What it is
TL;DRA cost segregation study is an engineering analysis that breaks a building's depreciable basis into its real components. Land improvements (15-year), tangible personal property (5- or 7-year), and structural building (39-year) get separated. The shorter-life categories qualify for bonus depreciation and accelerate massive deductions into the first year.
On a $4M building, a study typically reclassifies $800k-$1.4M into 5/7/15-year buckets. Most of that becomes first-year deduction under §168(k) bonus depreciation.
The strategy works on newly purchased buildings, ground-up construction, and look-back basis through Form 3115, meaning a building bought in 2018 can still get its catch-up depreciation in a current year.
Who it's for
Owners of commercial real estate, short-term rentals, and large residential rental portfolios with material taxable income to offset.
- ✓Buyer of $1M+ commercial building with $200k+ of taxable income
- ✓Short-term rental owners under the STR loophole
- ✓Real estate professionals (REPS-qualified) with rental losses to free up
- ✓Owners who placed a building in service in the last 5+ years and never accelerated
- ×Personal residences (depreciation is not allowed)
- ×Buildings under $500k where study cost ($5k-$15k) eats the benefit
- ×Owners with no taxable income to absorb the deduction
How it works
1. Engage a qualified engineering firm
The IRS expects an engineering-based study, not a tax-preparer's spreadsheet. Reputable firms use the IRS Cost Segregation Audit Techniques Guide as their playbook.
Study cost: typically $5,000-$15,000 depending on building size and complexity.
2. Reclassify components
Land improvements (parking lots, landscaping, exterior lighting) → 15-year.
Personal property (decorative lighting, cabinetry, carpet, specialty electrical) → 5- or 7-year.
Structural (walls, roof, HVAC, plumbing) → 39-year commercial or 27.5-year residential rental.
3. Apply bonus depreciation to the short-life buckets
§168(k) bonus depreciation: 60% in 2024, 40% in 2025, 20% in 2026, 0% in 2027, unless Congress extends. Recent legislative proposals have repeatedly extended; assume nothing.
The Tax Cuts and Jobs Act made used property eligible for bonus depreciation. This is the rule that makes look-back studies on existing buildings so valuable.
4. For prior-year property, file Form 3115
If you placed the building in service in a prior year and never did cost seg, a Form 3115 method change lets you take a §481(a) catch-up adjustment in the current year, capturing every missed dollar of accelerated depreciation since acquisition.
A worked example
Diego buys a $3,200,000 medical office building in 2026 ($2,800,000 building, $400,000 land). His other businesses generate $620,000 of taxable income. Bonus depreciation rate in 2026: 20% (assume no extension).
| Straight-line, no cost seg | Cost seg + 20% bonus | |
|---|---|---|
| Reclassified to 5/7/15-year | $0 | $780,000 |
| First-year bonus depreciation | $0 | $156,000 |
| First-year straight-line on reclassified | $0 | $78,000 |
| First-year regular building depreciation | $72,000 | $52,000 |
| Total first-year deduction | $72,000 | $286,000 |
| Incremental year-1 deduction | $214,000 |
At Diego's 37% federal + 4.65% Utah blended rate, the extra $214k of first-year deduction saves about $89,000. Study cost: $9,500. Net first-year benefit: roughly $80,000.
Limits and risks
Passive activity rules. Cost seg generates losses that for many investors are passive and can only offset passive income, unless you qualify under REPS or the STR loophole.
Depreciation recapture at sale. The accelerated dollars come back as §1245 recapture taxed at ordinary rates (capped at 25% for §1250 unrecaptured gain). Cost seg is a deferral, not a permanent saving, though deferral plus 1031 exchange can stretch indefinitely.
Bonus depreciation is phasing out. A study completed in 2026 captures 20% bonus; in 2027, 0% unless extended. Time matters.
Studies done by tax preparers without engineering credentials are the audit magnet. Use a firm that produces a defensible study following the IRS audit techniques guide and your audit risk is modest.
IRS reference
- IRC §168(k)Bonus depreciation, phasing down from 60% in 2024 to 0% in 2027 unless extended.
- IRC §1245 / §1250Definitions of personal property vs. structural components.
- Rev. Proc. 87-56Class lives and recovery periods for depreciable property.
- Rev. Proc. 2015-13Automatic accounting method changes, how to make up missed depreciation via Form 3115.