March 15 Partnership & S-Corp Tax Deadline (2026): A Washington Business Owner Guide
Quick answer (for busy owners)
If your business is a calendar-year Partnership (Form 1065) or calendar-year S Corporation (Form 1120-S), the “March 15” deadline moves this year: your federal return and owner K‑1s are due
Monday, March 16, 2026 (for the 2025 tax year).
Need more time? File an automatic extension using IRS Form 7004 to request a 6-month extension for partnerships and S-corporations by the same deadline.
Schedule a March deadline prep appointment with a Monroe, WA bookkeeping team
Who needs to file by March 15/16?
The March deadline generally applies to pass-through business returns where owners receive a Schedule K‑1.
If any of these describe your business, this is your deadline:
- Partnerships filing Form 1065
- Multi-member LLCs are taxed as partnerships (also typically file Form 1065)
- S Corporations filing Form 1120‑S
If you have a fiscal year (not a calendar year), the due date is usually the 15th day of the 3rd month after your year-end, not necessarily March 15.
What’s due (and what “K‑1s” mean)
By the deadline, the IRS expects the business return and the information owners need for their personal returns:
- Partnership: Form 1065 + Schedule K‑1 for each partner
- S‑Corp: Form 1120‑S + Schedule K‑1 for each shareholder
A Schedule K‑1 reports each owner’s share of income, deductions, credits, and other tax items. Owners often can’t finish their personal returns until K‑1s are ready.
Helpful official references: IRS instructions for Form 1065 partnership return and calendar-year filing timing, and IRS instructions for Form 1120-S showing the March 16, 2026, deadline for calendar-year S-corporations.
Not ready? How to file an extension the right way
Key point: An extension gives you more time to file, not more time to fix messy books.
If your books aren’t reconciled, you’re waiting on missing 1099s, or you need more time to review owner distributions, filing an extension can be a smart move.
Step 1: File Form 7004 by the due date
Use the IRS page explaining Form 7004 for automatic business extensions and submit it by the original deadline (March 16, 2026, for most calendar-year partnerships and S-Corps filing 2025 returns).
Step 2: Keep moving on the bookkeeping
Extensions work best when you use the extra time to finalize clean, supportable numbers—rather than rushing a return built on incomplete data.
Step 3: Remember: owners may still need their own extensions
If K‑1s won’t be ready before April’s individual filing deadline, owners may need to consider extending their personal returns, too.
Washington State reminders (no income tax ≠ no deadlines)
Washington State does not have a general individual or corporate income tax, but Washington businesses commonly still have state tax responsibilities—especially B&O (gross receipts) and sales/use tax.
- WA tax due dates vary by filing frequency (monthly, quarterly, annual). The Washington Department of Revenue explains filing frequencies and due dates here: Washington DOR filing frequencies and due dates for excise tax returns (B&O, sales tax, etc.).
- Many Washington cities also impose a local B&O tax, which is separate from the state B&O. (Not all local taxes are administered by the state.)
Washington owner-friendly takeaway: Even though WA doesn’t require a state income tax return, your federal Partnership/S‑Corp return is still required—and your K‑1s still drive what owners report on their federal personal returns.
March deadline checklist (Partnerships & S-Corps)
Use this checklist to reduce last-minute surprises and help your tax preparer move faster:
- Reconcile all bank and credit card accounts through year-end
- Confirm income is categorized correctly (avoid dumping everything into “miscellaneous”)
- Review owner distributions, contributions, and reimbursements (and make sure they’re documented)
- Verify payroll reports (especially if you’re an S‑Corp with shareholder wages)
- Gather 1099s you issued/received and confirm vendor W‑9s are on file
- Provide year-end loan balances and interest statements (business credit cards, vehicles, equipment)
- List fixed assets purchased (equipment, computers, furniture) with dates and amounts
- Confirm ownership percentages and each owner’s legal name/address for accurate K‑1s
- Pull prior-year return (if available), so your preparer can check carryovers and consistency
- Write down “what changed” this year (new owners, new states, new products, major expenses)
FAQ: March 15 Partnership & S-Corp deadline
When is the March 15 deadline for partnerships and S corporations in 2026?
For most calendar-year filers, the deadline falls on Monday, March 16, 2026, because March 15 is a Sunday.
What forms are due for partnerships and S corporations?
Partnerships typically file Form 1065 and provide each partner with a Schedule K-1. S‑corps file Form 1120‑S and provide a Schedule K‑1 to each shareholder.
Does Form 7004 give an automatic extension?
Yes. Filing Form 7004 on time generally provides an automatic extension to file. It’s still important to keep your bookkeeping moving so the extended return is accurate.
Does an extension extend the time to pay?
Not necessarily. Extensions are primarily for filing. Some entities may still owe certain taxes or payments by the original due date, depending on their situation.
Do Washington businesses file a state income tax return?
Washington does not have a general individual or corporate income tax, but businesses often still have WA excise tax obligations, such as B&O and sales/use tax (depending on the business activity).
What should I do if I can’t finalize numbers before March 16?
File an extension (Form 7004), then immediately focus on reconciliation, payroll review, and owner activity tracking so your K‑1s can be completed as soon as possible.
Disclaimer: This article is for general educational purposes and doesn’t constitute tax, legal, or accounting advice. Every business is different—please consult a qualified professional about your specific situation.
