Short answer

Cost segregation is the practice of classifying the parts of a building into the depreciation classes the law assigns, and its history is the history of two questions: what counts as tangible personal property rather than a structural component, and how fast each class may be recovered. The first question was worked out under the investment tax credit in the 1960s and 1970s, when the distinction decided whether an item earned a credit. The second was reset by the accelerated cost recovery system in 1981 and its 1986 successor, MACRS, which set the long recovery periods for buildings. The Tax Court's 1997 decision in Hospital Corporation of America joined the two by allowing the old investment-credit tests to classify building components under MACRS. The IRS answered with an Audit Techniques Guide in 2004, revised several times through February 2025. Bonus depreciation, first enacted in the early 2000s and expanded in 2017, made the classification worth more; the 2025 law made the 100 percent rate permanent. This article tells that story from the sources. It is general education, not tax advice.

Timeline

PeriodWhat changedWhy it matters to the practice
1962 onwardThe investment tax credit rewarded investment in tangible personal property; courts and regulations developed tests to separate such property from structural components of buildingsThe classification framework, decided item by item on function, was born here
1981The accelerated cost recovery system replaced useful-life depreciation with fixed recovery periods and composite treatment of buildingsComponent-by-component depreciation of a building was no longer the general rule
1986MACRS set the modern classes: 27.5-year residential rental property, 39-year nonresidential real property after 1993, and 3-, 5-, 7-, 10-, 15- and 20-year property; the investment tax credit was largely repealedThe long building periods created the incentive to identify shorter-lived property
1997Hospital Corporation of America v. Commissioner: the Tax Court held that the investment-credit tests could be used to classify building components as section 1245 property under MACRSThe legal foundation of modern cost segregation
2004The IRS issued the Cost Segregation Audit Techniques Guide for examinersThe practice acquired a published standard of what a quality study contains
2002 to 2017Bonus depreciation enacted, lapsed and re-enacted at various rates; in 2017 set at 100 percent for qualified property, including used property, with a phase-down scheduled from 2023The value of classifying property into short classes rose sharply
2020Qualified improvement property, meant to be 15-year, was confirmed as 15-year property after a drafting error had left it at 39 yearsInterior improvements to nonresidential buildings became bonus-eligible
2025The One, Big, Beautiful Bill made the 100 percent rate permanent for qualified property acquired after January 19, 2025; the IRS revised the audit guide in February and issued Notice 2026-11 in January 2026The current environment

The investment credit and the classification question

The distinction between tangible personal property and a building's structural components did not begin with depreciation. It was developed under the investment tax credit, where the question was whether an item earned the credit. The regulations and cases of that era asked whether an item was permanently attached, whether it served the operation of the building or the business conducted in it, and how easily it could be removed. The IRS guide's chapter on the legal framework traces those tests and the case law that applied them, which is why a modern study's legal analysis still speaks the language of the 1970s.

ACRS and MACRS: the recovery periods

The 1981 system replaced facts-and-circumstances useful lives with fixed recovery periods and treated buildings as composite assets, which ended the earlier practice of depreciating a building's components separately. The 1986 system, MACRS, set the classes still in force: residential rental property at 27.5 years, nonresidential real property at 39 years for property placed in service after mid-1993, and the shorter classes into which land improvements and personal property fall. Publication 946 reproduces the class lives and recovery periods. The long building periods are the reason a study is worth doing; the short classes are where its results land.

Hospital Corporation of America

In 1997 the Tax Court decided a dispute over a hospital company's classification of components of its buildings. The court held that the taxpayer could use the investment-credit tests to determine whether items were section 1245 property for depreciation purposes under MACRS, and then applied those tests item by item, permitting personal-property treatment for some items and denying it for others. The IRS later acquiesced in the result. The decision is the reason a study classifies by function rather than by building system, and the reason its legal analysis cites the investment-credit cases.

The audit guide

The IRS issued its Cost Segregation Audit Techniques Guide in 2004 to help examiners review studies. Its chapters describe the legal framework, the methodologies the IRS sees, from detailed engineering studies from actual records to rule-of-thumb allocations, the principal elements of a quality study, and how examiners review one. The guide has been revised several times, most recently in February 2025. It is the closest thing to a published standard for the practice and the document every practitioner should read in each revision.

Bonus depreciation and the value of classification

Additional first-year depreciation entered the Code in the early 2000s at partial rates, lapsed and returned repeatedly, and in 2017 was set at 100 percent for qualified property, extended to used property, with a phase-down scheduled to begin in 2023. Because bonus depreciation applies to property with a recovery period of 20 years or less and not to buildings, the classification a study performs became the gate to a first-year deduction rather than a timing difference spread over years. In 2020 the qualified improvement property category, intended as 15-year property, was confirmed as such after a drafting error. In 2025 the 100 percent rate was made permanent for qualified property acquired after January 19, 2025, with IRS guidance following in January 2026.

What the history teaches a practitioner

  • The classification question is old and case-driven; learn the cases, not just the classes.
  • The recovery periods are statutory and change rarely; bonus depreciation changes often. Keep the two straight in your models.
  • The IRS guide is a living document; each revision is a competence event.
  • The practice was built by taxpayers who documented their positions and won on the evidence. The elements the guide lists are the elements that won.

What people ask on Reddit and other forums

History comes up in "cost segregation reddit" threads on r/tax and r/Accounting when someone asks whether the whole thing is new or a gimmick. The searches that lead people to these threads are usually phrased "history of cost segregation", "Hospital Corporation of America cost segregation" and "component depreciation".

"Is cost segregation a new loophole?" No. The classification tests date to the investment credit era, the Tax Court applied them to depreciation in 1997, and the IRS has published a guide since 2004.

"Didn't component depreciation get banned?" The 1981 system ended the general practice of depreciating a building's components separately. The 1997 decision held that section 1245 property identified under the investment-credit tests is not part of the building for that purpose.

"Why does everyone cite a hospital case?" Because Hospital Corporation of America is the decision that joined the old classification tests to MACRS.

"When did 100 percent bonus start?" At 100 percent in 2017 for qualified property, with a phase-down from 2023, then permanently at 100 percent for property acquired after January 19, 2025 under the 2025 law.

Questions people also ask

When did cost segregation start?

The classification framework dates to the investment tax credit era; modern practice under MACRS dates to the Tax Court's 1997 decision in Hospital Corporation of America.

What is the Hospital Corporation of America case?

A 1997 Tax Court decision holding that investment-credit tests could be used to classify building components as section 1245 property for depreciation, decided item by item.

When did the IRS issue the cost segregation audit guide?

In 2004, with revisions since, most recently in February 2025.

Is component depreciation still allowed?

Not in the pre-1981 sense for a building's structural components. Property that is section 1245 property under the classification tests is depreciated in its own class.

What did the 2025 law change?

It made the 100 percent bonus depreciation rate permanent for qualified property acquired after January 19, 2025.

Sources