Should You Amend Your Tax Return? Common Mistakes to Fix After Filing

You hit submit on your tax return, breathe a sigh of relief…and then see a statement come through the mail that would’ve given you a pretty nice deduction.

What now? It’s not like you can call up the IRS with a quick “one last thing” message, right? 

Let us introduce you to Form 1040-X—otherwise known as the way you can amend your tax return post-filing.

Plenty of taxpayers file amendments every year, and you can too, if you discover new information after you submit your return.

In this blog, we’re breaking down:

• How you can amend your tax return

• The reasons you may want to—when you shouldn’t

• The deadlines you need to keep to

What is an Amended Tax Return?

An amended tax return is a corrected file you can use to change information filed in a previous tax year, without having to redo the entire return. 

You’ll use Form 1040-X to:

• Fix errors 

• Claim missed deductions or tax credits

• Report forgotten, missed, or underreported income

While you can opt for paper filing, the IRS allows you to file a 1040-X electronically for the current or two prior tax periods.

Of course, this isn’t a requirement if you got everything right the first time. There are a few common reasons you’d choose or need to file an amended tax return.

The Most Common Tax Mistakes That Require an Amendment

Sometimes, filing an amendment is optional, something that just works in your favor. Other times, it’s necessary to avoid receiving an IRS notice.

1. You Received a Corrected or Late Tax Document

If your employer sends a corrected 1099 or W-2, or you receive a late K-1, those figures may change the information you gave the IRS. 

This is particularly common for self-employed taxpayers who juggle multiple income streams and documents.

You’ll need to amend your return to update your total income and tax withholdings.

2. You Missed a Deduction

You want to make sure you take advantage of every deduction possible to lower your taxable income.  

You can amend your return to reflect missed items if you accidentally overlooked your:

• Mortgage interest

• Business expenses

• Charitable donations

3. You Claimed the Wrong Filing Status

Your filing status affects your tax bracket, standard deductions, and eligibility for credits.

If you filed under the wrong status, such as Single versus Head of Household or Married Filing Jointly versus Separately, it may be worth amending your return to ensure you don’t wind up with any surprises.

4. You Underreported or Overreported Income

You’re responsible for reporting all forms of income, not just what’s on your W-2. That means any money you bring from side gigs, freelance work, or rental properties. 

Underreporting increases your liability and can result in heavy penalties from the IRS. (Because, trust us, they will find out your real income.)

On the other hand, over-reporting means you paid too much and may be owed a refund.

Either way, an amended tax return puts you in safer waters.

5. You Missed Credits You’re Entitled To

There are plenty of tax credits you can claim to reduce the amount of tax you owe, such as Child Tax Credits, Earned Income Credit, and Education Credits.

These are often missed by self-filers, simply because they don’t know about them. Even so, if any apply to you, it’s worth filing a 1040-X to get money back in your pocket.

Now, there are some cases where you shouldn’t file an amended tax return.

When Not to Amend Your Tax Return

In some cases, corrections don’t fall on you to fix. 

These apply when you have: 

• Math Errors: The IRS will correct these automatically.

• Missing Forms: The IRS will request them separately.

• Waiting Periods for an IRS Notice: You’ll wait to amend things until after the IRS processes your original return.

If you’re going to file an amended tax return, you need to keep to certain timelines.

Key Deadlines for Amending Your Tax Return

Generally, you can file  1040-X:

• Within three years after the date you filed your original return

• Within two years after the date you paid the tax

It depends on whichever date is later.

For example, if you need to amend your 2025 tax return, you’ll have until April 2029.

Keep in mind that correcting your federal return can also impact your state tax liability, so you would need to contact your state tax agency to follow additional correction procedures.

Don’t Navigate Tax Return Amendments Alone

Amending a tax return sounds straightforward—until you realize one correction can ripple across your credits, deductions, and even your state returns. A missed step can trigger delays, penalties, or an unwanted IRS notice.

At AVM DeMars, our Long Island-based team has decades of experience guiding taxpayers through tax season so you get the best return possible. Whether you need help amending a past tax return or navigating dealings with the IRS, we are your partner.

Speak to one of our tax advocates today.