonus deduction for qualified production property under IRC Section 168(n), for tax years beginning on or after January 1, 2026. That provision, though, is aimed at nonresidential production real estate such as manufacturing and refining facilities — it has no application to residential rentals, short-term rental furnishings, decking, or landscaping. For the components this page's cost segregation studies actually reclassify, the addback that matters is the pre-existing Section 168(k) addback described above.

For a cost segregation study, this matters at the margin but doesn't erase the benefit. The engineering study still identifies the 5-, 7-, and 15-year components inside your Illinois rental property and reclassifies them out of 27.5- or 39-year straight-line treatment. On the federal return, those reclassified components remain eligible for 100% bonus depreciation in the placed-in-service year under current law, which can still produce a large first-year federal loss for an Illinois owner. On the Illinois return, that federal acceleration gets added back and then released over subsequent years — so the state-level cash benefit is smoothed out over time rather than delivered immediately, but it is not forfeited.

That timing difference is worth taking seriously in Illinois because carrying costs here are unusually high. Depending on the data source and year, Illinois's average effective property tax rate on owner-occupied housing runs roughly in the high-1% to low-2% range — recent Tax Foundation figures put it around 1.9%, tied with New Jersey for the highest in the country, while other national comparisons place it closer to 2.1% and rank it second behind New Jersey. Either way, it's roughly double the national average, driven heavily by Cook County and the collar counties. Investors already absorbing that property tax load have more reason, not less, to capture every available federal deduction — including the immediate bonus depreciation windfall a cost segregation study unlocks — even knowing the Illinois piece arrives on a longer timeline.

A cost segregation study is an engineering-driven reallocation of a property's depreciable basis. Left alone, the IRS default treats a residential rental building as a single asset depreciated straight-line over 27.5 years, or 39 years for a commercial property, regardless of how many different systems and finishes actually make up the building. A study sends an engineer through the property (or the construction documents, for a new build) to break out components such as flooring, cabinetry, decorative lighting, appliances, certain electrical and plumbing runs, fencing, parking areas, and landscaping, and assign each one to its correct IRS recovery class: typically 5-year and 7-year personal property, and 15-year land improvements.

Once those components are reclassified out of the 27.5- or 39-year bucket, they qualify for accelerated depreciation methods. Under the One Big Beautiful Bill Act (OBBBA), signed into law in July 2025, qualifying property that is both acquired and placed in service after January 19, 2025 is eligible for 100% federal bonus depreciation; property acquired on or before that date but placed in service later remains subject to the pre-OBBBA phase-down schedule (40% in 2025, 20% in 2026, and 0% after). In practical terms, for property meeting the post-January 19, 2025 acquisition-and-placed-in-service test, the reclassified 5-, 7-, and 15-year components can potentially be deducted in full in the year the property is placed in service, rather than depreciated a few percentage points at a time over decades. The result is a concentrated paper loss in year one that can offset rental income, and in some cases other income, depending on the owner's participation status.

For illustration only — your results depend on your property and tax situation, and this is not a projection of actual savings.

Say an investor buys a four-bedroom vacation cabin in Galena, one of Illinois's most established short-term rental markets, known for wine-country tourism and consistently strong summer and fall bookings, for $650,000, with roughly $520,000 of that allocated to the depreciable building after backing out land value.

Under standard straight-line depreciation, that $520,000 would be written off over 27.5 years, or about $18,900 per year.

A cost segregation study might reclassify roughly 25 to 30% of that basis — call it $140,000, illustratively — into 5-, 7-, and 15-year property: furniture and appliances that came with the cabin, area rugs and window treatments, the driveway and outdoor decking, and landscaping.

On the federal return, that $140,000 in reclassified components could potentially qualify for 100% bonus depreciation in the placed-in-service year (assuming both acquisition and placed-in-service occur after January 19, 2025), producing a large first-year federal deduction on top of ordinary depreciation. On the Illinois return, that same $140,000 bonus amount would need to be added back on Schedule M and Form IL-4562, then released through offsetting subtractions in later years rather than claimed all at once.

These are illustrative, round numbers only. Actual component percentages, recovery periods, and your ability to use the resulting loss depend on your specific property, your income, and your participation in the activity. Confirm every number with your CPA before relying on it.

Cost segregation isn't limited to the year you buy. If you've owned an Illinois rental property for a year or more and never had a study done, you can still capture the missed depreciation without amending a single prior-year return. The mechanism is a change in accounting method, filed on IRS Form 3115, which lets you claim the entire cumulative difference between what you've depreciated so far and what a proper component-based schedule would have allowed, all as one Section 481(a) adjustment in the current tax year.

That means an Illinois investor who bought a rental a few years ago and has been depreciating the whole building straight-line ever since can commission a look-back study today, reclassify the components as if the study had been done at acquisition, and take the full catch-up deduction on this year's federal return, subject to the same Illinois addback-and-subtraction timing described above for any bonus-eligible portion. No amended returns, and no reopening of closed tax years.

• Short-term rental hosts in Illinois's established vacation markets — including cabin and cottage owners in Galena and the wine-country corridor around it, and the cluster of cabins near Starved Rock State Park in Utica, Oglesby, and LaSalle-Peru, where strong weekend and seasonal demand supports healthy average daily rates.
• Chicago-area STR and long-term rental owners who understand the city's licensing rules, including the primary-residence requirement for buildings of four units or fewer, and the layered city and state lodging taxes that now apply to short-term stays, including possible surcharges on top of the base city and state rates.
• Long-term buy-and-hold landlords across Chicago's neighborhoods and suburbs, and in downstate markets like Springfield and Peoria, looking to offset rental income with a larger first-year deduction.
• Investors who recently purchased or substantially renovated an Illinois rental property, where the depreciable basis, and therefore the potential reclassification, is largest.
• High-income W-2 earners exploring the short-term rental material participation strategy, which can allow losses from a qualifying STR (average guest stay of seven days or less, plus material participation) to offset ordinary income — a strategy where the size of the loss cost segregation can generate matters even more given Illinois's high carrying costs.
• Owners weighing the federal-versus-state timing tradeoff, who want to understand, with their CPA, how much of the benefit lands this year at the federal level versus later at the Illinois level.