When does the three years start?

Generally when the return is filed — or its due date, if filed early. Amended returns generally don't restart it.

Can the IRS extend the period unilaterally?

No — extension needs the taxpayer's signed consent on Form 872, or a deficiency notice that suspends the clock during proceedings.

Is the collection clock the same thing?

No. Assessment (§6501) and collection (10 years from assessment) are separate clocks with separate rules — assessment first, collection after.

IRC §6501 · 26 U.S.C. §6501

IRC §6501: Limitations on Assessment

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

IRC §6501 gives the IRS three years from filing to assess additional tax — six years if gross income is understated by more than 25%, and forever if no return was filed or fraud is involved. Filing starts the clock; not filing leaves it running.

What it governs

Three-year general rule from filing (or due date, if early)
Six years for substantial understatements over 25% of gross income
Unlimited for unfiled, false, or fraudulent returns
Extended only by signed consent (Form 872) or deficiency proceedings

In the GuideFull treatment in Chapter 17 of the Guide — THE TAX CUTTERY® Guide to Federal Income Taxation, Professional Edition (564 pages, 24 chapters).

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Questions this section answers

When does the three years start?: Generally when the return is filed — or its due date, if filed early. Amended returns generally don't restart it.
Can the IRS extend the period unilaterally?: No — extension needs the taxpayer's signed consent on Form 872, or a deficiency notice that suspends the clock during proceedings.
Is the collection clock the same thing?: No. Assessment (§6501) and collection (10 years from assessment) are separate clocks with separate rules — assessment first, collection after.

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