Top Cost Segregation Firms In Washington State For 2027

Last updated: November 12, 2026

The best cost segregation firms in Washington State for 2027 are Kidder Mathews, CostSeg Northwest and Moss Adams, followed by Engineered Tax Services, R.E. Cost Seg and O’Connor. Kidder Mathews leads for a Seattle commercial real estate firm with in-house engineers, CostSeg Northwest for a local specialist working since 1994, and Moss Adams for a Seattle accounting firm with a dedicated cost segregation team.

Washington investors own Seattle apartments and office, Eastside and Tacoma multifamily, Spokane rentals and industrial property along the I-5 corridor. 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.

Washington has no personal or corporate income tax, so there is no state addback. The state does have a business and occupation tax and a capital gains tax that work differently, so the benefit of a study is mainly federal. Confirm your situation with your CPA.

Washington State Cost Segregation Firms At A Glance

Firm	Location	Founded	Specialty	Best For
1. Kidder Mathews	Seattle, WA	1965	Commercial real estate firm with an engineering cost seg team	Commercial owners
2. CostSeg Northwest	Seattle, WA	1994	Cost segregation only	Local owners
3. Moss Adams	Seattle, WA	1913	West Coast accounting firm	Larger owners
4. Engineered Tax Services	West Palm Beach, FL	2001	Licensed engineering firm for specialty tax	Owners of all types
5. R.E. Cost Seg	Houston, TX	2022	Investor-focused studies at two depths	Multifamily investors
6. O’Connor	Houston, TX	Not published	Property tax and cost segregation services	Owners who also appeal taxes
7. Cost Seg Smart	Los Angeles, CA	2024	Residential and commercial studies with audit support	Rental investors
8. KBKG	Pasadena, CA (seven US offices)	1999	Tax credits and incentives with engineered studies	Larger owners
9. CSA Partners	Lindon, UT	Not published	Cost segregation only	Western owners
10. Maven Cost Segregation	Offices in MI, UT, CO, PA and San Diego	Not published	In-person inspections for residential and commercial	Owners who want an inspection

How We Evaluated These Firms

We looked for firms with Washington offices or studies, experience with Seattle’s multifamily and commercial stock, and clear audit support.

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 Washington State For 2027

1. Kidder Mathews — A Seattle Real Estate Firm With In-House Engineers

Kidder Mathews is headquartered in Seattle, and its engineers handle the full study including the on-site inspection. The firm also works in brokerage, valuation and management.

That market knowledge helps on discounted acquisitions.

Best for: Washington commercial owners.

2. CostSeg Northwest — A Seattle Specialist Working Since 1994

CostSeg Northwest is based in Seattle and has focused on cost segregation since 1994.

It suits owners who want a local specialist.

Best for: Washington owners who want a local specialist.

3. Moss Adams — A Seattle Accounting Firm With A Cost Seg Team

Moss Adams was founded in Seattle and has a dedicated cost segregation team.

It suits larger owners who want the study coordinated with the return.

Best for: Larger owners who want a regional accounting firm.

4. Engineered Tax Services — A Seattle Page And Washington Studies

Engineered Tax Services serves Seattle and publishes Washington studies, including a retail property.

As a licensed engineering firm, it suits complex properties.

Best for: Washington owners who want an engineering firm.

5. R.E. Cost Seg — A Published Washington Multifamily Case

R.E. Cost Seg publishes a Washington multifamily case study with $142,762 in first-year savings and includes audit defense.

It suits Washington multifamily investors.

Best for: Washington multifamily investors.

6. O’Connor — Seattle Cost Segregation Plus Property Tax Help

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

It suits owners who want both reviewed.

Best for: Seattle owners who want a tax appeal review.

7. Cost Seg Smart — Seattle Coverage At Low Cost

Cost Seg Smart publishes a Seattle page and offers low-cost studies with written IRS responses included.

It suits single-family rental investors.

Best for: Seattle single-family rental investors.

8. KBKG — National Depth And A Strong Audit Record

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

It suits larger Washington owners.

Best for: Larger owners.

9. CSA Partners — A Western Cost Segregation Specialist

CSA Partners is based in Lindon, Utah and focuses only on cost segregation.

It suits western owners who want a specialist.

Best for: Western owners who want a specialist.

10. Maven Cost Segregation — In-Person Inspections With Audit Documentation

Maven Cost Segregation inspects properties in person and includes audit defense documentation.

It reports more than 5,000 studies delivered.

Best for: Owners who want an in-person inspection.

How To Choose A Cost Segregation Firm In Washington State

Does The Firm Know Washington State Property?

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

How Will Washington State Treat The Deduction?

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

Are You Planning A 1031 Exchange?

Washington investors often exchange. See our Washington 1031 exchange guide.

Is Audit Support Included?

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

Frequently Asked Questions

Does Washington have a state income tax?

No personal or corporate income tax, so the benefit of a study is mainly federal. Other state taxes apply, so ask your CPA.

Is cost segregation worth it in Seattle?

Often yes, because high property values mean large depreciable bases.

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 Washington State 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

Kidder Mathews, CostSeg Northwest and Moss Adams are the strongest picks for Washington investors, with Engineered Tax Services, R.E. Cost Seg and O’Connor close behind.

For related lists, see our Washington 1031 exchange guide, cost segregation firms in California and multifamily cost segregation firms.