What happens if I file my federal return after April 15?

If you owe tax and file after April 15 without an approved extension, the IRS assesses a failure-to-file penalty (5% of unpaid tax per month, up to 25%) plus a failure-to-pay penalty (0.5% per month) plus interest. If you're due a refund, there's no late-filing penalty — but you have only three years from the original deadline to claim it. CPAs should advise extension filers that an extension extends time to file, never time to pay.

Do state filing deadlines automatically follow federal?

Most states conform to the federal April 15 individual deadline, but conformity varies for corporate, pass-through, and franchise filings. Several states have unique deadlines: Louisiana (May 15), Virginia (May 1), Iowa (May 1 for individual), Delaware (April 30 for individual). Always verify against the state revenue department site — TaxScout links to the authoritative source on every state page.

Are estimated quarterly payments mandatory for CPA-firm clients?

Federally, individuals must pay estimated tax if they expect to owe $1,000 or more after withholding. C-corporations have a $500 threshold. Underpayment triggers Form 2210 / 2220 penalties calculated quarterly. Most CPA firms advise clients to safe-harbor at 100% of prior-year liability (110% for AGI over $150K) to avoid the penalty entirely.

Tax Year 2026 · Filed 2027State-revenue-department verified

Illinois tax deadlines 2026

Last updated May 13, 2026

Illinois individual income tax (Form IL-1040) conforms to the federal April 15 deadline.
Illinois Pass-Through Entity (PTE) tax election is filed quarterly with estimated payments mirroring federal estimated dates.
Illinois replacement tax (1.5% on partnerships, 2.5% on S-corps) applies in addition to standard income tax.

Illinois individual income tax (Form IL-1040)

Filing deadline

April 15, 2027

Extension deadline

October 15, 2027

Illinois imposes a flat 4.95% individual income tax. An automatic six-month extension is granted via Form IL-505-I if tax is paid by April 15.

Quarterly payments

Q1 2026

Jan 1 – Mar 31, 2026

April 15, 2026

Q2 2026

Apr 1 – May 31, 2026

June 15, 2026

Q3 2026

Jun 1 – Aug 31, 2026

September 15, 2026

Q4 2026

Sep 1 – Dec 31, 2026

January 15, 2027

Authoritative source

Illinois S-corp & partnership (Form IL-1120-ST / IL-1065)

Filing deadline

March 16, 2027

Extension deadline

September 15, 2027

Replacement tax: 1.5% on partnerships, 2.5% on S-corps. Pass-Through Entity (PTE) tax election (4.95%) provides federal SALT cap workaround.

Authoritative source

Federal individual income tax (Form 1040)

Filing deadline

April 15, 2027

Extension deadline

October 15, 2027

Quarterly payments

Q1 2026

Jan 1 – Mar 31, 2026

April 15, 2026

Q2 2026

Apr 1 – May 31, 2026

June 15, 2026

Q3 2026

Jun 1 – Aug 31, 2026

September 15, 2026

Q4 2026

Sep 1 – Dec 31, 2026

January 15, 2027

Authoritative source

Authoritative resources

File these Illinois returns with confidence

TaxScout's tax form library walks through every IRS form CPA firms file for Illinois clients — who must file, key fields, common mistakes, and step-by-step instructions.

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Frequently Asked Questions

Illinois-specific and federal questions for tax year 2026.

Illinois allows pass-through entities (partnerships, S-corps) to elect to pay the 4.95% Illinois income tax at the entity level rather than passing it through to owners on their personal Illinois returns. This provides a federal SALT cap workaround — the entity-level tax is deductible on the federal partnership / S-corp return, reducing federal taxable income passed to owners.

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