Most CPAs build their careers on individual returns and pass-through entities. Then a C corporation client shows up, and the rules shift. The flat 21% entity-level tax, double taxation on distributed earnings, compressed loss rules, and a completely different set of accounting method considerations make corporate returns a distinct practice area. Not harder, necessarily. Just different enough to trip up practitioners who assume the pass-through logic carries over.
Why C Corps Play by Different Rules
The fundamental difference is structural. S corporations and partnerships are pass-through entities: income flows to the owners, who pay tax at individual rates. One level of tax. C corporations pay tax at the entity level (a flat 21% under IRC Section 11(b), permanent since TCJA) and shareholders pay again when earnings are distributed as dividends. Two levels.
| C Corporation | S Corporation | Partnership | |
| Tax level | Entity + shareholder | Shareholder only | Partner only |
| Federal rate | Flat 21% at entity; dividends taxed to shareholder | Individual rates (up to 37%) | Individual rates (up to 37%) |
| QBI deduction | Not available | Up to 20% deduction | Up to 20% deduction |
| Loss limitations | Corporate NOL rules; 80% of taxable income | Basis, at-risk, passive activity, EBL | Basis, at-risk, passive activity, EBL |
| Tax return | Form 1120 | Form 1120-S | Form 1065 |
That double taxation is the defining feature. It’s also why compensation planning, dividend timing, and entity structure decisions matter more in the C corp context than anywhere else in the code.
Accounting Methods: Cash Isn’t Always Available
For individual and small business returns, the cash method is the default. Corporate taxation flips that assumption. C corporations with average annual gross receipts exceeding $30 million over the prior three tax years are generally required to use the accrual method. Smaller corporations may still elect the cash method, but the eligibility threshold matters because it determines when income is recognized and when deductions are available.
Accrual-basis corporations recognize income when all events have occurred to establish the right to receive it and the amount can be determined with reasonable accuracy. That’s the all events test, and it applies to the deduction side too (with the additional requirement of economic performance). The practical impact: a corporation can owe tax on income it hasn’t collected yet, and a deduction for services might not be available until the services are actually performed, regardless of when the liability was incurred.
Deductions: The Same Rules, Except When They’re Not
The general rule for corporate deductions is straightforward: ordinary, necessary, and reasonable. In practice, several deductions that individual and pass-through practitioners take for granted work differently at the corporate level.
Charitable contributions are capped at 10% of taxable income (computed before the deduction itself and before certain other adjustments). Excess contributions carry forward five years. The dividends received deduction, which has no analog on individual returns, allows corporations to deduct 50%, 65%, or 100% of dividends received from other domestic corporations, depending on the ownership percentage. Employer-provided meals follow their own substantiation and deduction rules.
On the capital expenditure side, the OBBBA restored 100% bonus depreciation for qualified property placed in service after January 19, 2025. Section 179 expensing increased to $2.5 million (with a $4 million investment limit). And the full expensing election for certain manufacturing property added another layer to the cost recovery analysis. For practitioners coming from pass-through work, the depreciation rules are mechanically similar. The difference is how those deductions interact with corporate-specific limitations like the charitable contribution cap and the NOL rules.
Losses: Three Categories, Three Rulesets
Corporate losses don’t all behave the same way. Net operating losses, capital losses, and passive activity losses each follow distinct rules.
Corporate NOLs carry forward indefinitely but can only offset 80% of taxable income in any given year. The OBBBA also restored a two-year carryback for certain NOLs. Capital losses are even more restrictive: C corporations can only deduct capital losses against capital gains (no $3,000 deduction against ordinary income like individuals get). Unused capital losses carry back three years and forward five.
Passive activity loss rules apply to closely held C corporations and personal service corporations, though the mechanics differ from how they work for individuals. A closely held C corporation (where five or fewer individuals own more than 50% of the stock) can offset passive losses against active income but not portfolio income. Personal service corporations face a stricter version: passive losses can only offset passive income, full stop.
Where Pass-Through Practitioners Get Tripped Up
The most common stumbles happen at the intersections between familiar pass-through concepts and corporate-specific rules.
Reasonable compensation runs in the opposite direction. S corp practitioners typically minimize officer salaries to reduce payroll tax exposure. With C corps, the incentive often runs the other way: higher salaries are deductible to the corporation, reducing the entity-level tax. But the IRS scrutinizes compensation that exceeds what’s reasonable for the services performed, and excess compensation isn’t deductible.
The accumulated earnings tax (a 20% penalty tax on earnings retained beyond the reasonable needs of the business) and the personal holding company rules catch practitioners who aren’t expecting them. Neither exists in the pass-through world. Both can create unexpected tax exposure for closely held C corporations, particularly those with significant passive income or investment portfolios.
Surgent CPE’s Understanding Corporate Taxation: Accounting Methods, Deduction Timing, and Losses (CTM2) with Dave Peters, CPA, covers these topics in detail, including the all events test, depreciation elections, and the full corporate loss framework.