What is cost segregation, in plain language

The one-line problem

When you buy a building, your depreciation schedule usually shows two lines: land, which never depreciates, and everything else, which sits on a single line and depreciates over 39 years for commercial property or 27.5 for residential rental.

That single line is a convenience, not a fact. Physically you bought hundreds of distinct assets, and the Internal Revenue Code has always recognised that they have different lives. Carpet is not a roof. A parking lot is not a foundation. The dedicated circuit feeding a walk-in cooler is not the building's electrical service.

What the study actually does

An engineering-based cost segregation study identifies each component, establishes what it cost, classifies it under the correct asset class, and documents the reasoning well enough that a third party can follow it years later.

The output is a revised depreciation schedule in which some of your basis sits at 5 years, some at 7, some at 15, and the rest stays on the building. Nothing is invented. The components were always there; the study is the evidence that assigns them correctly.

Where the money comes from

Two places. First, timing: a deduction taken in year one is worth more than the same deduction spread across four decades, and you can deploy the cash you keep.

Second, and much larger since 2025: bonus depreciation. Property with a recovery period of 20 years or less is eligible for 100% first-year expensing, which is permanent again for property acquired and placed in service after 19 January 2025. A building does not qualify. The 5-year finishes and 15-year parking lot inside it do. The study is what moves basis from the ineligible bucket to the eligible one.

A worked shape

Take a $3,000,000 depreciable basis on a commercial building. Without a study, first-year depreciation is roughly $64,000 after the mid-month convention.

With a study reclassifying 25% into short-life classes and 100% bonus applying to that portion, the first-year deduction is roughly $806,000. At a 35% combined rate, that is about $260,000 of tax deferred into year one instead of trickling out over thirty-nine years.

What it is not

It is not a permanent tax saving. Section 1245 components recapture as ordinary income on a taxable sale, and the building portion produces unrecaptured section 1250 gain capped at 25%. It is a timing and rate benefit, which is usually a very good one, and occasionally is not.

It is also not free money if you cannot use the deduction. Passive activity rules limit rental losses for many owners, and a large accelerated loss that suspends for six years is worth much less than the headline number suggests. Any firm that will not raise this with you before taking your fee is not being straight with you.