The 40-Year IRS History That Explains Why the Cost Segregation Audit Techniques Guide Works the Way It Does

I spent 90 minutes on the phone with James C. Peacock in June 2026. James was an IRS General Engineer for 38.5 years. He was among the first IRS engineers to examine cost segregation. He contributed to the Cost Segregation Audit Techniques Guide from its original development through the February 2025 update. He trained roughly 200 IRS engineers in cost seg and Section 179D before he retired in September 2025.

Most people who write about the ATG have read it. James helped write it.

This article covers what he told me about where the ATG came from, why it was built the way it was, and what that history means for any property owner getting a cost seg study done today.

Before Cost Segregation, There Was the Investment Tax Credit

Most people think of cost segregation as a depreciation strategy. It is. But its roots are in a different tax benefit entirely: the Investment Tax Credit.

The ITC was a direct tax credit, not a deduction, available on certain tangible personal property used in a trade or business. It was enacted in 1962, suspended, reinstated, expanded, and modified several times through the 1970s and early 1980s. At various points the credit ran from 6 to 10 percent of the cost of qualifying property.

To claim the ITC, a taxpayer had to identify which components of a building qualified as personal property under Section 1245, not structural components. The same classification question that sits at the center of cost segregation today sat at the center of ITC claims forty years ago.

The IRS noticed. From the mid-1970s forward, the agency deployed engineers specifically to examine ITC claims. At the peak, James told me, there were roughly 300 IRS engineers working ITC examinations. These were not revenue agents guessing at property classifications from a desk. These were engineers with construction backgrounds doing physical inspections and applying construction cost databases to determine what qualified.

That is the institutional knowledge that eventually became the ATG.

Two Decades of Contested Litigation

ITC examinations were contentious. The IRS and taxpayers fought over the same classification disputes that come up in cost seg audits today: What is a structural component? When does a specialty electrical panel belong to the building versus the equipment it serves? Does a partition wall move freely enough to qualify as personal property?

These disputes generated a significant body of case law. The major court cases in cost segregation history that practitioners cite today, including the cases that define what personal property means in a real estate context, were largely decided during the ITC era. The White Coat test for movability. The Scott Paper rule on dedicated electrical panels. The Amerisouth ruling on kitchen cabinets. These cases were not abstract. They came out of real audits where the IRS and taxpayers went to court.

James saw a lot of that litigation from inside the agency. He started at the IRS in 1986, during the middle of the ITC examination period. The ITC itself was largely phased out by the Tax Reform Act of 1986 for most property, but the examinations of prior-year claims continued for years after that. Engineers were still working ITC issues through the mid-1990s.

"The methodology we used on ITC examinations was the same methodology. We were doing construction cost allocation. We were looking at the same types of property. We were asking the same questions about what qualified and what didn't."

— James C. Peacock, former IRS Engineer SME

The ITC and cost segregation shared a technical foundation. The difference was the tax benefit at stake: a direct credit for ITC versus accelerated depreciation for cost seg. But the engineering work was nearly identical.

The HCA Decision and What It Settled

The Hospital Corporation of America case, decided in 1997, was the inflection point.

HCA was a major ITC case involving a large healthcare operator with extensive real property holdings. The dispute covered the classification of building components across multiple hospital facilities. The resolution validated the engineering-based methodology that IRS engineers had been using and that taxpayers' engineers had been using on their side of the same disputes.

In practical terms, HCA closed the era of contested ITC litigation. With the ITC itself largely gone and the landmark classification dispute resolved, the IRS no longer had hundreds of engineers working a dwindling portfolio of ITC examination cases.

At the same time, cost segregation was starting to emerge as a formal practice. The same engineering methodology that had been applied to ITC claims was now being applied to depreciation on newly placed-in-service property. The IRS engineers who had spent years on ITC work recognized it immediately.

James presented at the 1997–98 IRS Engineers National Convention on the CSI MasterFormat and cost segregation methodology. That presentation was an early attempt to translate the institutional knowledge accumulated during the ITC era into a framework for examining the next generation of property classification claims. The audience was IRS engineers who had worked ITC and were now seeing cost seg studies come across their desks.

Building the ATG: 2000 to 2002

Cost segregation grew rapidly in the late 1990s and early 2000s. Engineering firms that had been doing ITC work pivoted to cost seg. New firms entered the market. The IRS started seeing cost seg studies in audits with more regularity, and the quality varied widely.

The agency decided it needed a formal examination guide. Not an internal memo. A comprehensive technical document that would give IRS examiners a consistent standard against which to evaluate any cost seg study they encountered.

James was part of the team that built it. The development ran from approximately 2000 to 2002. The group included more than a dozen IRS engineers and attorneys. They drew on the case law from the ITC era, the engineering methodology refined over two decades of examinations, and the accumulated experience of engineers who had spent careers doing this work.

"We had people who had worked ITC for years. We had attorneys who had litigated the cases. The ATG was not someone sitting down and writing what they thought the rules should be. It was a documentation of what the law actually was, built from decades of examination experience."

— James C. Peacock, former IRS Engineer SME

The ATG was released publicly in 2004. James contributed to every major update that followed, including the February 2025 update that is the current version. The core methodology has remained consistent. What changes between updates are refinements to reflect new court decisions, legislative changes like bonus depreciation and the QIP fix in 2020, and the addition of new property types that come up in examinations.

The February 2025 update added guidance on short-term rental property classification, Section 179D updates, and the treatment of AI-generated cost seg studies.

The CSI MasterFormat Connection

One thing most investors do not realize: the standardized cost reporting system that underpins every quality cost seg study has nothing to do with cost segregation. It predates it by decades.

The CSI MasterFormat is the construction industry's standard system for organizing project specifications and cost data. It was developed by the Construction Specifications Institute. The divisions organize construction work into logical categories: concrete, masonry, mechanical, electrical, and so on. RS Means, the construction cost database that every serious cost seg study references, is built on the same organizational framework.

IRS engineers adopted MasterFormat for ITC examinations because it provided a common language between the engineer doing the study and the engineer reviewing it. When both sides are working from the same cost database with the same organizational structure, classification disputes become resolvable. You can point to a specific line item and debate whether it belongs in one category or another. Without that shared framework, the conversation has no common ground.

James's 1997 convention presentation on CSI MasterFormat was essentially an argument that the IRS needed to standardize on this framework for cost seg examinations the same way it had for ITC. That argument won. The ATG's guidance on engineering methodology reflects the MasterFormat approach, which is why IRS examiners today look for 12- or 16-digit RS Means codes in cost seg reports. Vague references to "mechanical" without a code generate Information Document Requests. Specific codes demonstrate that the study is grounded in actual construction cost data, not estimates pulled from thin air.

What the History Tells You About How Audits Work Today

Understanding that the ATG was built from 40 years of examination experience changes how you read it. Every chapter covers a classification issue that came up repeatedly in ITC and cost seg audits. The IRS did not write the ATG speculatively. They wrote it from an accumulated record of what taxpayers get wrong.

The land allocation check comes first in an examination because missing land was a recurring problem in ITC studies. Engineers would allocate costs meticulously across building components and forget that the land has to come out of the purchase price first. An acquisition price of $1 million with a study that adds up to $1 million tells the IRS examiner immediately that something is wrong. That is an automatic adjustment with no room to negotiate.

The 1245/1250 balance check exists because studies repeatedly overweighted personal property while treating structural components as an afterthought. James reviewed a high-rise study where the personal property allocation looked defensible on the surface, but when he examined the 1250 side of the ledger, the structural steel framework and concrete slabs were missing. Tens of millions of dollars in structural components had been omitted. That generated an automatic adjustment on the entire 1245 position, not just the missing items.

The guidance on contingency fees exists because the IRS saw studies where the fee structure created an incentive to classify aggressively. An examiner who sees a contingency fee engagement agreement in the first IDR is going to look harder at every classification in that study.

None of this is random. Every item the IRS engineer looks for in a cost segregation audit traces back to something that went wrong in a real examination. The ATG is not a list of rules the IRS invented. It is a record of the patterns that generate adjustments.

The Mantra That Ran Through 38 Years of Examinations

James came back to one phrase several times in our conversation. It was a saying that circulated among IRS engineers throughout his career. He heard it from colleagues when he started and repeated it to the engineers he trained before he retired.

"It's the support, not the report."

— IRS engineer mantra, as cited by James C. Peacock

The report is the cost seg study document. The support is everything behind it: the contractor's final application for payment, the plans and specifications, the site visit notes, the RS Means source data with specific line item codes, the documentation of how each classification was determined.

A well-formatted report with defensible-looking numbers and vague sourcing fails the same test a sloppy report fails. An IRS examiner who sends an IDR asking for contractor invoices and plans and specs and gets a cost seg study in response has not received the support. They have received the report again. That generates more IDRs. More IDRs means a longer, harder audit.

This principle was true during ITC examinations. It was true when the ATG was being written. It is still true today. James trained roughly 200 IRS engineers on cost seg and 179D before he retired. The engineers entering IRS service now have the same training he received and the same documentation standards. The bar has not dropped.

The Audit Rate Reality

James shared the audit rate data from IRS Statistics of Income. The average business audit rate runs around 0.07 percent. For businesses with a cost seg study, the rate is approximately 0.078 percent.

"Two out of a thousand instead of one," he told me. "It doubled. But it's still very, very, very low."

Cost seg does not trigger audits. The IRS does not scan returns for cost seg studies and flag them. Studies get discovered inside audits that are already open, triggered by DIF scores and IRS examination campaigns. By the time a cost seg study comes to an examiner's attention, the audit was already happening for a different reason.

The risk is not getting audited. The risk is getting audited and having a study that does not hold up. That is where the history matters. The ATG was built from examination experience specifically to give examiners a consistent standard. A study built to meet that standard has 40 years of institutional knowledge working in its favor. A study that shortcuts the documentation has 40 years of examination patterns working against it.

Why This History Matters When Choosing a Provider

The cost seg industry ranges from engineering firms with decades of examination experience to software products that generate allocation reports without a site visit. The ATG explicitly addresses this: studies require a physical inspection of the property. An engineer who has not walked the building has not done the work the ATG was designed to evaluate.

James's view on AI-generated cost seg studies was direct. They "go against every IRS rule" because no physical inspection is possible. But he sees a legitimate use for AI as a completeness check. Studies repeatedly undervalue 1250 property because engineers focus on identifying 1245 property. AI can flag when the structural component estimate looks too thin relative to the overall property value. That is a quality control function, not a classification function.

Square footage models present the same problem as AI-generated studies, for the same underlying reason. They tell an IRS examiner that the preparer used industry averages rather than examining the actual property. That generates IDRs. Property-specific RS Means codes with specific line item references demonstrate the opposite: that someone actually looked at the building and applied construction cost data to what they found.

For more on what an IRS engineer checks when a cost seg study lands in an audit, see our guide to cost segregation audit risk and the major court cases that shaped cost segregation classification rules.

The Practical Takeaway

The ATG is not a bureaucratic document. It is the distilled record of what happened when IRS engineers examined cost seg and ITC claims over four decades. Every standard it sets reflects something that went wrong in a real audit.

A property owner who understands that history asks different questions when evaluating a cost seg study. Not just "what is the total benefit" but:

• Did the preparer conduct a physical site visit?
• Are RS Means codes present for individual line items, or just category references?
• Is the 1250 property estimate complete and internally consistent with the structure type?
• Does the land allocation come out of the purchase price first?
• Can the support documents be produced if an IDR arrives?

James trained 200 IRS engineers on these questions before he retired. The examiners now entering the field learned the same methodology from the same standards. The history is still in the room every time an IRS engineer opens a cost seg study during an audit.

A study built to survive that examination is the only kind worth having.