The last date for extensions has come and gone. Tax returns have been submitted. For several businesses, September 16 is merely a deadline where one can breathe a sigh of relief. Nevertheless, that tendency is not only understandable but is also where a whole range of entirely avoidable complications arise.
The days following September 15 are crucial for the whole of the tax season calendar - not because anything is actually due for submission but rather because several things are about to be due; some things have just changed, and some things require action on the part of clients when they do not actually need to.
The companies that regard this week as a period for relaxing and taking things easy from then on will be busy solving issues in the course of October, November, and December, whereas businesses that treat this week as a turning point will have a very busy time making profits, establishing trust and preparing for the next season in January.
1. Confirm Every Extended Return Actually Cleared - Not Just That It Was "Filed"
The terms "filed" and "accepted" cannot be regarded as synonymous terms. Delays, mistaken omissions of e-file authorizations, and lack of state acknowledgment may remain undetected for a long time if no one is actively keeping a track of acknowledgment status on the batch. Make sure you reconcile every extended return at 1120-S, 1065, and trust with your e-file confirmations before you decide that the deadline is completely over. Missing an IRS notification about a rejected return in November is a more inconvenient matter than checking everything before the deadline.
2. Don't Let September 15 Slide Past You Twice - It Was Also a Q3 Estimate Deadline
The September 15 date marked not only the due date for the extended entity deadline but also the date for the third quarter individual and calendar year corporate estimated tax payments. There is a tendency among clients who are busy with business return filing to overlook this other less noticeable obligation entirely. An effective way of reminding clients when the third quarter payments were made is by reviewing the same before the penalties for underpayment begin appearing in April.
3. Start October 15 Countdown Immediately, Not in Two Weeks
The due date for personal extensions is October 15 -which is just a month away. A common pitfall for many firms is considering it a “next month” concern immediately after the exhaustion of the filing season. In fact, planning for the October 15 due date should be done this week: take stock of which tax returns are complete, which ones are pending information, and which ones have issues that need to be resolved right away, and not by October 10. If the staff in the tax preparation department is still recovering from the rush of September, this is often the time when firms start to bring in staff from outside of the firm to handle the work without putting too much pressure on their own employees before they close the tax year.
4. Check Whether Your State's PTET Election Window Is Closing
One of the most frequently overlooked items, even among existing companies, is the fact that a lot of states' Pass-Through Entity Tax (PTET) election or estimated payment deadlines are based on the extended federal due date. This means that some PTET elections need to be acted on around September 15, and others have deadlines linked to that date. Unfortunately, it is very easy to forget deadlines when there are multiple jurisdictions involved. PTET elections can have a significant impact on the clients' SALT deduction workaround benefits; therefore, if the deadline is missed, the clients lose money. If your business does not have a tracker for paying various deadlines state-by-state, create it now because otherwise you will be risking using your memory.
5. Update Your Client Communications on Beneficial Ownership (BOI) Reporting - the Rule Has Changed Again
Practitioners find this a surprising twist because it has been transformed multiple times in a period of two years. As per FinCEN's final ruling in August 2026, the BOI report for U.S. domestic entities, as per the Corporate Transparency Act, has been revoked permanently. The obligation now remains in the purview of certain foreign entities registered to work in the U.S.
If your company has sent any instructions to clients in 2024 or early 2025 indicating that the clients must submit BOI reports, then the information is no longer valid. It is recommended to send short and effective communication to clients, particularly the newly formed LLCs and corporations, to resolve this important issue.
6. Mine the Extended Returns You Just Filed for Q4 Planning Opportunities
Every long-term return that your team just completed presents multiple chances for planning, but it’s simple to lose track of it. The knowledge about carryforward losses, unused credits, the space for pension payments, and inefficiencies in the structure of organizations are stored in the minds of your specialists right now but require plenty of time to be reacquainted in December. That’s the type of work that makes a compliance-only relationship different from the advisory one. It’s the same principle that works in the case of virtual CFO and FP&A offering making use of existing knowledge about a company for the purposes of Q4 planning.
If the clients in question are multi-state sellers or those working online, then they should also be checked for their exposure to sales tax on their sales in various states before outsourcing virtual CFO and FP&A services.
7. Look at Your Own Firm Before You Look at Anyone Else's Return
After the deadline, it’s easy to go completely client-side. Doing so would mean that you miss out on the internal review that will help you protect your margins moving forward. Here’s what to contemplate:
Staff capacity and burnout - this is the period of highest risk for attrition post-season. A brief check-in with your team at this point is free and prevents a lot of turmoil.
Workflow issues from this time - where documents were stuck, reviews were delayed, and what you will change before next season, instead of just logging into it.
Bandwidth shortages - if the last six months made it evident your team is too thinly spread to do both compliance and advisory work well, this is the time to check whether you should go for outsourced accounting & bookkeeping or hire specialized staff for tax preparation instead of repeating the same mess next April.
Firms that carry out this kind of review regularly see their filing seasons run much smoother than those that don’t.
Why This Week Matters More Than It Feels Like It Does
These seven responsibilities must not be perceived as urgent deadlines like the one we have for April 15. This is exactly the reason why it is so easy to let them slide until later -and yet, exactly why it’s wrong to do so. A missed PTET election, unresolved rejected return, or a client using obsolete BOI guidance will not announce itself right away. Instead, it shows up weeks or months down the road as a penalty, dissatisfied client, or missed planning opportunity.
Where Can This Take Your Practice Next
Many of the key tasks suggest a single conclusion: compliance and advisory work should ideally complement each other, yet this can only be possible when a company has enough human resources to connect the two fields instead of closing the file. Should September witness your firm’s limitations in this regard, it is important to consider the way Xconcile provides its outsourced tax preparation, virtual CFO and FP&A services as well as bookkeeping and accounting help directly to companies like yours and what is to be built specifically to alleviate this post-deadline pressure, thus giving your in-house team an opportunity to concentrate on client relationships and advisory work necessary for the growth of the company.
