Part III · Resolution — When Things Go Wrong · pp. 375–388

Chapter 20: Expenses, Substantiation, and the Cohan Line

By Paul D. Diaz, EA, MBA · Chapter summary · Updated September 20, 2026

As the IRS deploys automated and AI-enhanced enforcement against poorly documented expenses, contemporaneous recordkeeping built into practice workflow is the decisive defense for taxpayers and practitioners alike.

Overview

This chapter addresses the substantiation requirements for ordinary business expenses—the most common source of disputes between taxpayers and the IRS. It traces the doctrine from the Cohan rule, which allowed courts to estimate deductible expenses when exact records were unavailable, through Congress's response imposing strict substantiation requirements under Section 274(d) for travel, meals, entertainment, gifts, and listed property. The chapter explains how these strict rules eliminate judicial estimation for covered categories, meaning taxpayers who fail to maintain adequate records lose the deduction entirely.

The chapter then examines the current and emerging enforcement landscape, including the Automated Underreporter program, the Automated Collection System, and the growing use of artificial intelligence to identify suspicious deduction patterns at scale. It argues that substantiation is not merely a compliance burden but a strategic asset, and it demonstrates how contemporaneous recordkeeping integrated into the Examination File methodology transforms a vulnerable taxpayer position into a defensible one. The chapter concludes that the future of expense enforcement is automated notices with minimal human involvement, making systematic recordkeeping architecture essential for practitioners and their clients.

In This Chapter

• 20.1 The Cohan Rule: Foundation and Limits — The judicial estimation doctrine from Cohan v. Commissioner and how Section 274(d) eliminated it for travel, meals, entertainment, gifts, and listed property.
• 20.2 The Automated Enforcement Terrain — How AUR computer matching, ACS collection automation, and AI-enhanced case selection are replacing labor-intensive examinations with scalable notice-based enforcement.
• 20.3 Substantiation as Strategic Defense — Why contemporaneous records are a strategic asset: they enable fast, thorough responses to notices that close cases, while missing or reconstructed records leave taxpayers defenseless.
• 20.4 Integration with Planning and Compliance Architecture — How substantiation fits into the Monthly Trident Cadence and shapes expense planning decisions such as mileage reimbursement versus company car and actual meals versus per diem.
• 20.5 The Future Is More Letters, Fewer People — The enforcement shift toward automated notices and reduced human review, making systematic recordkeeping the defining difference between practices that resolve matters quickly and those that face protracted disputes.

Key Terms

• Cohan rule — A judicial doctrine allowing courts to estimate deductible business expenses when a taxpayer incurred them but cannot prove exact amounts, now largely overridden by statute.
• Section 274(d) — The code provision imposing strict substantiation requirements for travel, entertainment, gifts, and listed property, eliminating the Cohan rule for those categories.
• Listed property — Property defined under Section 280F(d)(4) including automobiles, transportation property, entertainment property, and computers, subject to strict substantiation rules.
• Adequate records — Contemporaneous documentation such as logs, diaries, or account books created at or near the time of the expense, supported by receipts or similar evidence.
• Automated Underreporter program (AUR) — An IRS computer-matching system comparing return data to third-party information documents and generating notices proposing adjustments for discrepancies.
• Automated Collection System (ACS) — A computer-driven IRS collection process that sends escalating notices and initiates enforced collection actions with minimal human discretion.
• CP2000 — The most common AUR notice, which explains a discrepancy, proposes additional tax, and gives the taxpayer a limited period to respond.
• Examination File — A pre-organized collection of substantiating documents and schedules assembled before filing, structured to provide immediate responses to likely examination issues.

Who Needs This Chapter

Tax practitioners and advisors serving small business owners, rental property owners, and self-employed individuals who claim deductible expenses on Schedules C, E, or F and face increasing automated scrutiny of those deductions.

Questions This Chapter Answers

What was the Cohan rule?

Courts could estimate deductible expenses when exact records were missing — judicial mercy for the documented-but-imprecise taxpayer.

What killed Cohan for travel and meals?

Section 274(d): strict substantiation for travel, entertainment, gifts, and listed property. No records, no deduction, no estimation.

What records actually survive an exam?

Contemporaneous ones — built into workflows, dated when made, corroborated by third parties. Reconstructed logs persuade no one.