Missed the September 15 tax deadline: What happens next and how to reduce the penalty

Quick Answer

• Missing the extended September 15 deadline for Form 1065 or Form 1120-S triggers a $255-per-owner monthly penalty, capped at 12 months, even if the business owes no tax.

• Small partnerships with 10 or fewer partners may qualify for relief under Revenue Procedure 84-35, while S corporations must instead request reasonable cause relief.

• Filing as soon as possible limits the penalty and lets partners meet their own October 15 personal filing deadline.

Missing a major tax deadline is stressful, especially when you are running a service business without a dedicated internal finance team. But waiting to address it only increases the cost.

The good news is that the penalty clock can often be reduced, and in some cases avoided entirely, once your books are accurate enough to file. The fastest way to limit the damage is simply to get a correct return filed.

What was due on September 15

As covered in our original September 15 deadline guide, calendar-year partnerships and S corporations that filed a six-month extension back in March faced their federal filing deadline on September 15. If that date has now passed without a filed return, the failure-to-file penalty is already accruing.

How the late-filing penalty is calculated

Under IRC Section 6698 for partnerships and Section 6699 for S corporations, the IRS charges a flat $255-per-owner monthly penalty for each month or part of a month the return is late, for up to 12 months, according to the IRS Instructions for Form 1065. Any part of a month counts as a full month, and the partner or shareholder count is based on everyone who held an interest at any point during the tax year, even if they left mid-year.

State filing requirements and penalties for late partnership or S corporation returns vary by state and are not covered here.

Partnership relief under Revenue Procedure 84-35

Small partnerships have an avenue for relief that S corporations do not. Per the IRS's explanation of Notice CP162E, the IRS will presume reasonable cause and waive the penalty for a partnership if it meets all of the following:

• The partnership had 10 or fewer partners for the taxable year. A married couple filing a joint return counts as one partner toward that limit.

• Each partner during the tax year was a U.S. citizen, a lawful permanent resident, or the estate of either one.

• Each partner's share of every partnership item is identical.

• Every partner fully reported their share of partnership income on their own timely filed personal return.

If any one partner fails to meet that last condition, the partnership can lose its ability to claim this relief entirely, so it is worth confirming with each partner before requesting it.

S corporation relief: Reasonable cause

Revenue Procedure 84-35 does not extend to S corporations. An S corporation that files late must instead request standard reasonable cause relief, which generally means writing directly to the IRS explaining why the delay happened. Acceptable causes typically include natural disasters, serious illness, or unavoidable destruction of records. A simple oversight, or relying on a preparer who made a mistake, is rarely accepted on its own.

Why October 15 still matters

The September 15 entity deadline and the October 15 personal deadline are connected. Partners cannot accurately complete their own extended personal returns without a correct Schedule K-1 from the partnership. Late or incorrect K-1s carry their own separate penalty, too, up to $340 per K-1 under the same Form 1065 instructions cited above. Filing the entity return quickly, even after September 15, gives partners the best chance of still meeting October 15 without needing an amended personal return later.

What to do right now

Get your books current enough to file an accurate return today rather than waiting for a "complete" picture. Every additional month adds another full round of the penalty, and the fastest way to stop the clock is to file, then pursue Revenue Procedure 84-35 or reasonable cause relief afterward if you qualify. For service firms already running lean without a dedicated finance team, this is often exactly the kind of deadline that slips.

Frequently asked questions

Can my partnership still qualify for penalty relief after filing late?

Yes. Relief under Revenue Procedure 84-35 and reasonable cause relief are both requested after the fact, once the return has already been filed or the IRS has already assessed the penalty. There is no requirement to ask for relief in advance, though filing the actual return as soon as possible still limits how many months of penalty accrue before relief is granted.

Does the penalty apply if the partnership owes no tax?

Yes. Unlike an underpayment penalty, which is tied to how much tax was actually owed, the failure-to-file penalty for Form 1065 and Form 1120-S is a flat charge per owner per month. A partnership or S corporation with zero taxable income for the year can still face the full penalty simply for filing its informational return late.

What happens to my partners if K-1s are late?

Each partner needs an accurate Schedule K-1 to complete their own personal return by October 15. If K-1s are not ready in time, partners may need to file their own extension or amend their personal return later, which creates extra work and cost beyond the entity-level penalty the partnership itself already owes.

Numetix uses an expert-led, AI-powered, and human-in-the-loop approach to get your books current fast, so you can file an accurate return and stop the penalty clock rather than waiting on a perfect picture. Explore our bookkeeping services or see how we support tax filing readiness.

Disclaimer: This article is for informational purposes only and does not constitute formal legal or tax advice. Always consult a qualified tax professional regarding your specific situation.