What an S corporation owner should expect from a year-round CPA

The appeal of an S corporation is well understood: profit distributed to the owner is not subject to self-employment tax. The cost of that treatment is a set of obligations that a sole proprietor never faces, and the owners who run into trouble are almost always the ones who elected S status for the tax benefit without absorbing the administrative side of it.

The filings, and how they connect

For a calendar-year S corporation, a normal year involves:

• Form 1120-S, the federal S corporation return, due March 15. For the 2026 tax year that is Monday, March 15, 2027.
• The state pass-through return, which varies by state. In Idaho that is Form 41S; in Montana, Form PTE.
• Schedule K-1, which reports your share of income, deductions, and credits and flows onto your personal return.
• Form 1040 with the K-1 included, due April 15. For 2026 that is Thursday, April 15, 2027.
• The state individual return. Idaho Form 40, Montana Form 2, and so on.
• Payroll filings through the year: Forms 941 quarterly, Form 940 annually, W-2 and W-3 by January 31, plus state withholding and unemployment filings.

The entity return is due a month before the personal return for a reason. The K-1 has to exist before the 1040 can be finished. When an S corporation return goes on extension, the owner return usually follows it, because the owner cannot file without the K-1.

An extension moves the filing date, not the payment date. Form 7004 extends the 1120-S to September 15 and Form 4868 extends the 1040 to October 15, but tax owed is still due in April, and interest runs from there.

Reasonable compensation is the issue that actually matters

An owner who works in the business must be paid a reasonable wage through payroll before taking distributions. This is not optional and it is the most common area of S corporation exposure. The IRS position is that compensation for services comes first, and distributions come after.

Set the wage too low and you are exposed to reclassification, back payroll tax, interest, and penalties. Set it too high and you have given up the benefit that made the election worthwhile. The number is not a percentage rule, despite how often one gets repeated. It depends on what the work would cost to hire out, the hours involved, the skill required, and what the business can support.

What year-round attention looks like here: the wage is set deliberately at the start of the year rather than backed into in December, the reasoning is documented, and it is revisited when the business changes shape.

Basis is tracked continuously or it is reconstructed badly

Shareholder basis governs how much loss you can deduct and whether a distribution is tax-free. It begins with what you put in, rises with income allocated to you, and falls with losses and distributions. Distributions in excess of basis are taxable.

Basis is the owner's responsibility to track, not the corporation's, and Form 7203 is filed with the personal return in the years it is required. Practically, a preparer who is engaged for the year maintains the schedule as part of the annual cycle. A preparer engaged in March of the third year is reconstructing it from whatever records still exist, and that reconstruction is expensive and frequently incomplete.

Loans from the shareholder to the corporation create basis. Loans the corporation obtains from a bank generally do not, which surprises owners who have personally guaranteed the debt.

Payroll details owners routinely get wrong

• Health insurance for a more-than-2% shareholder. Premiums paid by the corporation must be included in W-2 wages, in Box 1 but not subject to Social Security and Medicare, in order for the owner to claim the self-employed health insurance deduction. Miss this on the W-2 and the deduction is at risk. Fixing it after January 31 means a corrected W-2.
• Expense reimbursements. Business expenses paid personally should be reimbursed under an accountable plan rather than deducted on the personal return, which is no longer available for employee business expenses. Mileage, home office, and travel all run through this mechanism.
• Retirement contributions. Employer contributions are tied to W-2 wages, so the compensation decision constrains the retirement decision. Deciding in April what you should have done in October is the usual failure.
• Distributions taken unevenly among shareholders. An S corporation may have only one class of stock. Disproportionate distributions can threaten the election itself.

Estimated payments, and the safe harbor

S corporation income is generally not withheld on, so the owner pays quarterly estimates. For a calendar year those fall on April 15, June 15, September 15, and January 15 of the following year.

The safe harbor is worth understanding because it converts an unknowable number into a known one. Penalties are generally avoided by paying the lesser of 90% of the current year tax or 100% of the prior year tax, and that second figure rises to 110% when prior-year adjusted gross income exceeded $150,000. Paying to the prior-year safe harbor means the size of a good year does not create a penalty, even if the balance due in April is large.

Idaho and Montana each have their own estimated payment rules, and a business with activity in both states may owe in both.

What "year-round" should actually mean

The phrase gets used loosely. In practice it should mean a small number of specific touchpoints rather than unlimited access:

• A compensation decision made at the start of the year, with the reasoning written down.
• Estimated payments calculated and sent to you before each due date rather than guessed at.
• A projection in the fourth quarter, while there is still time to act on it.
• Basis and depreciation schedules maintained as the year goes, not rebuilt at filing.
• A call before a large purchase, a new state, a second owner, or a sale, because those are the decisions where timing changes the answer.

Everything on that list is cheap in October and expensive in March. That asymmetry is the entire argument for working with someone through the year rather than handing over a folder in February.