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 verifiedNo state income tax

South Dakota tax deadlines 2026

Last updated May 14, 2026

South Dakota: No state individual or corporate income tax — South Dakota is one of two US states with no income tax of any kind (along with Wyoming).
South Dakota relies on sales tax (4.5% state + local) and bank franchise tax for revenue. CPA firms with SD trust clients should note SD's favorable trust law (no rule against perpetuities, asset-protection trust statutes).

South Dakota state income tax

South Dakota does not impose an individual or wage income tax. Federal individual deadlines still apply to South Dakota residents — see the federal section below.

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

Penalty summary

Failure-to-file: 5%/month up to 25%. Failure-to-pay: 0.5%/month. Interest accrues on unpaid balances.

Authoritative source

Federal S-corp & partnership (Form 1120-S / 1065)

Filing deadline

March 16, 2027

Extension deadline

September 15, 2027

Authoritative source

Federal C-corporation (Form 1120, calendar year)

Filing deadline

April 15, 2027

Extension deadline

October 15, 2027

Authoritative source

Federal payroll (Form 941)

Quarterly payments

Q1 941

Jan – Mar 2026 wages

April 30, 2026

Q2 941

Apr – Jun 2026 wages

July 31, 2026

Q3 941

Jul – Sep 2026 wages

November 2, 2026

Q4 941

Oct – Dec 2026 wages

February 1, 2027

Authoritative source

Authoritative resources

File these South Dakota returns with confidence

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

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Frequently Asked Questions

South Dakota-specific and federal questions for tax year 2026.

South Dakota is widely considered the most trust-favorable US state because: (1) no state income tax means trust income isn't taxed at state level, (2) no rule against perpetuities allows trusts to last forever (dynasty trusts), (3) strong asset-protection trust statutes including self-settled spendthrift trusts, (4) favorable directed-trust statutes separating trustee responsibilities. CPA firms advising high-net-worth families on trust structures often consider SD situs even when the grantor lives elsewhere.

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