Overview

Attachments for business tax returns are not one‑size‑fits‑all. The exact documents you must include depend on the return you file and specific items you claim (depreciation, credits, pass‑through income, etc.). Missing required attachments can trigger notices, adjustments, or audits. Follow IRS form instructions and keep organized records to back up every line item (IRS, About Form 1040; IRS, About Form 1120; IRS, About Form 1065).

Common required attachments by entity

  • Sole proprietor (Form 1040 with Schedule C): Attach Schedule C to Form 1040 to report business profit or loss. If you claim depreciation or amortization, include Form 4562. Self‑employment tax is calculated on Schedule SE (filed with Form 1040) when applicable (see Schedule C guidance).

  • Related FinHelp resources: Schedule C (Profit or Loss from Business) and Which Schedules Go With Your Form 1040?

  • Partnership (Form 1065): File Form 1065 and attach Schedule K‑1 for each partner. The partnership return shows partnership totals; partners use K‑1s to report their shares on individual returns. Include supporting schedules and statements the Form 1065 instructions require (IRS, About Form 1065).

  • S corporation (Form 1120‑S): File Form 1120‑S and attach Schedule K‑1 for each shareholder. Attachments often include depreciation (Form 4562) and any statements required by the 1120‑S instructions.

  • C corporation (Form 1120): Corporations generally attach financial statements or balance sheets, depreciation schedules (Form 4562) when claiming depreciation, and a reconciliation of book to tax income (Schedule M‑1 or Schedule M‑3). Large corporations (generally those with $10 million or more in assets or meeting other thresholds) must file Schedule M‑3 instead of M‑1—follow the 1120 instructions for exact thresholds (IRS, About Form 1120).

Other common attachments

  • Form 8995 or 8995‑A when claiming the qualified business income (QBI) deduction (pass‑through filers).
  • Form 8283 for noncash charitable contributions over the reporting threshold.
  • Form 8825 for rental real estate income reported by partnerships and S corps.
  • Form 4562 for depreciation and Section 179 deductions.
  • Statements supporting credits (e.g., research credit) as required by the credit’s instructions.

What not to do

  • Do not send every receipt with the return. You must summarize expenses on the tax forms and retain receipts in your records. Keep originals or digitized copies for the retention period the IRS recommends (see IRS recordkeeping guidance).
  • Don’t assume information returns (like Forms 1099) must be attached to an individual 1040. Report the income on the correct schedule; the payer files 1099s with the IRS/SSA as required.

Practical filing tips (from practice and common pitfalls)

  • Read the form instructions line by line: each IRS form lists required attachments and statements. That is your primary checklist (IRS, About Form 1120; IRS, About Form 1065).
  • Keep a single organized packet for mailed returns: include the return, required schedules, and any IRS‑requested statements in the order the instructions recommend.
  • For e‑filing, some attachments can be uploaded electronically; others require mailing a paper attachment per the e‑file instructions — confirm on the form’s current filing instructions.
  • Reconcile book to tax: attaching a clear Schedule M‑1 or M‑3 reconciliation reduces follow‑up questions from the IRS.
  • Keep depreciation schedules and business asset lists current year‑to‑year; missing or inconsistent schedules are a frequent audit trigger. In my practice, clients who maintain year‑end fixed‑asset ledgers avoid last‑minute reconstruction and missed deductions.

Checklist before filing

  • Confirm the primary form (1040, 1065, 1120, 1120‑S, etc.).
  • Review that return’s instructions for required attachments and thresholds.
  • Attach all required Schedules (C, K‑1s, M‑1/M‑3) and Form 4562 when relevant.
  • Include statements for credits or unusual items as the instructions require.
  • Retain supporting documents (receipts, bank statements, payroll records) for your records — don’t mail everything to the IRS.

Real‑world examples

  • A sole proprietor claimed significant depreciation but did not include Form 4562; the IRS requested the depreciation schedules and delayed processing until they were supplied.
  • A partnership forgot to issue one partner’s K‑1; the partner’s individual return showed different income and generated reconciliation requests.

Common mistakes

  • Attaching unnecessary original receipts rather than summarized schedules.
  • Failing to attach Form 4562 with depreciation or Section 179 claims.
  • Mixing up information returns (payer responsibilities) with taxpayer attachments.

Authoritative sources

Professional disclaimer

This article is educational and not a substitute for professional tax advice. For complex situations or large transactions, consult a tax professional or the IRS instructions for the form you are filing.

If you want help matching required attachments to your exact return, a tax preparer can review your financials and provide a filing checklist tailored to your business.