Cost Segregation After the 2025 Tax Law: A Plain-English Guide for Small Commercial Owners

For years, cost segregation sounded like something for institutional owners with 40-building portfolios and in-house tax teams. If you own a small strip center, a two-story office building, a flex warehouse or a medical office, you may have assumed it wasn't worth the effort.

The 2025 tax law changed that math. Between permanent 100% bonus depreciation and a much larger Section 179 limit, small commercial owners now have more ways to deduct building costs early than at any point in years. Here's what changed, what qualifies, and how to tell whether a study makes sense for your property.

What Changed in 2025

The One Big Beautiful Bill Act, signed July 4, 2025, made three changes that matter to commercial owners:

1. 100% bonus depreciation is permanent. It applies to qualifying property acquired and placed in service after January 19, 2025. Property bought under a binding written contract signed on or before that date still falls under the older phase-down rates. The IRS laid out the details in Notice 2026-11, summarized by BDO.
2. Section 179 more than doubled. The expensing limit rose to $2.5 million for 2025, phasing out once qualifying purchases exceed $4 million. With inflation adjustments, the 2026 figures are $2.56 million and $4.09 million.
3. Certainty. Before 2025, bonus depreciation was shrinking every year, so owners had to rush. Permanence means you can plan acquisitions and improvements around your business, not a sunset date.

How Cost Segregation Works on a Commercial Building

The IRS depreciates a commercial building over 39 years. That's slow: on a $1.2 million building, it's about $30,800 a year.

But not everything you paid for is "building." A cost segregation study, typically prepared by engineers and tax specialists, separates out components with shorter lives:

Anything in the 5-, 7- and 15-year classes qualifies for 100% bonus depreciation, so it can be deducted in the year the property is placed in service. Retail centers with large parking fields, medical offices with specialized plumbing and electrical, and buildings with heavy tenant finishes often see the largest reclassifications.

The study needs to hold up to scrutiny. The IRS publishes a Cost Segregation Audit Techniques Guide describing what examiners expect: documented methodology, cost allocations tied to real data, and usually a site inspection.

Three Tools Small Commercial Owners Can Combine

Commercial owners have options that residential landlords don't. Used together, they cover most of what you spend on a building.

1. Cost segregation plus bonus depreciation. Best at acquisition or after a major renovation. The study identifies short-life components; bonus depreciation writes them off in year one.

2. Qualified improvement property (QIP). Interior improvements to a nonresidential building made after the building was first placed in service, such as new walls, ceilings, flooring, lighting and tenant build-outs, are generally 15-year property and eligible for bonus depreciation. Enlargements, elevators and escalators, and the internal structural framework don't qualify. If you regularly build out space for new tenants, QIP can turn those costs into current deductions.

3. Section 179 for roofs, HVAC, fire and security systems. For nonresidential property, Section 179 can cover roofs, HVAC systems, fire protection and alarm systems, and security systems, as well as QIP. Two limits matter: the deduction can't exceed your business taxable income (unused amounts carry forward), and rental owners should confirm with their CPA that their activity qualifies as a trade or business.

The practical difference: bonus depreciation can create a loss, while Section 179 is capped by income but lets you choose exactly how much to expense. Many owners use bonus depreciation for components identified in a study and Section 179 selectively for a specific roof or HVAC replacement.

Is a Study Worth It for a Small Building?

Studies aren't free. Fees typically range from about $2,000 for a simple desktop report to $15,000 or more for a detailed engineering study on a complex property, and a common rule of thumb is that a full engineering study starts to clearly pay off around $500,000 of building basis.

Here's a simplified example for a small strip center (it ignores the mid-month convention and state tax):

That's about $234,000 of additional first-year deductions. If the owner can use them at a 35% federal rate, it's roughly $82,000 of tax deferred into later years, against a study fee in the low five figures.

Three questions decide whether those numbers are real for you:

• Can you use the deduction now? Rental losses are often passive and may only offset passive income. If your losses would be suspended, the benefit is delayed.
• How long will you hold? When you sell, depreciation on short-life components is generally recaptured as ordinary income. Longer holds, or a 1031 exchange into similar property, make the trade more favorable.
• Is this a high-income year? Deductions are worth the most when your marginal rate is highest.

Timing: The Date That Matters

Bonus depreciation follows the date property is placed in service, meaning ready and available for its intended use, not the contract or closing date. A stabilized building is usually placed in service at closing. A renovation or tenant build-out counts when the space is ready for occupancy.

That makes the fourth quarter important. If you're closing a purchase or finishing a build-out late in the year, coordinate your closing, contractor and cost segregation firm early. Our year-end playbook walks through the timing in detail.

If you already own a building that never had a study, you haven't missed out. A look-back study can catch up depreciation from prior years through a change in accounting method, generally without amending old returns. The bonus rate depends on when you acquired the property.

Common Mistakes Small Owners Make

• Using a template instead of a study. Percentages pulled from the internet won't survive an audit.
• Ignoring replaced components. When you replace a roof or HVAC unit, you may be able to write off the remaining value of the old one through a partial disposition election. Skip it, and you keep depreciating something you've already thrown away.
• Forgetting the exit. Plan for recapture when you buy, not when you list.
• Assuming Section 179 works like bonus depreciation. Section 179 is limited by business income; bonus depreciation isn't.
• Waiting until April. By then the year's decisions are already made.