How to amend a business tax return

Which form does each entity use?

See Form 1040-X, Form 1120, Form 1120-S and Form 1065.

The amended return replaces the original figures in full, not just the changed lines.

Why are partnerships different?

Partnerships are generally covered by a centralized audit regime. Under it, a partnership cannot file an ordinary amended return; it files an administrative adjustment request, which may result in the partnership paying an imputed underpayment or in partners taking the adjustments into account in the year the request is made, often through statements similar to K-1s. Small partnerships with eligible partners can elect out of the regime each year on a timely filed return, and then amend normally.

Because the rules differ by year and by election, check whether the partnership elected out of the centralized regime for the year being corrected before choosing the route. The election is made on the original return each year.

How do you prepare an amendment?

1. Confirm the error and its effect

   Tax, owner allocations and state returns.

2. Choose the right route

   By entity type, as in the table.

3. Explain each change

   Clearly, with supporting schedules.

4. File electronically where possible

   Many amended business returns can be e-filed.

5. Amend related returns

   Owners' returns and state returns.

Keep a copy of the original return and the amendment together.

How long do you have?

To claim a refund, generally three years from the date the original return was filed, or two years from when the tax was paid if later. Adjustment requests for partnerships also must be filed within three years of the later of the return's filing date or due date, and before the IRS starts an examination of that year. See the IRS statute of limitations.

Track the dates for each open year in one place.

What does an example look like?

An S corporation with two equal shareholders filed its 2025 Form 1120-S, then found a $20,000 Form 1099-NEC for consulting income it had missed. It files an amended Form 1120-S with the amended box checked, adding the income, and sends each shareholder an amended K-1 showing an extra $10,000. Each shareholder then files Form 1040-X to report the additional income, and both amend their state returns. The corporation's own state return is amended too, if the state requires it.

Do state returns need amending?

Usually, whenever a federal change affects state taxable income or owners' state income. Many states require an amended state return within a set period after a federal change is filed or finalized, so check each state's rules and deadlines.

What if the amendment shows more tax?

Pay with the amendment to stop interest growing. Penalties may apply, but correcting voluntarily usually reduces them and supports any request for relief. See reasonable cause.

When should you not amend?

For arithmetic errors the IRS corrects itself, or for small classification errors with no tax effect, which can be corrected going forward in the books. If the IRS has already sent a notice proposing changes, respond to it instead. See CP2000 notice.

Where an error affects several years, amend each open year separately, starting with the earliest.

This guide is general information. It is not tax or legal advice for your situation.