Onboarding Business Tax Clients: Partnerships, S Corps and C Corps
Business returns have earlier deadlines and more moving parts than individual returns. Everything to collect when onboarding a partnership, S corporation or C corporation client, and when to collect it.
Quick answer: When onboarding a business tax client, collect entity and ownership documents, the EIN letter, the last two or three years of business returns, year-end financial statements or a trial balance, depreciation and loan schedules, payroll filings, prior K-1s, and contact details for every owner. Start early: calendar-year partnership and S corporation returns for 2026 are due March 15, 2027, a month before individual returns, and owners need their K-1s to file their own returns.
Key takeaways
- Pass-through returns are due a month before individual returns.
- The condition of the books determines how long the return takes. Check it before quoting.
- Prior returns and depreciation schedules carry forward into this year's return.
- Late business returns delay every owner's personal return through late K-1s.
What to collect
Entity documents
- Formation documents and state registration
- EIN confirmation letter
- S corporation election acceptance, if applicable
- Operating, partnership or shareholder agreements
- Owners, ownership percentages and changes during the year
Prior filings
- The last two to three years of federal and state business returns
- Prior-year K-1s
- Payroll tax filings and year-end payroll reports
- Any IRS or state notices
Financial records
- Year-end balance sheet and profit and loss, or a trial balance
- Bank, card and loan statements for year end
- Fixed asset and depreciation schedules
- Loan agreements and amortization schedules
- Records of owner contributions and distributions
- Information for 1099s the business issued. See how to collect W-9s.
Access and authorizations
- Accountant access to the accounting software
- Form 8821 or 2848 for business authorizations
Key dates (tax year 2026)
| Entity | Return | Due | Extended due |
|---|---|---|---|
| Partnership | Form 1065 | March 15, 2027 | September 15, 2027 |
| S corporation | Form 1120-S | March 15, 2027 | September 15, 2027 |
| C corporation | Form 1120 | April 15, 2027 | October 15, 2027 |
Extensions use Form 7004. Full calendar: 2027 tax deadlines.
Late filing penalties for pass-throughs
Partnership and S corporation returns have a late-filing penalty charged for each owner, for each month the return is late, even when no tax is due, as described on the IRS failure to file penalty page. That makes on-time filing or extension especially important for new business clients whose books may need work.
A timeline that works
- Onboarding (fall): engagement letter, entity documents, prior returns, access
- December: review books, flag cleanup, confirm 1099 obligations
- Early January: year-end document request
- Mid-February: internal document deadline
- March 1: decide on extension
- March 15: file or extend; issue K-1s
When K-1s are late, every owner waits. See late K-1s: what to do.
Business clients often need documents from several people: owners, office managers and outside payroll providers. Correctdocs sends one secure link, checks each statement and form as it arrives for the right business, account and period, and follows up automatically.
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Frequently asked questions
When are S corporation returns due for 2026?
March 15, 2027 for calendar-year S corporations, or September 15, 2027 with an extension on Form 7004.
What documents do I need from a new business tax client?
Entity documents, EIN letter, prior returns and K-1s, year-end financials, depreciation and loan schedules, payroll filings and access to their accounting software.