Key takeaways
- Washington has no personal or corporate income tax, so an S corporation files no state income tax return.
- The S election does nothing for B&O tax, which is reported on the Combined Excise Tax Return based on gross receipts.
- Washington has no owner-level tax on pass-through income, so there is no PTE elective tax, composite return or nonresident withholding to run.
- The capital gains excise tax is an individual filing, but it can reach a shareholder's share of gains allocated to Washington.
What an S corporation in Washington actually files
The entity files Form 1120-S with the IRS and furnishes Schedule K-1 (Form 1120-S) to each shareholder. That is the whole income tax return stack for the corporation. Washington has no personal income tax and no corporate income tax, so there is no state S election, no state composite return and no state version of the 1120-S.
What Washington adds is the Business and Occupation tax. It is a gross receipts tax on the value of products sold or income earned, with no deduction for labor or materials, and it applies to S corporations, C corporations and LLCs alike. The return is the Combined Excise Tax Return, filed through MyDOR.
The third piece belongs to the shareholders, not the company: the capital gains excise tax. When a client says the Washington S corporation return is done, ask which of the three they mean.
Federal due dates for the tax year 2026 return
Form 1120-S is generally due the 15th day of the 3rd month after year end. For a calendar-year 2025 return that date was March 16, 2026, because March 15 fell on a Sunday. For tax year 2026 the 15th is a Monday, March 15, 2027, so no shift applies.
Form 7004 gives an automatic six-month extension to file, not to pay. The K-1 deadline is tied to it: the instructions say Schedule K-1 must be provided to each shareholder on or before the day the Form 1120-S is required to be filed.
A missed Form 7004 is expensive. The current instructions list $255 per shareholder for each month or part of a month, up to 12 months, when no tax is due. Three shareholders and a return filed two and a half months late is nine shareholder-months at $255 each. The amount is adjusted periodically, so confirm it on the tax year 2026 instructions.
A corporation that must file 10 or more returns of all types in a year has to e-file Form 1120-S, so count the client's payroll and information returns before assuming paper is allowed.
B&O tax: the state filing that applies to the S corporation
The B&O tax has more than 50 classifications, and a company with several activities can owe under more than one. A bookkeeping client that also resells software might report service income in one classification and retail sales in another. Check the Multiple Activities Tax Credit and the Small Business B&O Tax Credit on each return.
The Department of Revenue assigns the filing frequency. By annual tax liability, $1,050 or less is annual, $1,051 to $4,800 is quarterly and $4,801 or more is monthly. Monthly returns are due the 25th of the following month, quarterly returns at the end of the month after the quarter (the first quarter is due April 30) and annual returns April 15. A weekend or holiday moves the date to the next business day.
A small company may not need to file. For calendar years starting January 1, 2023 or later, a business can qualify for active non-reporting status if gross income across all B&O classifications is under $125,000, it does not collect retail sales tax, its public utility tax gross income is under $24,000 and it owes no other DOR tax or fee. A client with a sales tax permit for resale items fails the test.
Out-of-state S corporations are not exempt. A business must register for B&O if it has physical presence in Washington, more than $100,000 of gross receipts sourced to Washington, or is organized or domiciled there. An Idaho consultant with no office in the state crosses the receipts line once Washington clients pay more than $100,000.
Capital gains excise tax and the S corporation shareholder
The capital gains excise tax is 7 percent on the sale or exchange of long-term capital assets and applies only to individuals. The company does not file it. Shareholders can owe it on their share of long-term gains passed through from the S corporation, but only for gains allocated to Washington.
Starting with tax year 2025 there are two tiers: 7 percent on the first $1 million of taxable Washington capital gains and 9.9 percent above that, which is the 7 percent plus an additional 2.9 percent. The standard deduction was $270,000 for 2024 and $278,000 for 2025, adjusted for inflation, so look up the 2026 amount before quoting a number.
Real estate and depreciable or Section 179 business assets are among the exempt categories, so most equipment sales never reach the calculation. Stock, business interests and goodwill-type gains can. A credit equal to the B&O tax paid on the same sale is available starting with tax year 2025, so put the S corporation's B&O on that sale in the shareholder's file.
Why there is no PTE election or nonresident withholding
Washington does not have owner-level personal income tax on pass-through entity income. That removes the usual state machinery: no elective pass-through entity tax to model around the SALT deduction cap, no nonresident shareholder withholding and no composite return.
The question comes back through the owners' home states. A Washington S corporation with an Oregon-resident shareholder can have an Oregon resident return on the K-1 income, and a Washington resident shareholder in a company operating in another state is the nonresident there. Use the state-by-state overview to find the right state guide.
Documents to collect from the client
For the federal return, collect the year-end trial balance, shareholder basis worksheet, loan balances, fixed asset additions and the shareholder list with percentages and any ownership changes. Confirm the Form 2553 acceptance letter: the election must generally be filed within 2 months and 15 days of the start of the tax year it takes effect, and an unaccepted election is a bigger problem than any state form.
For Washington, ask for each Combined Excise Tax Return filed for 2026 with the MyDOR confirmation, the DOR frequency notice and any credit carried forward. Request the notice instead of asking which frequency they file on, because clients guess and the answer decides whether a quarterly return was due April 30. Compare gross income across all returns to the Form 1120-S. A gap means unreported receipts or a classification error.
For shareholders, ask: "Did the company sell any equipment, real estate, a business interest or investments in 2026? If yes, upload the closing statement or bill of sale." Also ask whether any shareholder lives outside Washington. Usual can hold this as a request with a yes or no gate on each item and a not-applicable answer, so clients with no sales do not leave blanks that look like missing documents.
Failure modes to catch before filing
The most common miss is a client who elected S status and assumes B&O tax disappeared. The tell is gross receipts on the 1120-S and no excise returns in MyDOR. Check registration status and the non-reporting test before assuming nothing is owed.
The second is a shareholder gain that never reaches the Washington side. If a K-1 shows long-term gain from selling an entity interest, the individual may have a capital gains return due the same day as Form 1040, April 15, even though the S corporation filed on time.
The third is the extension mismatch. A capital gains extension must be requested through MyDOR on or before April 15 and requires a federal extension, but it does not extend payment. Paying less than 80 percent of the tax due counts as substantially underpaid and carries its own penalty. Starting with tax year 2026, payment can be made up to six months before the due date, which helps a client who just closed a sale.