If you’ve ever wondered whether a cost segregation study is truly IRS-approved — or whether you should be worried about an audit — this article is for you. The IRS doesn’t just tolerate cost segregation. It has published a 261-page guide telling its own examiners exactly how to evaluate one. That document is called the Cost Segregation Audit Techniques Guide — or ATG — and understanding it can change the way you think about both the strategy and your audit risk.

What Is the IRS Audit Techniques Guide?

The IRS publishes a series of Audit Techniques Guides (ATGs) on specialized tax topics — everything from tanning salons to construction accounting. These guides are written for IRS examiners to use when auditing returns in specific industries or involving specific deductions.

The Cost Segregation ATG is one of the most detailed guides in the entire series. It was first published in 2004 and has been updated several times since, with the most recent edition released in February 2025. It spans 261 pages and includes chapters on legal history, accepted methodologies, documentation standards, property classification rules, and step-by-step examination procedures.

A Brief History: How We Got Here

Cost segregation wasn’t always mainstream. For decades, property owners could only depreciate buildings as a single unit — walls, wiring, flooring, and parking lots all lumped together over 39 years. That changed with a landmark 1997 U.S. Tax Court ruling in Hospital Corporation of America (HCA) v. Commissioner, which confirmed that certain building components could be classified as personal property with shorter recovery periods.

That decision opened the floodgates. Studies grew in popularity through the late 1990s and early 2000s, and the IRS responded in 2004 by publishing the first Cost Segregation ATG — giving both examiners and taxpayers a consistent framework. The guide has since been updated to reflect major tax law changes, including the Tax Cuts and Jobs Act of 2017, the Inflation Reduction Act, and most recently the One Big Beautiful Bill Act (OBBBA) signed into law in 2025, which permanently restored 100% bonus depreciation.

For a detailed look at the historical development of the ATG and accepted methodologies, KBKG’s cost segregation history page is a useful reference.

What the ATG Actually Covers

The guide is organized into several chapters. Here’s a plain-English overview of what each section covers:

• Chapter 1 – Introduction: Explains why cost segregation studies are done, who performs them, and what the IRS’s goals are when reviewing them
• Chapter 2 – Legal Framework: Covers the tax code sections and court cases that give cost segregation its legal footing, including the landmark HCA ruling
• Chapter 3 – Cost Segregation Methodologies: Describes the six accepted approaches for conducting a study, ranked roughly from most to least reliable
• Chapter 4 – Principal Elements of a Quality Study: Lists the 13 specific elements the IRS expects to see in any well-prepared study
• Chapter 5 – Review and Examination: Gives examiners a step-by-step process for evaluating a study when a return is selected for audit
• Chapter 6 – Deferred Issues: Addresses related tax issues like uniform capitalization rules, accounting method changes, and sampling techniques
• Chapters 7–9 – Industry-Specific Guidance: Covers unique considerations for specific property types, including parking structures, electrical systems, and more

The most important chapters for property owners and their advisors are Chapters 3, 4, and 5. Together, they define what a good study looks like — and what the IRS will focus on if your return is reviewed.

The 13 Elements of a Quality Study

Chapter 4 of the ATG is the most referenced section in the entire guide. It lists the 13 principal elements the IRS uses to determine whether a cost segregation study is “quality” — meaning it’s accurate, defensible, and worthy of the deductions it supports.

A study that hits all 13 elements, according to the IRS, “greatly expedites the Service’s review, thereby minimizing the audit burden on all parties.” In other words, a quality study doesn’t just protect you — it actually makes it easier for the IRS to close a review quickly.

For a deeper breakdown of each element, Leyton’s guide to the 13 vital elements and the American Society of Cost Segregation Professionals (ASCSP) are both excellent resources.

Six Accepted Study Methodologies

Chapter 3 of the ATG describes six different methods a cost segregation study can use to classify and cost assets. The IRS does not mandate a single approach — but it ranks them clearly by reliability. Where you land on this spectrum matters a great deal if your return is ever reviewed.

• IRS Preferred
  Detailed Engineering from Actual Cost Records — Uses actual construction invoices, pay applications, and accounting records. The IRS calls this “the most methodical and accurate approach.” Best for new construction where records are complete.
• IRS Preferred
  Detailed Engineering Cost Estimate Approach — Similar to the above, but uses cost estimates (based on RSMeans or other industry data) when actual costs aren’t available. Common for acquired or older properties. Requires strong documentation of the estimation methodology.
• Acceptable
  Survey or Letter Approach — Involves contacting contractors and subcontractors directly to verify what they installed and what it cost. Can be accurate but depends on third-party responsiveness and documentation.
• Acceptable
  Residual Estimation Method — Calculates the cost of short-lived assets and subtracts them from total project cost to determine long-lived property value. Simpler but harder to document and more susceptible to challenge.
• Use with Care
  Sampling or Modeling Approach — Applies results from a model property to similar properties (e.g., a fast-food franchise chain). Can be statistically valid if done properly but is subject to IRS challenge over statistical methodology.
• High Risk
  Rule of Thumb Approach — Applies a fixed percentage (e.g., “25% of this building type is personal property”) with no supporting engineering analysis or documentation. The ATG explicitly warns against this method. Studies relying solely on rules of thumb are flagged as high audit risk.

The ATG is clear: engineering-based approaches carry the most weight. A study built on actual records or detailed cost estimates — prepared by professionals with construction knowledge — is far more defensible than one built on industry averages or software-generated percentages.

Section 1245 vs. Section 1250 Property: Why It Matters

One of the key concepts the ATG focuses on is the distinction between two types of property under the tax code: Section 1245 property and Section 1250 property. Getting this right is the heart of what a cost segregation study does.

Section 1250 property is real property — the building structure itself, including walls, roof, foundation, and other structural components. This is what gets depreciated over 27.5 or 39 years under standard rules. You can’t accelerate this.

Section 1245 property is personal property — assets that are either removable without significant damage to the structure, or that supply specific utilities or services. This category includes things like specialized electrical systems, removable flooring, signage, kitchen equipment, and most items with 5- or 7-year tax lives. These can be accelerated — and under 100% bonus depreciation, they can be fully expensed in Year 1.

Red Flags the IRS Looks For

The ATG is explicit about what makes an examiner take a closer look. These aren’t secrets — the IRS publishes them openly. Knowing them helps you evaluate any firm before you hire them.

The ASCSP’s overview of common misconceptions about cost segregation addresses several of these red flags in detail, as does Scarpello Consulting’s guide to selecting a quality provider.

How an IRS Examination Actually Works

Chapter 5 of the ATG lays out exactly what happens if a return with a cost segregation study is selected for examination. For most property owners, the process is far less frightening than they imagine — especially when the study was done well.

1. Initial Risk Analysis Most examinations end here. The IRS examiner reviews the study and performs a preliminary check for obvious problems: missing documentation, aggressive allocations, unqualified preparers, or flat-percentage rule-of-thumb methods. A quality study typically passes this stage with no further action.
2. Request for Documentation If the examiner proceeds, they’ll request a copy of the full cost segregation report, the letter of engagement, fee arrangement details, and supporting documents like invoices and construction drawings. This is why documentation is so critical.
3. Preparer Interview The examiner may contact the study’s preparer to ask about methodology and classification rationale. This is another reason to hire a firm that stands behind its work and offers audit support.
4. Property Inspection (If Warranted) For complex or high-value properties, an IRS engineer may inspect the property in person to verify the assets identified in the study still exist and are classified appropriately.
5. Findings and Resolution The examiner compares the study’s classifications to IRS-approved asset classes, reviews cost allocations, and prepares a summary. If no issues are found, the matter closes. If adjustments are proposed, the taxpayer has the opportunity to respond.

What This Means for You as a Property Owner

If you’ve been hesitant about cost segregation because of audit concerns, the ATG should actually be reassuring. The IRS has gone out of its way to document exactly what a good study looks like — and that transparency works in your favor when you hire the right firm.

Here’s what to take away from the guide:

• Ask your provider how they address each of the 13 ATG quality elements. A reputable firm should be able to walk through them confidently.
• Avoid any firm using rule-of-thumb percentages without a proper engineering analysis. This is the single biggest red flag in the ATG.
• Ask about contingency fees. The ATG treats contingency-fee arrangements as an audit trigger. Flat-fee studies are more defensible.
• Make sure your provider offers audit support. If the IRS does ask questions, you want the firm that did the study to be available to respond — ideally at no additional cost.
• Keep your documentation. Construction drawings, invoices, and the full study report should be retained for as long as the depreciation deductions remain on your return — and potentially longer.

The bottom line: cost segregation is not a gray area. It’s a clearly established, IRS-approved strategy backed by decades of case law and a 261-page guide written by the IRS itself. Done correctly, it’s one of the most well-documented tax strategies available to real estate and commercial property owners.