If you’re a business owner, especially operating as an S Corporation, partnership, or multi-member LLC, the September 15, 2025 tax deadline is one date you can’t afford to ignore. Missing it could mean unnecessary penalties, interest charges, or worse — a letter from the IRS that derails your end-of-year plans.
Whether you’re a general contractor, tradesman, or run a small business in San Diego County or beyond, getting a handle on this deadline now can save you stress later.
Who Needs to File by September 15?
The September 15 deadline primarily applies to:
- S Corporations (Form 1120S)
- Partnerships (Form 1065)
- Multi-member LLCs treated as partnerships or S Corps
This deadline is for calendar-year businesses that filed for an extension back in March 2025. If you didn’t file for an extension, your return was originally due on March 15. Either way, this September filing marks your last shot to stay in compliance without incurring late-filing penalties.
Why This Deadline Matters
There are several reasons the September 15 deadline deserves your full attention:
- Avoid IRS Penalties
Late filing of Form 1120S or 1065 can result in $245 per shareholder or partner, per month, up to 12 months. That adds up quickly, especially for businesses with multiple owners. - K-1s for Your Owners or Partners
Business tax returns generate Schedule K-1s, which partners and shareholders need for their personal tax returns. A delay here can cause a ripple effect and complicate individual filings. - Year-End Tax Planning Starts Now
Once your return is filed, you can assess profits, losses, and deductions — all of which are critical inputs for strategic fourth-quarter planning. - Banks and Lenders Want Tax Docs
Thinking of applying for a business loan or refinancing equipment? Lenders typically request the most recent business tax return. Missing the September deadline can delay your funding process.
Common Mistakes Business Owners Make
Here’s what we often see this time of year:
- Assuming the CPA Filed
Just because you handed off your documents doesn’t mean your return has been filed. Always confirm with your CPA or accountant. - Misunderstanding Entity Type
Many small business owners don’t realize they elected S Corp status or have a partnership classification — and that comes with unique filing deadlines. - Forgetting State Requirements
In California and many other states, state business returns also have fall deadlines. Don’t focus only on federal returns.
What You Should Be Doing Now
If you haven’t filed yet, here’s how to prioritize the next few weeks:
1. Get All Financials in Order
Make sure your bookkeeping is current. Gather profit & loss statements, balance sheets, payroll reports, and any large expense documentation. If you use accounting software like QuickBooks, reconcile all bank and credit card accounts through June or July.
2. Schedule Time With Your CPA
Don’t wait until the week of the deadline. Your CPA needs time to review documents, check for missing information, and submit an accurate return.
3. Ask About Tax Strategy
If this is the first time you’re working with a tax professional like James D. Miller CPA, it’s the perfect time to talk tax strategy — not just filing. What’s your estimated liability? Are you maximizing deductions for vehicles, tools, or depreciation? Should you shift to a different business structure for 2026?
4. Review Estimated Taxes
If your business is doing better than expected, you may owe more in Q3 estimated tax payments, also due on September 16, 2025. Your CPA can help you avoid underpayment penalties.
Need Help Meeting the Deadline?
At James D. Miller CPA, we specialize in helping business owners — especially those in construction, trade services, and blue-collar industries — stay compliant and reduce tax liability. We know your time is valuable, especially during peak project seasons.
Our firm offers:
- Fast turnaround for extended business returns
- Personalized tax planning consultations
- Industry-specific deductions and strategies
- Local expertise with San Diego County and California tax laws
Final Thoughts
The September 15 business tax deadline isn’t just another box to check — it’s a strategic opportunity. By taking action now, you not only avoid penalties but also lay the groundwork for smarter year-end planning and a stronger 2026.Need a hand finishing your business return or planning ahead? Contact James D. Miller CPA today to schedule your consultation. Let us help you take the stress out of tax season.