September 15 Extension Deadline: 5 Critical Steps to Take

September 15 Extension Deadline: There's No Second Extension Behind This One

The September 15 extension deadline is the end of the line for calendar-year S corporations and partnerships. Your client's Form 1120-S and Form 1065 are originally due the 15th day of the 3rd month after the tax year closes, March 15 for calendar-year filers, shifted to March 16 in years when March 15 falls on a weekend. A Form 7004 extension filed by that original date buys an automatic six-month extension, pushing the filing deadline to September 15. There's no further extension available after that.

At BusAcTa Advisors, we support audit and tax documentation for U.S. CPA firms, and the calls we get every September from your team are some version of the same question: the extended deadline just passed and your client's return still isn't ready, what now.

This is general information about federal late-filing penalties, not tax advice for any specific client. Confirm current penalty amounts and abatement eligibility with IRS guidance for your client's exact situation.

Step 1: Calculate the Exact Penalty Exposure, Per Owner, Per Month

The Form 1120-S late filing penalty and its partnership counterpart aren't flat fees. It's calculated per shareholder or partner, per month, which is what makes this penalty grow so fast for any entity with more than one or two owners. Form 1065 late filing penalty rates work the same way: for returns required to be filed in 2026, the rate is $255 for each month or part of a month the return is late, up to a maximum of 12 months, multiplied by the total number of people who were shareholders or partners at any point during the tax year.

This penalty applies to your client whether or not the entity owes any tax. Your client's pass-through entity, even with zero net income, still faces this exposure if the return is late, because the penalty is based on ownership count, not tax liability. S-corp partnership penalty per shareholder math works out fast: a five-shareholder S corporation that files three months past the extended deadline owes $255 multiplied by 5 multiplied by 3, or $3,825, before any consideration of unpaid tax. Run this calculation immediately for any client past the deadline. Does the number itself change how urgently the return gets finished?

Step 2: Don't Forget the Separate K-1 Penalty, Because It Stacks on Top

Missing the September 15 deadline doesn't just trigger the entity-level failure-to-file penalty. The Schedule K-1 late penalty applies separately: every K-1 not furnished to a shareholder or partner by the due date carries its own penalty, separate from and in addition to the failure-to-file penalty. For returns required to be filed in 2026, that penalty is $340 per Schedule K-1 not furnished correctly and on time. If the IRS determines the failure was intentional, the penalty increases to the greater of $680 per K-1 or 10 percent of the items required to be reported, with no maximum cap on intentional-disregard penalties.

For your client with five owners who's both late on the entity return and late getting K-1s out, the two penalties stack. Does your firm's late-filing checklist account for the K-1 penalty as a separate line item, or does your process stop at the entity-level failure-to-file calculation? Both deadlines run off the same date, but they're assessed independently.

Step 3: File the Return Now, Because Every Additional Month Adds to the Bill

The failure-to-file penalty accrues for each month or partial month the return remains unfiled, capped at 12 months. That structure means a return filed one day into a new month triggers that entire month's penalty, the same as if it were filed on the last day of that month. There's no proration. Filing your client's return today instead of in two weeks, if that gap crosses into a new month, can be the difference between one additional month of penalty and two.

The instinct your team will feel most often when a deadline is already missed is to wait until the return is perfect before filing, gathering every last document, resolving every open question. That instinct costs your client money here. The penalty clock keeps running on an unfiled return regardless of how thorough the eventual filing turns out to be. Get your client's return filed as soon as it's reasonably complete, then amend later if something material surfaces, rather than holding the filing hostage to perfection while the per-month penalty keeps accruing on them.

If your client's entity owes tax at the entity level, which is uncommon for partnerships but can apply to S corporations with built-in gains tax or excess net passive income tax, a separate failure-to-pay penalty also applies to them, generally one-half of one percent of the unpaid tax per month, up to a maximum of 25 percent. That penalty and the per-owner failure-to-file penalty run on parallel but separate tracks, and you stop both by getting your client's return filed and any tax paid.

Step 4: Check First-Time Abatement Before Assuming the Penalty Is Final

The IRS offers First-Time Penalty Abatement for taxpayers with a clean compliance history. An entity generally qualifies if it filed all required returns, or valid extensions, for the prior three tax years, and had no significant penalties assessed during that period. If your client meets that history, you can eliminate the entire late-filing penalty with a single First-Time Abatement request, typically a phone call to the IRS or a written request referencing the relevant tax period.

First-Time Abatement is generally available once per taxpayer, so it's worth confirming your client hasn't already used it for a different tax year or penalty type before you assume it applies here. Has your firm checked whether your client's compliance history over the prior three years actually supports a First-Time Abatement request, or is your assumption just that the penalty will need to be paid?

Step 5: Build the Reasonable Cause Case if First-Time Abatement Doesn't Apply

Reasonable cause IRS penalty relief is the remaining path if a client doesn't qualify for First-Time Abatement, perhaps because of a penalty assessed in a prior year. The IRS may waive your client's penalty if the entity can demonstrate it exercised ordinary business care but was unable to file on time due to circumstances genuinely outside its control, a serious illness, a natural disaster, the death of a person central to preparing the return, or comparable disruptions.

Your reasonable cause request needs to be specific and documented, not a general assertion that the year was difficult. Your supporting documentation might include medical records, disaster declarations, correspondence showing you were actively working toward compliance, or evidence the delay stemmed from circumstances your client couldn't have reasonably anticipated or controlled. Generic explanations rarely succeed. Your well-documented, specific account of what happened and why it prevented timely filing has a meaningfully better chance.

You can see how we support tax compliance documentation for CPA partners on the how it works page. Our LLC and partnership tax service and corporate tax preparation service both cover late-filing situations, including penalty calculation and abatement request preparation. Our tax planning and advisory service can help your firm build a deadline-tracking process that catches at-risk clients before September 15 arrives, not after.

For the current failure-to-file penalty structure and abatement procedures, see the IRS failure to file penalty page, which is the authoritative source for these requirements.

Missing September 15 Is Expensive, Not Unrecoverable

A client whose S corporation or partnership return missed the September 15 extension deadline is facing a real and growing penalty, calculated per owner, per month, with a separate penalty stacking on top for late K-1s. None of that changes your priority order: calculate the actual exposure, file the return immediately rather than waiting for it to be perfect, and then work the relief options, First-Time Abatement first, reasonable cause if that doesn't apply. Your clients come out of this with the smallest bill when you move on all three fronts at once instead of treating the filing and the penalty conversation as separate problems to solve one after the other.

If you'd like to discuss how we support pass-through entity late filing situations and deadline tracking for CPA partners' clients, book a scoping call with BusAcTa Advisors, and we'll walk through their specific exposure before you commit to anything.