How to Start an S Corp in Illinois (and How It Compares to an LLC)

An S Corporation (S Corp) is a federal tax classification that can save you thousands of dollars in self-employment taxes, but it comes with more paperwork than an LLC. If you're considering an S Corp in Illinois, you need to understand both the formation process and whether this structure actually makes sense for your situation. This guide walks you through the exact steps to form one, the Illinois S Corp tax obligations, and how it stacks up against an LLC.

What Is an S Corp, and Why Does It Matter?

An S Corp is not a legal structure you register with Illinois. Instead, it's a tax election. You first form a corporation or LLC under Illinois law, then elect S Corp tax treatment at the federal level (IRS Form 2553). The appeal is significant: S Corps allow you to split your income into a reasonable salary and distributions, potentially reducing the 15.3% self-employment tax on a portion of your earnings.

In Illinois, an S Corp still owes the 1.5% personal property replacement tax (filed on Form IL-1120-ST), but avoids the 7% corporate income tax that C corporations pay. This is where the real savings emerge, and where confusion sets in.

Illinois LLC vs. S Corp: The Direct Comparison

When comparing LLC vs S Corp in Illinois, you're really comparing two different animals. An LLC is a legal structure (registered with the Illinois Secretary of State). An S Corp is a tax classification you layer on top of any business entity.

LLC Taxed as a Partnership (Default)

• Formation: File Articles of Organization with Illinois Secretary of State ($150 filing fee, 10 business days).
• Taxes: The LLC pays the 1.5% replacement tax on net income (a single-member LLC owned by an individual is disregarded and does not). No corporate income tax. Self-employment tax applies to all profits.
• Annual burden: $75 annual report; simpler bookkeeping.

LLC Taxed as an S Corp (S Corp Election)

• Formation: Same LLC filing ($150, 10 business days), then file IRS Form 2553 federally, no separate Illinois form needed.
• Taxes: Owner draws a reasonable salary (subject to payroll taxes), takes the rest as distributions. Only the salary portion is subject to self-employment tax. Still pays 1.5% replacement tax, but on a smaller net income figure if structured right.
• Annual burden: Payroll processing, separate corporate tax return (Form 1120-S federally, IL-1120-ST in Illinois), more accounting.

The Real Difference in Illinois S Corp Tax

Here's the financial truth: If you expect to net $80,000 as a sole owner, an LLC taxed as a partnership means you pay 15.3% self-employment tax on the full $80,000 (approximately $12,240 per year). With an S Corp election in Illinois, you might pay yourself a $50,000 salary (payroll tax approximately $7,650) and take $30,000 as a distribution (no self-employment tax). Total: approximately $7,650. That's about $4,590 in annual savings, but only if your accountant structures it correctly and you handle payroll properly.

The catch: You must pay yourself a "reasonable salary" for work performed. The IRS will challenge S Corps that try to reduce it too far. Factor in payroll processing, separate tax returns, and added accounting fees, and the math often only works at higher profit levels, so have a CPA run your numbers.

S Corp vs. C Corporation in Illinois

You might also hear about LLC vs. corporation in Illinois, these terms describe legal structures registered with the Secretary of State. An actual corporation (C Corp) or S Corp corporation is the entity itself. An LLC is an entity that can elect S Corp tax treatment. Don't confuse them.

C Corporation (Default Corporate Tax)

• Formation: File Articles of Incorporation ($150, 10 business days).
• Taxes: 7% corporate income tax plus 2.5% replacement tax equals 9.5% effective rate on net income. Dividends to owners are taxed again at the individual 4.95% rate (double taxation). Franchise tax obligation, though the first $10,000 of franchise tax liability is exempt as of 2025, so most small corporations owe $0.
• Annual report: $75, due in the 60 days before the first day of the anniversary month.

S Corporation (Taxed as Pass-Through)

• Formation: File Articles of Incorporation or Articles of Organization ($150), then elect S Corp tax status federally (Form 2553). No separate Illinois election form required.
• Taxes: 1.5% replacement tax (filed on IL-1120-ST). No corporate income tax. A corporation that elects S status still files the franchise tax with its annual report (first $10,000 of liability exempt); an LLC taxed as an S Corp owes no franchise tax. Income passes through to owners and is taxed at the individual 4.95% rate. No double taxation.
• Payroll: Owner must take a reasonable salary; payroll taxes apply to that salary portion.

Most small businesses in Illinois that choose to elect S Corp status will form an LLC, not a C corporation. The LLC structure is simpler and more flexible for member agreements. The S Corp tax election, available to both LLC and corporations, is what saves the money.

Step-by-Step: How to Start an S Corp in Illinois

Forming an S Corp in Illinois is really two processes: forming an LLC or corporation, then electing S Corp tax treatment. Here's how:

Step 1: Choose Your Business Name

Your name must be unique and distinguishable in the Secretary of State database. If you're forming an LLC, it must include "Limited Liability Company," "LLC," or "L.L.C." If you're forming a corporation, it must include "Corporation," "Corp.," "Incorporated," "Inc.," or similar. You cannot use "Ltd.," "Co.," or "L.P." in an LLC name.

Search the business database at https://apps.ilsos.gov/businessentitysearch/ to verify availability. You can also reserve a name for $25 for 90 days via Form LLC-1.15 if you want to lock it in before filing.

Step 2: Gather Required Information

Before filing, you'll need:

• Business name (meeting IL requirements above)
• Registered agent name and Illinois street address (must be a real street address, not a P.O. Box; can be your business address)
• Registered agent must be an Illinois resident or an Illinois-authorized entity
• Principal business address in Illinois
• Manager(s) or member(s) names (for LLC) or incorporator/initial director information (for corporation)
• Your federal Employer Identification Number (EIN), apply at https://www.irs.gov before or after filing (it's free)

Step 3: File Articles of Organization (LLC) or Articles of Incorporation (Corporation)

For an LLC, file Form LLC-5.5 (Articles of Organization) with the Illinois Secretary of State at https://apps.ilsos.gov/llcarticles/index.jsp.

For a corporation, file Form BCA 2.10 (Articles of Incorporation) through the Secretary of State's online incorporation application.

Filing fees:

• Standard (10 business days): $150
• Expedited (24 hours): $250

You can file online or by mail. Online is faster. You'll receive a certificate of organization or incorporation in the mail once approved.

Step 4: Obtain an EIN from the IRS

Apply for an Employer Identification Number (EIN) at https://www.irs.gov/ein. You can get one online immediately; fax and mail applications take longer. It's free. You'll need the EIN before you can hire employees or set up a business bank account.

Step 5: File IRS Form 2553 to Elect S Corp Tax Status

This is where the "S Corp" part becomes official, at the federal level. File Form 2553 (Election by a Small Business Corporation) with the IRS. You have two windows:

• Same year: File by March 15 of the tax year you want the election to take effect (or within 2 months and 15 days of formation).
• Late election: File anytime, and it typically takes effect the following tax year, though some circumstances allow late elections to apply retroactively. Consult a CPA for timing.

Important: There is no separate Illinois state form for S Corp election. Illinois automatically recognizes the federal 2553 election.

Step 6: Open a Business Bank Account

Bring your certificate of organization/incorporation, EIN letter, and photo ID to a bank. Keeping business and personal finances separate is not optional, it protects your liability shield.

Step 7: Register for Illinois Department of Revenue Taxes

Register at https://tax.illinois.gov/businesses/registration.html using MyTax Illinois (Form REG-1). This is free and takes one to two business days online. You'll receive a Certificate of Registration if you make retail sales. Even if you don't collect sales tax, register for withholding before you run payroll, since an S Corp pays its owner a salary.

Illinois Tax Obligations After S Corp Election

Once your S Corp is live, here's what you file in Illinois:

Personal Property Replacement Tax (IL-1120-ST)

S Corps file the Replacement Tax return annually. The rate is 1.5% of net income. This is in addition to federal S Corp taxes, but it's lower than what a C corporation pays (2.5% replacement plus 7% corporate income tax). File with the Illinois Department of Revenue by the federal S Corp return deadline (typically March 15 of the following year).

Annual Report (Form BCA 14.05-D for Corporations, or Annual Report for LLCs)

If you formed a corporation, file the annual report with the Illinois Secretary of State. If you formed an LLC that elected S Corp status, you still file the LLC annual report. The fee is $75. It's due within 60 days before your anniversary month (the month you filed the original articles). Miss the deadline and the entity falls out of good standing. An LLC owes a $100 penalty if the report is not filed before the first day of the second month after the anniversary month; a corporation owes a 10% penalty on delinquent franchise tax plus interest. Either can face administrative dissolution.

Federal S Corp Taxes

File IRS Form 1120-S (U.S. Income Tax Return for S Corporation) annually. You'll also need to issue K-1 forms to yourself and any other owners. Your accountant handles this, but understand that S Corp tax preparation is more complex than a simple partnership return.

Tips and Common Mistakes to Avoid

Mistake 1: Skipping the Reasonable Salary

Many S Corp owners try to minimize the salary portion to avoid payroll taxes. The IRS looks for this. Your salary must be reasonable for the work you perform. If you're a consultant earning $150,000, taking a $30,000 salary and $120,000 in distributions invites IRS scrutiny. Ask your CPA what salary is reasonable for your role and industry.

Mistake 2: Forming Too Early

Don't elect S Corp status before your profit is high enough for the savings to cover the added costs; have a CPA run the numbers. The extra accounting and payroll processing costs will eat any tax savings at lower income levels. If you're just starting, an LLC taxed as a partnership is simpler and cheaper until you scale.

Mistake 3: Forgetting to Register with Illinois Department of Revenue

Even if you have no sales tax, register for withholding once you run payroll. Skipping this creates compliance gaps and can result in back taxes plus penalties.

Tip 1: Consider a PTE Election as an Alternative

Illinois offers a Pass-Through Entity (PTE) elective tax. An S Corp or partnership that still owes the 1.5% replacement tax can also elect to pay the optional 4.95% PTE tax at the entity level on its income. Whether it helps depends on the owners. Run the numbers with your CPA.

Tip 2: Get Payroll Software or a Service

Once you elect S Corp status, you must file payroll taxes quarterly (federal Form 941 and Illinois Form IL-941). Use software like Gusto, ADP, or Paychex, or hire a bookkeeper. DIY payroll is how mistakes happen.

Tip 3: Keep Clean Accounting Records

S Corps require separate accounting from day one. Commingling personal and business expenses muddies your books and can invite IRS scrutiny. If an audit happens and you can't demonstrate clean records, you may face adjustments, penalties and a harder time defending your S Corp treatment.

When to Consult a Professional

This article is informational and does not constitute legal or tax advice. Before electing S Corp status, consult a CPA or tax professional who understands Illinois requirements. They should:

• Model your specific income scenario to confirm S Corp savings actually outweigh the added complexity.
• Advise on the reasonable-salary threshold for your role and industry.
• Set up payroll and tax filings to stay compliant.
• File the Form 2553 correctly and on time.
• Handle IL-1120-ST and all other state-level filings.

A qualified Illinois business attorney should also review your LLC operating agreement or corporate bylaws to ensure governance and liability protections are solid.

Disclaimer

This content is educational and informational only. It does not constitute legal, tax, or accounting advice. Business formation and tax classification decisions depend on your specific facts, industry, and financial situation. Always consult a qualified attorney and certified public accountant (CPA) before forming an S Corp or making tax elections. The fees, timelines, and requirements outlined here are accurate as of the publication date but may change. Verify current information with the Illinois Secretary of State (https://www.ilsos.gov/departments/business-services.html) and the Illinois Department of Revenue (https://tax.illinois.gov/) before filing.