Professional Corporation Taxation: C-Corp, S-Corp, and Section 199A

A professional corporation is taxed as a C-corporation by default, paying a flat 21% federal income tax on any profit it retains, with several restrictions that ordinary corporations do not face. Most professional firms respond to that default in one of two ways: they distribute nearly all profit as deductible compensation before year-end, or they elect S-corporation status so income passes through to the owners’ personal returns. Professional corporation taxation carries its own rulebook, and choosing the wrong structure, or missing an election deadline, can be an expensive mistake.

Which Firms Are Treated as Professional Service Corporations

Federal tax law applies Professional Service Corporation rules to firms where nearly all employee time goes toward services in specific fields: health, law, engineering, architecture, accounting, actuarial science, performing arts, or consulting.¹ The IRS reads those categories broadly. “Health” covers physicians, dentists, nurses, veterinarians, physical therapists, and psychologists. “Accounting” includes enrolled agents and tax return preparers alongside CPAs. “Consulting” sweeps in lobbyists and professional advisors but specifically excludes architecture and engineering firms from that bucket.²

To qualify, at least 95% of employee time must be spent performing services in one of those fields. The stock must also be substantially all owned by current employees who perform the services, retired employees, or their estates.¹

A newly formed professional corporation is automatically classified as a C-corporation for federal tax purposes. It files Form 1120 each year to report income and deductions unless it elects S-corporation treatment.³

How the Default C-Corp Tax Works

A professional service corporation taxed as a C-corp pays a flat 21% federal income tax on all taxable income, starting with the first dollar.⁴ Before the 2017 Tax Cuts and Jobs Act, PSCs faced a punitive flat 35% while other C-corporations could use graduated brackets starting at 15%. The 21% rate now applies uniformly to every corporation, but several PSC-specific restrictions make the C-corp structure more constraining than the headline rate suggests.

Stock in a corporation that performs services in the PSC fields does not qualify as Qualified Small Business Stock under Section 1202. The statute excludes businesses in health, law, engineering, architecture, accounting, actuarial science, performing arts, consulting, athletics, financial services, and brokerage services from the definition of a “qualified trade or business.”⁵ Owners who eventually sell their shares cannot exclude up to $10 million in capital gains the way founders of product-based startups can.

C-corporations that stockpile profits beyond reasonable business needs face a 20% accumulated earnings tax on the excess, stacked on top of the 21% corporate tax.⁶ Most C-corporations can accumulate up to $250,000 before this becomes an issue. Service corporations in the PSC fields get only a $150,000 cushion.⁷ That lower threshold leaves little room for retained earnings.

If a PSC generates substantial passive investment income and meets certain ownership concentration tests, it can also trigger a separate 20% personal holding company tax on undistributed passive income like dividends, interest, and royalties.⁸ This is rare for active practices but becomes a risk when the firm holds significant investments. The combined message from these rules is blunt: the tax code does not want professional service corporations hoarding cash.

Zeroing Out Corporate Income Through Compensation

The standard playbook for a C-corp PSC is to distribute every dollar of profit as deductible compensation before year-end. Salaries, bonuses, and employer retirement plan contributions are ordinary business expenses that reduce corporate taxable income. Done precisely, corporate taxable income lands at or near zero, and the owners pay tax only once, on their personal returns at individual rates.

The IRS scrutinizes this strategy under the “reasonable compensation” doctrine. If the agency decides an owner-employee’s pay exceeds what the market would support, it can reclassify the excess as a nondeductible dividend. That triggers the double taxation the zero-out strategy was designed to avoid: the corporation loses its deduction and owes 21% corporate tax, and the owner still owes individual income tax on the same dollars treated as a dividend. Reasonableness depends on the owner’s responsibilities, hours worked, the firm’s revenue, and what comparable professionals in the same market earn. The burden of proof falls on the corporation, so keeping documentation, whether board resolutions, compensation studies, or a written rationale, matters more than most owners realize.

Every dollar of W-2 wages triggers Federal Insurance Contributions Act taxes. The employer and employee each pay 6.2% for Social Security and 1.45% for Medicare, a combined rate of 15.3%.⁹ Social Security tax applies only up to the annual wage base, but Medicare has no cap. Owners earning above $200,000 in wages owe an additional 0.9% Medicare tax on the excess, with no employer match.¹⁰

One genuine advantage of C-corp status is the tax treatment of fringe benefits. Health insurance premiums and up to $50,000 in group-term life insurance coverage can be fully deductible by the corporation and excluded from the owner-employee’s income, with no W-2 inclusion and no payroll taxes. That benefit disappears when the firm elects S-corp status, which is worth weighing before making the switch.

Calendar Year and Cash Method Rules

A professional service corporation must use a calendar tax year, January 1 through December 31.¹¹ The IRS imposes this to prevent income deferral. A PSC can elect a fiscal year with up to three months of deferral by filing Form 8716 under Section 444, but the election comes with minimum distribution requirements and possible deduction limits under Section 280H.¹² For most practices, the calendar year is simpler.

The PSC catches a break on accounting method. Most C-corporations above a gross receipts threshold must use accrual accounting, but PSCs may use the cash method regardless of revenue.¹ Cash-method accounting gives the firm more control over the timing of income and deductions, which matters when managing year-end compensation distributions.

Electing S-Corporation Status

Many professional corporations elect S-corp status to escape the C-corp regime. The election converts the PC into a pass-through entity: the corporation itself generally pays no federal income tax, and all profits flow through to the owners’ personal returns via Schedule K-1.¹³ Owners report the income on their individual returns at personal ordinary income rates.

To make the election, the corporation files Form 2553 no later than two months and 15 days after the start of the tax year in which the election should take effect. For a calendar-year corporation, that deadline is March 15. The election can also be filed any time during the preceding tax year.¹⁴ Miss the deadline and the firm waits another full year, spending it under C-corp PSC rules.

Ongoing eligibility is limited:¹⁵ no more than 100 shareholders, all of whom must be U.S. citizens or residents (corporations, partnerships, and nonresident aliens cannot hold shares), and only one class of stock, meaning all shares carry identical rights to distributions and liquidation proceeds.

The corporation still files an annual return on Form 1120-S, but it is informational; the tax liability passes through to the shareholders.¹⁶ Pass-through treatment eliminates the double-taxation risk and sidesteps the accumulated earnings tax, the personal holding company tax, and the PSC calendar year requirement. Those rules simply stop applying once the corporation is no longer taxed as a C-corp.

The S-Corp Payroll Tax Split

The main tax-planning lever for an S-corp professional firm is splitting income between W-2 wages and distributions. Both show up as ordinary income on the owner’s personal return, but only the W-2 wages trigger FICA taxes.⁹ Distributions of remaining profit after paying a reasonable salary are not subject to the 15.3% payroll tax split. For a high-earning physician or attorney, that saving can reach five figures annually.

The IRS knows this and audits the split aggressively. The reasonable compensation standard applies here in reverse from the C-corp context: S-corp owners face scrutiny for paying themselves too little. If the IRS determines a salary is unreasonably low, it can reclassify distributions as wages and impose back payroll taxes, interest, and penalties.

The safest approach is to set compensation at a level defensible as fair market value for what a similarly qualified professional would earn in a comparable role. Document the rationale in a written employment agreement, review it annually, and adjust as the practice grows. Once reasonable W-2 compensation is paid, the remaining net income passes through as distributions subject only to income tax.

Fringe Benefits Change Under S-Corp Status

Electing S-corp status trades some fringe benefit advantages for the payroll tax savings. Any shareholder owning more than 2% of the stock is treated more like a partner than a traditional employee for benefit purposes.¹⁷ That changes the treatment of several common benefits:

Health insurance. The corporation can still pay premiums and deduct them, but the amount must be included in the shareholder-employee’s W-2 as income. The shareholder then claims an above-the-line deduction on their personal return, which usually washes out the income inclusion, but only if neither the shareholder nor their spouse had access to a subsidized employer plan elsewhere.¹⁸
Group-term life insurance. The full cost of coverage must be included in the shareholder’s W-2 wages. Rank-and-file employees can exclude up to $50,000 of coverage from income; shareholders above the 2% threshold get no exclusion.¹⁷
Health Savings Accounts. Shareholder-employees cannot make pre-tax contributions through a cafeteria plan. Contributions go in after-tax, and the deduction is claimed on the personal return instead.¹⁷

For solo practitioners and small firms where the owner is the primary beneficiary of these plans, the lost C-corp exclusions can partially offset the payroll tax savings. Run the numbers both ways before making the election, particularly with high health insurance costs or substantial life insurance needs.

Section 199A and the Specified Service Business Phaseout

S-corp professional firms should factor in the Section 199A qualified business income deduction, which allows eligible owners to deduct up to 20% of their share of the firm’s qualified business income on their personal return.¹⁹ Originally set to expire after 2025, the deduction was made permanent by legislation enacted in 2025. C-corp PSCs get no Section 199A deduction at all; the provision applies only to pass-through income.

There is a significant catch for most professionals. The PSC fields (health, law, accounting, consulting, performing arts, financial services, actuarial science, and athletics) are classified as Specified Service Trades or Businesses.² Owners of specified service businesses lose the deduction entirely once their taxable income exceeds certain thresholds, which are adjusted annually for inflation. Below those thresholds the deduction can be claimed in full or in part, depending on where income falls within the phase-out range.

The calculation also excludes any reasonable compensation paid by the S-corporation from the owner’s qualified business income.¹⁹ Higher W-2 wages satisfy the IRS’s reasonable compensation requirement and reduce FICA exposure on distributions, but they also shrink the pool of income eligible for the 20% deduction. Finding the right level requires running projections with both variables.

Engineering and architecture firms are a notable exception. Both are PSC fields for corporate tax purposes but are explicitly excluded from the specified service business category under Section 199A.² Owners of engineering or architecture PCs can claim the full deduction without the income-based phaseout, a meaningful tax advantage over their peers in law or medicine.

The Passive Income Trap After a C-to-S Conversion

A professional corporation that converts from C-corp to S-corp status needs to watch one more trap. If the S-corp still carries accumulated earnings and profits from its C-corp years and more than 25% of its gross receipts come from passive investment income (dividends, interest, rents, or royalties), the corporation owes a tax on the excess net passive income at the highest corporate rate.²⁰ If the problem persists for three consecutive years, the IRS can terminate the S-corp election entirely, pushing the firm back into C-corp status and every PSC rule that comes with it.

The fix is straightforward. Distribute the accumulated C-corp earnings and profits as a taxable dividend early in the S-corp years, or keep passive investment income below 25% of gross receipts. Most active practices generate enough service revenue that this never becomes an issue, but firms holding significant investment portfolios or rental real estate should run the numbers before the three-year clock runs out.

1
Office of the Law Revision Counsel. 26 U.S. Code 448 – Limitation on Use of Cash Method of Accounting
2
eCFR. 26 CFR 1.199A-5 – Specified Service Trades or Businesses and the Trade or Business of Performing Services as an Employee
3
Internal Revenue Service. Instructions for Form 1120
4
Internal Revenue Service. Instructions for Form 1120-W
5
Office of the Law Revision Counsel. 26 U.S. Code 1202 – Partial Exclusion for Gain From Certain Small Business Stock
6
Office of the Law Revision Counsel. 26 U.S. Code 531 – Imposition of Accumulated Earnings Tax
7
Office of the Law Revision Counsel. 26 U.S. Code 535 – Accumulated Taxable Income
8
Office of the Law Revision Counsel. 26 USC 541 – Imposition of Personal Holding Company Tax
9
Internal Revenue Service. Topic No. 751, Social Security and Medicare Withholding Rates
10
Internal Revenue Service. 2026 Publication 926
11
Office of the Law Revision Counsel. 26 U.S. Code 441 – Period for Computation of Taxable Income
12
Internal Revenue Service. Publication 538 – Accounting Periods and Methods
13
Internal Revenue Service. Instructions for Form 1120-S (2025)
14
Internal Revenue Service. Instructions for Form 2553
15
Internal Revenue Service. S Corporations
16
Internal Revenue Service. About Form 1120-S, U.S. Income Tax Return for an S Corporation
17
Internal Revenue Service. Publication 15-B (2026), Employer’s Tax Guide to Fringe Benefits
18
Internal Revenue Service. S Corporation Compensation and Medical Insurance Issues
19
Internal Revenue Service. Qualified Business Income Deduction
20
Office of the Law Revision Counsel. 26 USC 1375 – Tax Imposed When Passive Investment Income of Corporation Having Accumulated Earnings and Profits Exceeds 25 Percent of Gross Receipts