The Six Cost Segregation Approaches: What the ATG Recognizes

A cost segregation study is only as reliable as the method used to identify property and allocate its costs. The 2025 IRS Cost Segregation Audit Technique Guide identifies six common approaches used in cost segregation studies, ranging from detailed engineering based on actual cost records to rule-of-thumb methods. The ATG does not require one specific approach, but it explains that some methods produce more accurate and reliable allocations than others. For commercial real estate investors, the important question is not simply which approach was used, but what evidence supports the resulting classification and cost allocation.

Wondering how much tax you could save with cost segregation? Let’s find out. Schedule a Cost Segregation Strategy Call today.

Key Takeaways

• The ATG identifies six common approaches for performing cost segregation, from detailed engineering to rule-of-thumb methods.
• The detailed engineering approach from actual cost records generally provides the most methodical and accurate allocation when reliable construction records are available.
• The ATG does not require taxpayers to use one specific approach, but accuracy and documentation remain critical.
• Different property situations may require different approaches, particularly when comparing newly constructed and acquired properties.
• Engineers evaluate the available evidence, property facts, construction records, cost data, and appropriate estimating techniques before selecting or applying an approach.
• The same property can produce different cost-allocation results depending on the quality of the underlying cost information and methodology.
• The approach determines how evidence is developed, but the property facts and supporting documentation still determine the defensible classification.

What Are the Six Cost Segregation Approaches?

The 2025 ATG identifies six common approaches used to perform cost segregation studies:

1. Detailed Engineering Approach from Actual Cost Records
2. Detailed Engineering Cost Estimate Approach
3. Survey or Letter Approach
4. Residual Estimation Approach
5. Sampling or Modeling Approach
6. “Rule of Thumb” Approach

The ATG notes that other approaches may also be used, although many are derivatives of the approaches discussed in Chapter 3.

The ATG uses the term “approach” to simplify its discussion of methodology. The approach refers to the process used to allocate total project costs among the appropriate assets and property classes.

That distinction matters.

Cost segregation is not simply a calculation that produces a percentage of short-life property.

It is an allocation of property costs supported by engineering analysis, documentation, tax authority, and property-specific facts.

The CostSegRx engineering philosophy follows the same basic principle: begin with engineering, determine what exists and why it exists, understand how it is installed, identify the supporting documentation, and then determine how costs should reasonably be allocated.

How Do the Six Approaches Differ?

The approaches differ primarily in how the property is identified and how its costs are established.

The Detailed Engineering Approach from Actual Cost Records uses contemporaneous construction and accounting records. The ATG describes it as generally the most methodical and accurate approach because it relies on solid construction-cost documentation and minimizes cost estimating.

The Detailed Engineering Cost Estimate Approach follows much of the same engineering process but uses contractor estimates or published estimating data when actual costs are unavailable or insufficient. The ATG emphasizes that estimating data should be clearly referenced and consistently applied.

The Survey or Letter Approach obtains cost information from contractors and subcontractors. The information can then be incorporated into an engineering or residual approach. The ATG cautions that contractor information from other projects may not be comparable or reliable.

The Residual Estimation Approach determines the costs of short-lived assets and assigns the remaining cost to the building or other long-lived property. The ATG describes it as simpler and less time-consuming than engineering approaches, but potentially less accurate.

The Sampling or Modeling Approach uses a representative sample of substantially similar properties to develop a model that can be applied to a larger population. The ATG identifies statistical validity and sampling error as important considerations.

The “Rule of Thumb” Approach relies on fixed percentages, industry averages, or other simplified assumptions. The ATG says the documentation associated with these methods is typically limited and should be examined carefully rather than automatically accepted or rejected.

Does the IRS Require One Specific Approach?

No.

The ATG expressly recognizes that there is no single required approach for performing cost segregation.

That does not mean every approach is equally reliable.

The ATG states that the Service does not prescribe a specific methodology, while also recognizing that approaches based on actual costs or proper estimation techniques generally produce more accurate and reliable allocations.

This is an important distinction for investors.

The question should not be:

“Does the IRS require this exact type of study?”

The better question is:

“Does the methodology produce an accurate, supportable allocation based on the facts and available evidence?”

The ATG emphasizes that cost segregation is factually intensive and that there are no bright-line tests for separating § 1245 property from § 1250 property.

That means the methodology cannot substitute for engineering judgment.

A sophisticated model does not make an incorrect asset classification correct.

Likewise, a simple approach can still contain supportable conclusions if the underlying evidence is appropriate and adequately documented.

When Is Each Approach Used?

The property itself helps determine what information is available.

New construction often provides extensive contemporaneous cost information.

Construction drawings, specifications, contracts, change orders, payment requests, vendor invoices, and other project records can provide the evidence needed for a detailed engineering approach.

Acquired property can be different.

The ATG explains that an acquired property may have extensive construction records, or it may have little more than the purchase price. When construction cost information is unavailable, replacement cost new less depreciation and other appraisal-based techniques may be necessary to estimate the costs of the acquired assets.

Multiple substantially similar properties can create another situation.

A retail or restaurant owner may own dozens or hundreds of facilities with similar construction and use.

A properly designed sampling or modeling approach may allow engineers to study a representative population rather than conduct a full individual study of every property. The ATG recognizes sampling for situations involving large numbers of substantially similar properties, while emphasizing population definition, sample size, stratification, and sampling error.

The key is matching the methodology to the evidence and the property population.

This is consistent with how engineers perform cost segregation: the property is investigated first, then the available evidence is used to support the conclusions.

How Should an Investor Evaluate the Approach?

Investors do not need to become cost estimators to ask useful questions.

Start with the study's methodology.

What information did the preparer use?

Were actual construction records available?

Were the properties inspected?

Were engineering drawings and specifications reviewed?

Were actual costs used, estimated costs used, or both?

Were contractor or vendor costs incorporated?

Was sampling used?

Were the results reconciled to the total project cost?

The ATG's quality-study guidance says a quality study should describe the methodology used, identify the steps taken to classify assets and determine costs, and use the best available documentation.

The ATG also states that a quality study should reconcile total allocated costs to total actual costs.

That is where methodology becomes practical.

The investor should be able to understand where the numbers came from and how they connect back to the property.

At CostSegRx, that means engineering is not treated as a layer added after the tax analysis.

Engineering is how the property is understood in the first place.

As the CostSegRx engineering philosophy puts it, “Contractors Build. Engineers Classify.” Trade categories describe construction activity, while engineers evaluate what the installed systems actually support.

Can Methodology Change the Cost Allocation?

Illustrative example only. Figures shown are estimated for demonstrative purposes only. Actual classifications, costs, asset classes, recovery periods, depreciation deductions, and tax results depend on the specific property, business activity, supporting documentation, engineering analysis, applicable authority, and taxpayer circumstances.

Assume a newly constructed commercial property has a total depreciable project cost of $10 million.

A detailed engineering study using contemporaneous construction records identifies $1.8 million of property that qualifies for shorter recovery periods based on the property's specific facts and applicable tax authority.

Now assume a simplified methodology estimates the same category using generalized assumptions and produces a $2.4 million allocation.

The difference is:

$2.4 million − $1.8 million = $600,000

The point is not that the detailed approach will always produce the lower or higher number.

The point is that methodology affects how the evidence is translated into cost allocations.

The ATG cautions that rule-of-thumb methods may have limited documentation and that estimated costs should be reviewed against actual costs where possible. Its example demonstrates how an estimated unit cost can materially differ from the actual contractor cost for the same type of asset.

A defensible study therefore does not begin with the desired allocation.

It begins with the property, the evidence, and the engineering analysis.

What Should Investors Remember About the Six Approaches?

The six approaches are not six different tax rules.

They are different ways of developing and allocating the cost information used in a cost segregation study.

Some rely heavily on actual construction records.

Some rely on engineering estimates.

Some obtain information from contractors.

Some use residual calculations.

Some use sampling or modeling.

Some use simplified assumptions.

The ATG does not require one specific approach.

But it does make clear that accuracy, documentation, reconciliation, and appropriate methodology matter.

The best approach is not necessarily the most complicated one.

It is the approach that appropriately matches the property, available evidence, engineering analysis, and cost information.

For CostSegRx engineers, that leads to a simple principle:

The approach determines how the evidence is developed, but the property facts still determine the defensible classification.