Key takeaways
- Idaho Form 41S is due April 15, 2027 for a calendar-year 2026 S corporation, a month after the federal Form 1120-S due date of March 15, 2027.
- The Idaho extension is six months to file, but only with a valid extension, and it never extends the time to pay.
- Every nonresident individual owner with $1,000 or more of Idaho distributable income needs a decision: Form PTE-NROA, composite, or withholding.
- The Affected Business Entity election is made on the return each year and is not mailed to the Tax Commission.
Who files Form 41S and when it is due
An S corporation files Idaho Form 41S if it does business in Idaho or is registered with the Idaho Secretary of State to do business there. Idaho counts owning or leasing property, soliciting business, having Idaho income, or having an agent acting for the company in the state. An S corporation protected by Public Law 86-272 is exempt, so the first question for a remote seller is what it does in Idaho, not where it is incorporated.
Idaho and the IRS use different dates for a calendar-year 2026 return. Form 1120-S is due on the 15th day of the third month, March 15, 2027. Idaho Form 41S is due on the 15th day of the fourth month, April 15, 2027. Quote the client both dates in one sentence so no one assumes the Idaho filing can wait on a late federal return. Idaho treats the next business day as timely when its date lands on a weekend or holiday. For tax year 2025 the federal date was March 16, 2026 because March 15 fell on a Sunday; that is history and does not apply to 2026 returns.
On the first Idaho return, attach a copy of the federal S election approval or Form 2553. Idaho accepts the federal election, so there is no separate Idaho election. Ask for the old IRS acceptance letter in the first document request.
What goes in the filing package
Idaho lists the order: Form 41S pages 1 and 2, Form 42, Form 75, Form 44, Form PTE-12 if applicable, Form ID K-1 for each owner if applicable, additional schedules in alphabetical order, additional forms in numerical order, and a complete copy of the federal return. Because the federal copy is required, the Idaho return cannot be finished before the federal return is final.
Idaho does not let the S corporation carry a loss forward at the entity level. S corporations cannot deduct NOL carrybacks or carryforwards, and the loss passes through to shareholders. An S corporation that makes the Affected Business Entity election is the exception and may claim an NOL.
Form 42 appears in the Idaho filing order, and apportionment for a multistate S corporation is reported there. The Idaho page confirms one rule you can act on: the apportionment attributes of a disregarded entity or a qualified subchapter S subsidiary are added into the parent's Idaho factor, and the return needs a schedule listing disregarded entities by name and federal ID number. For a client with single-member LLC subsidiaries, ask for each one's Idaho sales and total sales before building Form 42. I did not verify the factor formula itself for 2026, so use the current Form 42 instructions for it.
Decide how each nonresident owner is handled
Form PTE-12 is the schedule of pass-through owners, and each owner gets a filing code. Resident and part-year resident individuals are code R and report the income themselves. Corporations, partnerships, trusts, estates and nonresident individuals with under $1,000 of Idaho distributable income are code N. Neither group triggers composite filing or withholding.
A nonresident individual with $1,000 or more of Idaho distributable income gets one of three treatments. Code A means the owner signed Form PTE-NROA, the Idaho Nonresident Owner Agreement, and will file an Idaho return and pay tax on the entity's Idaho-source income. Code C means the entity pays tax at the corporate rate on the entity return as a composite payment. Code W means the entity withholds at the highest individual rate and files a separate Form PTE-01 for each owner, due by the 15th day of the fourth month after year end.
Worked example. A Boise S corporation has a resident owner with 60 percent and an Oregon owner with 40 percent. If the Oregon owner's share of Idaho distributable income is $20,000, that owner needs a treatment. Composite tax is that share times the corporate rate. Idaho's S corporation page lists 5.3 percent for 2025 and no 2026 rate as of November 10, 2026, so using 5.3 percent for a 2026 return is a planning number only, so quote the client a range and not a final figure. If the Oregon owner's share were $900, code N would apply and nothing is paid at the entity level.
Composite has a cost. Composite filers cannot use NOL or capital loss carryovers or personal exemptions for that owner, so an owner with a large carryover is usually better off signing Form PTE-NROA and filing personally. Keep every signed agreement in the file.
Owners who move. An owner who lived in Idaho for part of 2026 is a part-year resident and falls under code R, not the nonresident choices, so confirm the move date and whether the owner will file as a part-year resident before you assign a code. An owner who lived outside Idaho all year but holds Idaho-source income is the $1,000 test case above. Ask for the December 31 address and the move date for each owner.
The Affected Business Entity election
Idaho's pass-through entity tax is the Affected Business Entity, or ABE, election. A partnership or S corporation can elect to pay Idaho income tax at the corporate rate, and owners receive a credit against their Idaho income tax. The election is made by checking the ABE box on the return and must be renewed every year. The election form is kept on file and not sent to the Tax Commission, so the file copy is your only evidence.
The ABE payment deadline is the same as the return, April 15 for a calendar-year filer. A first-time ABE with no prior Idaho return must pay 80 percent of the tax due by that date to qualify for an extension. Estimated payments are voluntary. An ABE cannot claim the Idaho capital gains deduction on sales of Idaho property, because that deduction passes to the owners, so a year with an Idaho property sale needs a side-by-side calculation.
Idaho's conformity bill, House Bill 559, conforms state law to most provisions of the federal One Big Beautiful Bill Act, according to a Tax Commission press release dated February 17, 2026. As of November 10, 2026, the Idaho S corporation page lists no 2026 tax rate, so treat any ABE figure as an estimate until the 2026 rate and forms are posted.
Extension rules and the payment test
Idaho lets an entity file up to six months after the original due date, but only with a valid extension. The test is prepayment: withholding, estimated payments or amounts applied from a prior refund must equal 80 percent of the estimated tax on the current return or 100 percent of the income tax on the prior-year return. With no prior-year return, use 80 percent of the current year. No payment is needed if the prior-year tax was zero or if the required payment is $50 or less.
Worked example. The prior-year Idaho tax was $4,000 and the 2026 estimate is $10,000. The 80 percent test needs $8,000, and the 100 percent prior-year test needs $4,000, so $4,000 paid by April 15, 2027 satisfies the rule. Form 41ES is the worksheet that documents it. If a payment is due, pay through the state's payment tools or mail the voucher at the bottom of Form 41ES. If none is due, Idaho says not to send Form 41ES.
The extension is not an extension to pay. Interest runs from the original due date on unpaid tax, and Idaho's extension page says penalties run at 0.5 percent per month up to 50 percent of the tax due. The federal extension is separate: Form 7004 must be filed by the regular Form 1120-S due date, and an Idaho extension does not substitute for it.
Documents to collect from the client
Request these before March: the federal S election approval or Form 2553 for a first Idaho return; the prior-year Form 41S and Form PTE-12; the shareholder list with each owner's state of residence on December 31 and any mid-year move date; any signed Form PTE-NROA from last year; and the prior-year ABE election form if one was filed. Add Idaho sales and total sales by entity if the client has disregarded subsidiaries.
Message to send: "For each owner, please send their home address on December 31, 2026 and tell us if anyone moved states during the year. If an owner lives outside Idaho, we will decide between a signed nonresident agreement, a composite payment, or withholding before the April 15, 2027 filing date." Mid-year moves are the most common reason a PTE-12 code is wrong.
If the client resists, point out that each nonresident owner at or above $1,000 needs one of the three treatments, and that the package needs a complete copy of the federal return, so delays in federal work delay Idaho.
After filing: amendments and records
Amend Idaho whenever the federal return is amended, using Form 41S with the Amended Return box checked and a reason stated. If the IRS changes the federal return, notify the Tax Commission in writing within 180 days of the final federal determination. Late reporting of additional tax carries a 5 percent penalty plus interest.
Keep returns and supporting documents at least seven years. For each year, keep the owner treatment record: the PTE-12 code, signed agreements, ABE election form and extension worksheet.