Residual Estimation: Why the ATG Warns About the Shortcut

The 2025 IRS Cost Segregation Audit Technique Guide identifies the Residual Estimation Approach as one of six common approaches to cost segregation. The ATG describes it as an abbreviated method in which only short-lived asset costs are determined. Those costs are added together and subtracted from the total project cost, with the remaining residual assigned to the building and other long-lived assets. The approach is simpler and less time consuming than the engineering approaches, but the ATG also warns that it can be less accurate. Most importantly, the residual cost should still be independently evaluated for reasonableness and reconciled to the total project cost.

---

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

---

Key Takeaways

• The Residual Estimation Approach identifies selected short-lived assets and assigns the remaining project cost to the building and other long-lived assets.
• The residual is calculated by subtracting identified short-lived asset costs from the total project cost.
• The ATG warns that residual costs may not be independently estimated or checked for reasonableness, which can reduce accuracy.
• The approach can simplify cost segregation, but the underlying property and short-lived asset costs still require supportable analysis.
• Different estimation techniques can skew an allocation toward § 1245 property, particularly when short-lived assets receive more detailed costing than the building.
• Reconciliation should test whether the residual remains reasonable for the property that remains after identified short-lived assets are removed.
• The residual is a mathematical result. Engineering determines whether the result makes sense.

What Is the Residual Estimation Approach?

The Residual Estimation Approach starts by identifying certain short-lived assets and determining their costs.

Those costs are then subtracted from the total project cost.

The remaining amount is called the residual.

The residual is then assigned to the building and other long-lived assets.

In simplified form:

Total Project Cost − Identified Short-Lived Asset Costs = Residual

For example, if a project costs $10 million and the study identifies $1.5 million of qualifying short-lived assets:

$10 million − $1.5 million = $8.5 million residual

The $8.5 million becomes the amount assigned to the building and other long-lived property.

The mathematics are simple.

The engineering question is not.

The key question is:

Does the $8.5 million residual reasonably represent the property that remains after the identified short-lived assets are removed?

That is where the ATG's warning becomes important.

Why Is It Called a Residual Approach?

The approach gets its name from the way the remaining cost is determined.

Instead of developing the cost of every asset independently, the study determines selected short-lived assets and lets the remaining project cost become the residual.

The process can be summarized as:

Identify short-lived property

↓

Determine its cost

↓

Subtract that cost from total project cost

↓

Assign the remaining cost to the building and other long-lived assets

This can reduce the amount of detailed cost development required.

But it also means the residual is heavily influenced by the accuracy of the short-lived asset estimates.

If the identified short-lived costs are too high, the residual becomes too low.

If the identified short-lived costs are too low, the residual becomes too high.

The residual therefore inherits the consequences of the assumptions used to calculate the assets removed from the total.

What Does the ATG Say About Accuracy?

The ATG makes a direct comparison between the Residual Estimation Approach and engineering approaches.

It says the residual method is simpler and less time consuming, but it can also be less accurate.

That does not mean the ATG says the method can never be used.

Instead, the ATG identifies a weakness that needs to be understood.

The engineering approaches attempt to determine costs through detailed analysis.

The residual approach determines selected costs and assigns the balance to the remaining property.

That difference matters because the remaining property is not necessarily one homogeneous asset.

A commercial property can contain:

• Structural components
• Building systems
• Land improvements
• Equipment
• Furniture and fixtures
• Other tangible property

The residual amount may encompass many different types of property.

The engineering challenge is determining whether that residual is reasonable for what remains.

What Is the Biggest Weakness?

The ATG identifies the biggest weakness directly:

Residual costs generally are not estimated or checked for reasonableness.

That creates a potential blind spot.

The short-lived property may receive detailed attention.

The residual may simply become whatever is left.

That can produce a result where:

The short-lived assets are analyzed.

But:

The remaining property is not independently tested.

The ATG says this is not sufficient.

A proper and reasonable residual should be determined and then added to the short-lived asset costs to determine whether the total project cost reconciles.

That is a very important distinction.

Reconciliation is not just:

“Our numbers add up.”

It is:

“Does the residual itself make sense for the property that remains?”

Why Can the Method Skew Toward § 1245 Property?

The ATG specifically warns that different estimation techniques for short-lived assets can produce a result skewed in favor of § 1245 property.

The ATG provides an example of the problem:

A study might use single-unit costs for high-quality construction when estimating § 1245 property while using gross square footage to estimate the building.

Those two methods can operate at different levels of precision.

For example:

§ 1245 property

May be estimated asset by asset.

Building

May be estimated broadly using square footage.

If the unit costs used for the short-lived assets are aggressive or poorly matched to the project, the amount removed from the total project cost can become too large.

The residual then becomes smaller.

The result can shift too much cost toward § 1245 property.

That is precisely why the ATG cautions about the methodology.

Why Does Function Still Matter?

Cost segregation is ultimately about property classification.

The Residual Estimation Approach does not eliminate the need to understand what the property actually is.

An engineer still needs to know:

• What exists
• What each asset does
• What it serves
• How it is installed
• Whether it is building property or separate property
• What documentation supports the conclusion

The residual method changes how costs may be developed.

It does not change the underlying need for accurate classification.

How Does the Residual Approach Affect Engineering?

Consider a commercial property containing specialized equipment.

The study identifies the equipment and several associated components as short-lived property.

The associated costs are then subtracted from the total project cost.

But the remaining property could include:

• Building structure
• Electrical distribution
• HVAC
• Plumbing
• Flooring
• Site improvements
• Other building components

The residual does not tell the engineer what those remaining assets are.

It only tells the engineer how much cost remains.

That is why the residual should not be confused with an engineering classification.

A residual is a mathematical result.

An asset classification is an engineering and tax conclusion.

Those are different things.

Why Is Cost Reconciliation So Important?

The ATG's warning about reconciliation is one of the most important lessons from this methodology.

Suppose total project cost is:

$10 million

Identified short-lived property:

$2 million

Residual:

$8 million

The numbers reconcile mathematically.

But what if the property is a specialized manufacturing facility where the building itself contains substantial structural and infrastructure costs?

The $8 million residual might be reasonable.

Or it might not.

The mathematical calculation cannot answer that question.

The engineer needs to consider the property facts.

This is where the principle “percentages are the result, not the methodology” becomes useful.

The residual percentage should be an outcome of the analysis.

It should not be treated as the starting assumption.

How Does This Differ From Detailed Engineering?

The difference is primarily how costs are developed.

Detailed Engineering from Actual Cost Records

The engineer works from contemporaneous construction and accounting records and develops individual asset costs using those records.

Detailed Engineering Cost Estimates

The engineer develops estimated costs when actual records are unavailable or insufficient.

Residual Estimation

The engineer determines selected short-lived asset costs and assigns the remaining project cost to the building and other long-lived assets.

The approaches therefore differ in how much of the project cost is independently developed.

The ATG specifically identifies the residual approach as abbreviated.

That makes it important for investors to understand what the study actually did.

Two studies can produce similar percentages using very different methodologies.

The percentage alone does not explain the quality of the analysis.

What Should an Investor Ask?

An investor reviewing a study that uses residual estimation should ask:

Which assets were independently identified?

How were those asset costs determined?

Were actual costs used?

Were estimates used?

What estimating sources were used?

How were the quantities established?

How was the residual calculated?

Was the residual independently evaluated for reasonableness?

Was the total project cost reconciled?

Does the residual make sense given the actual property?

These questions help reveal whether the residual is simply the mathematical remainder or a reasonably supported cost allocation.

Why Can High-Quality Construction Create a Problem?

The ATG's specific warning about high-quality construction is important.

Suppose a property contains expensive specialized equipment and complex building infrastructure.

An engineer estimates the specialized equipment using detailed single-unit costs.

The building is estimated using a broad gross-square-foot approach.

Those methodologies do not necessarily provide the same level of detail.

The specialized property may receive a highly specific cost.

The building may receive a broad estimate.

If the short-lived property estimate becomes disproportionately high, the residual automatically decreases.

That can create a skewed allocation.

The problem is therefore not simply the amount assigned to the short-lived assets.

It is the relationship between the methods used to estimate both sides of the equation.

Illustrative Example

Illustrative example only. Figures shown are estimated for demonstrative purposes only. Actual land allocations, asset classifications, depreciation, and tax results depend on the specific property, supporting documentation, engineering analysis, and taxpayer circumstances.

Assume a newly constructed commercial facility has total project costs of $12 million.

Cost Category	Amount
Identified short-lived assets	$2,400,000
Residual assigned to building and other long-lived assets	$9,600,000
Total project cost	$12,000,000

Mathematically, the study reconciles.

But now suppose further engineering review indicates that the short-lived assets were estimated using detailed single-unit costs that did not align with the project's actual construction costs.

After reviewing the construction documentation, the engineer determines that the supportable cost for those identified assets is $1.8 million rather than $2.4 million.

The residual then changes:

$12 million − $1.8 million = $10.2 million

The difference is:

$600,000

That $600,000 did not appear because the building changed.

It appeared because the cost assigned to the short-lived assets changed.

This illustrates the core weakness of a residual calculation.

The residual is only as reliable as the costs removed from the total.

The purpose of the example is not to suggest that either allocation is typical.

It demonstrates why the underlying cost methodology matters.

What Does a Reasonable Residual Look Like?

The ATG does not provide a universal percentage for a reasonable residual.

That is important.

There is no ATG rule saying that a particular percentage of a commercial property should remain as the residual.

The appropriate result depends on the facts.

Those facts can include:

• Property type
• Construction
• Systems
• Equipment
• Site improvements
• Documentation
• Actual costs
• Engineering analysis
• Applicable tax treatment

The ATG's focus is on reasonableness and reconciliation, not a predetermined percentage.

This is consistent with the CostSegRx principle that percentages are the result, not the methodology.

How Does This Fit Into the ATG Encyclopedia?

The Residual Estimation Approach sits between detailed engineering and the more abbreviated approaches in the ATG.

The methodology sequence is:

Detailed Engineering from Actual Cost Records

↓

Detailed Engineering Cost Estimates

↓

Survey or Letter Approach

↓

Residual Estimation Approach

↓

Sampling or Modeling Approach

↓

Rule-of-Thumb Approach

The ATG identifies all six as common approaches and explains the attributes and potential drawbacks of each.

For the AIO Encyclopedia, that sequence matters.

The approaches should not be reduced to:

Good method vs. bad method.

Instead, the reader should understand:

How does the method develop costs?

What evidence does it use?

Where can the methodology introduce uncertainty?

What controls does the ATG expect?

That creates a much more useful reference system.

Engineering Principle

The Residual Is a Result, Not a Classification.

The Residual Estimation Approach can simplify cost segregation by determining selected short-lived asset costs and assigning the remaining project cost to the building and other long-lived assets.

But the ATG warns that the residual can be less accurate because it generally is not independently estimated or checked for reasonableness.

The proper question is therefore not:

“Does the residual add up?”

It is:

“Does the residual make sense for the property that remains?”

That requires engineering judgment, documentation, cost analysis, and reconciliation.

The residual is a mathematical result. Engineering determines whether the result makes sense.