Cost Segregation Fundamentals: How the Study Actually Works

#Why default depreciation costs you money

When you buy a rental property, the IRS makes you depreciate the building, not the land. The schedule is 27.5 years if it’s residential, or 39 years if it’s commercial. That straight-line schedule gives you a small deduction each year.

Example: $500K residential rental, $400K building / $100K land.

• Default depreciation: $400K ÷ 27.5 = ~$14,545/yr for 27.5 years

That’s it. It’s slow. It doesn’t match how a building’s parts actually wear out.

#What cost segregation does

Cost segregation breaks the building into its actual parts. Each part gets the depreciation life the IRS says it should have:

A qualified cost segregation study sends an engineer (or other qualified specialist) to inspect the property. They allocate the basis among the parts. Here’s a typical allocation for a $500K residential property:

In this example, 100% bonus depreciation lets you deduct the $60,000 of land improvements and the $80,000 of personal property in year one. Add about $9,000 for the building, and year one comes to near $149,000. A normal schedule, with no study, gives you about $14,500.

#The bonus depreciation lever

The Tax Cuts and Jobs Act (2017) set bonus depreciation at 100% for property with a recovery period of 20 years or less. Then it started to phase down 20 points a year, starting in 2023 (80% in 2023, 60% in 2024, 40% in 2025). The One Big Beautiful Bill Act (OBBBA) ended that phase-down. For qualified property acquired and placed in service after January 19, 2025, bonus depreciation is back to 100%, permanently. That means everything classified as 5, 7, or 15-year property gets fully deducted in the year you place it in service. It no longer rides a declining percentage.

#When cost seg makes financial sense

A cost seg study costs $4,000-$15,000. The price depends on property type, complexity, and study depth. For the study to be worth it, the front-loaded depreciation has to deliver real present-value tax savings.

Quick break-even math:

• Property basis: $500K
• Cost seg incremental year-1 deduction: ~$60K (over default)
• Taxpayer marginal rate: 32%
• Year-1 tax savings: $60K × 32% = $19,200
• Study cost: $5,000
• Net year-1 benefit: $14,200

For a single $500K property, cost seg pays for itself in year one, if you have income to offset.

Property size where cost seg starts to make sense:

• Residential rental: $300K+ basis (smaller properties: math gets tight)
• Commercial property: $500K+ basis
• Look-back study (Section 481(a)) on previously placed-in-service property: $400K+ basis

For a closer look, see what drives the price of a study, the full break-even math, and what can go wrong with a do-it-yourself study.

#How to use the deduction

Cost seg creates a big paper loss. You can only USE that loss if you have other income to offset it:

Passive income (other rentals’ income, syndication K-1 income): say you actively take part in your rentals and your income is under $150,000. Then you may use up to $25,000 of rental loss against other income. The allowance phases out from $100,000 to $150,000 of income. Active income (W-2, business income, ordinary 1099): you can’t offset this without one of two unlocks:

Unlock 1: STR Loophole. Short-term rentals (average stay ≤7 days) with material participation are not passive activities under Section 469. Losses offset W-2 income directly. See our full guide.

Unlock 2: Real Estate Professional Status (REPS). If you meet the REPS tests and also materially participate in your rentals, the losses are no longer passive. Other limits still apply, including a yearly cap on business losses. See our REPS guide.

If you DON’T have one of these unlocks, the cost-seg-accelerated loss carries forward as suspended passive loss. It waits there until you have passive income to offset it, or you sell the property.

#What a study includes

A qualified cost segregation study delivers:

1. Site inspection (in-person or documented virtual)
2. Component-level basis allocation: every depreciable component identified, classified, and assigned its share of basis
3. Bonus depreciation schedule: year-by-year depreciation accounting for current law
4. Written engineering report: 50-100 page document with methodology, citations, IRS-defensible
5. Tax preparer integration: output flows directly to Form 4562 + Schedule E
6. STR / REPS positioning memo (where applicable), documenting the non-passive characterization

For a $500K residential property, the in-house version of the study takes about 3-4 weeks. For a $1M+ commercial property, we work with engineering-firm partners, and the study takes 6-10 weeks.

#Look-back studies (Section 481(a))

If you’ve owned the property for 1-15+ years and never ran cost seg, a look-back study changes how you depreciate it. You file Form 3115 with your return. Under Section 481(a), you get the whole catch-up amount as one deduction, in that one year, without amending prior returns.

Say you bought a $500K property in 2018 and missed 7 years of accelerated depreciation. The catch-up deduction often runs $40K-$80K. It’s a one-time event that closes the gap between what you depreciated and what you should have.

#Common questions

Does cost segregation create audit risk? Properly-done cost seg studies are well-supported in IRS guidance (Cost Segregation Audit Techniques Guide). The IRS expects engineering-grade studies with documented methodology. Cheap “online cost seg calculators” without engineering backup do create risk.

What about depreciation recapture when I sell? Recapture applies at sale, no matter whether you ran cost seg. The personal-property and land-improvement parts get Section 1245 recapture at ordinary rates. The gain on the building that comes from past depreciation is taxed at up to 25% as unrecaptured section 1250 gain. A 1031 exchange can delay much of the tax. Some recapture on the 5 and 7-year parts may still be due if the new property doesn’t have similar parts. Plan this with your preparer before you sell.

Can I do cost seg on a primary residence converted to rental? Yes. Once you convert it to a rental, the property is depreciable, and cost seg applies. The basis is the lower of its fair market value at conversion or your adjusted cost basis.

Does cost seg work on a property I’m flipping? No. A property held for sale (a flip) isn’t depreciable. It’s inventory. Cost seg needs the property to be a depreciable asset, used as a rental or for business.

Can I do cost seg myself? In theory, yes. In practice, no. The IRS expects a study to follow set methods and stay well documented. A study done without that support is easier to challenge. The savings far outweigh the study’s cost.

What’s the difference between cost seg and bonus depreciation? Cost segregation is the engineering study that reclassifies property into shorter recovery periods. Bonus depreciation is the IRS rule that lets you fully deduct property in year one. It applies to property with a recovery period of 20 years or less. They work together: cost seg creates the eligible buckets, and bonus depreciation speeds up the deduction.

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If you own a rental property that hasn’t had cost seg run, the Discovery call is the right next step. We model the year-one savings + study fee against your specific property and quote the engagement.