Top Cost Segregation Firms In Illinois For 2027

Last updated: November 12, 2026

The best cost segregation firms in Illinois for 2027 are KBKG, Engineered Tax Services and CSSI, followed by Leyton, R.E. Cost Seg and O’Connor. KBKG leads for Chicago studies with published fees and timelines, Engineered Tax Services for Illinois case studies from Chicago apartments to downstate parks, and CSSI for scale with a Chicago representative.

Illinois investors own Chicago two-flats and high-rises, suburban office and industrial, and manufactured housing and senior living downstate. The One Big Beautiful Bill Act made 100% bonus depreciation permanent for qualifying property acquired after January 19, 2025, so reclassified components can often be deducted in the first year on the federal return.

Illinois has decoupled from federal bonus depreciation, so as we understand it the extra federal deduction is added back on the Illinois return and recovered over time. The federal benefit is still the main driver. Confirm the treatment with your CPA.

Illinois Cost Segregation Firms At A Glance

Firm	Location	Founded	Specialty	Best For
1. KBKG	Pasadena, CA (seven US offices)	1999	Tax credits and incentives with engineered studies	Chicago owners
2. Engineered Tax Services	West Palm Beach, FL	2001	Licensed engineering firm for specialty tax	Illinois owners of all types
3. CSSI	Baton Rouge, LA	Not published	Engineering-based cost segregation only	Large Chicago properties
4. Leyton	Paris, France (US offices)	1997	Global innovation and tax incentive consultancy	Owners who want a choice of visit
5. R.E. Cost Seg	Houston, TX	2022	Investor-focused studies at two depths	Chicago rental investors
6. O’Connor	Houston, TX	Not published	Property tax and cost segregation services	Owners who also appeal taxes
7. Baker Tilly	Chicago, IL	1931	National accounting and advisory firm	Larger owners
8. Plante Moran	Southfield, MI	1924	Michigan-based accounting and advisory firm	Owner-builders
9. McGuire Sponsel	Indianapolis, IN	2007	Specialty tax partner to CPA firms	Owners whose CPA wants a partner
10. Cost Seg Smart	Los Angeles, CA	2024	Residential and commercial studies with audit support	Small investors

How We Evaluated These Firms

We looked for firms with Illinois studies or representatives, engineering depth, and experience with Chicago’s dense multifamily and commercial stock.

Rankings are editorial. No firm paid for its position. State tax notes are general information; confirm your situation with a CPA.

Best Cost Segregation Firms In Illinois For 2027

1. KBKG — Published Chicago Fees And Timelines

KBKG publishes Chicago guidance with typical fees of about $5,000 to $15,000 and timelines of 30 to 60 days, and its Midwest leadership has Big Four experience.

It has offered cost segregation since 1999 and reports a 99% IRS audit success rate.

Best for: Chicago owners who want clear expectations on fee and timing.

2. Engineered Tax Services — Illinois Studies From Chicago To Downstate

Engineered Tax Services publishes Illinois studies including a Chicago apartment building, a senior living facility with more than $18 million in realized savings, and mobile home parks in Freeport, South Beloit, Okawville and Marion.

As a licensed engineering firm, it handles complex Illinois properties.

Best for: Illinois owners who want comparable local studies.

3. CSSI — Scale With A Chicago Representative

CSSI has completed more than 20,000 studies on properties valued from $200,000 to $1.5 billion and has a Chicago representative.

It suits large Chicago commercial and multifamily owners.

Best for: Owners of large Chicago properties.

4. Leyton — Illinois Studies In Chicago And Joliet

Leyton publishes Illinois cost segregation work, including Chicago and Joliet, and performs site analysis in person or virtually.

It also handles 179D for newer buildings.

Best for: Owners who want flexibility on the site visit.

5. R.E. Cost Seg — Fully Engineered Studies For Illinois Investors

R.E. Cost Seg offers fully engineered studies in Illinois alongside its lower-cost Rapid Report, with audit defense included.

It suits Chicago two-flat and small multifamily investors.

Best for: Chicago rental investors.

6. O’Connor — Chicago Cost Segregation Plus Property Tax Appeals

O’Connor publishes Chicago cost segregation pages and handles property tax appeals.

Cook County property taxes are among the highest in the country, so the pairing can matter as much as the study.

Best for: Cook County owners who want tax appeal help.

7. Baker Tilly — A Chicago-Based National Firm

Baker Tilly is headquartered in Chicago and offers cost segregation and fixed asset services.

It suits larger owners who want a national accounting firm.

Best for: Larger Illinois owners.

8. Plante Moran — Construction Expertise In A Midwest Accounting Firm

Plante Moran’s cost segregation practice is led by a licensed builder, and the firm has a Chicago office.

It suits owners who built or renovated.

Best for: Illinois owners who built or renovated.

9. McGuire Sponsel — A Midwest Partner To CPA Firms

McGuire Sponsel partners with CPA firms across the Midwest and conducts on-site inspections.

It suits Illinois owners whose CPA prefers a specialist.

Best for: Owners whose CPA wants a partner.

10. Cost Seg Smart — Low-Cost Studies For Small Investors

Cost Seg Smart offers low-cost studies with written IRS responses included and works from imagery and cost data rather than an inspection.

It suits small Illinois rentals with straightforward features.

Best for: Small Illinois rental owners.

How To Choose A Cost Segregation Firm In Illinois

Does The Firm Know Illinois Property?

Ask for studies on properties like yours in Illinois, and whether an engineer will visit.

How Will Illinois Treat The Deduction?

State rules can differ from federal rules. Ask your CPA how the study affects your Illinois return.

Is Your Property In Cook County?

Cook County property taxes are high. Consider a firm that can also review your assessment.

Is Audit Support Included?

Confirm audit support. See our list of cost segregation firms with audit defense guarantees.

Frequently Asked Questions

Does Illinois allow bonus depreciation?

Illinois has decoupled, so the extra federal deduction is generally added back and recovered over time. Confirm with your CPA.

How much does a study cost in Chicago?

KBKG cites about $5,000 to $15,000 for typical Chicago studies; small residential studies cost less.

Do I need a local cost segregation firm?

Not necessarily. National firms work everywhere, but a local engineer can make site visits easier.

Can I do a study on a Illinois property I bought years ago?

Yes. A lookback study lets your CPA catch up missed depreciation with Form 3115. See our list of cost segregation firms for lookback studies.

Conclusion

KBKG, Engineered Tax Services and CSSI are the strongest picks for Illinois investors, with Leyton, R.E. Cost Seg and O’Connor close behind.

For related lists, see our Chicago real estate CPA firms, cost segregation firms in Michigan and multifamily cost segregation firms.