California Self-Employed — Schedule C, Form 540 & SMLLC 568

Establish the taxpayer's full-year California residency status, entity structure (sole proprietor vs. SMLLC), and filing status. Confirm whether a Form 568 is required for an SMLLC and identify any part-year or non-resident complications that push outside this workflow's scope.

Gather and reconcile the business profit-and-loss for the year. Categorize all income (gross receipts from 1099-NEC, 1099-K, and direct invoices) and deductible expenses across Schedule C Parts I–V, including home office (Form 8829 or simplified method), vehicle mileage (Form 4562 if actual method), and contract labor that may trigger 1099-NEC issuance obligations.

Compute Schedule SE self-employment tax on net Schedule C profit (15.3% on first $176,100 of net earnings, 2.9% Medicare above that for 2025), then derive the above-the-line deductions for half of SE tax, self-employed health insurance premiums (Form 7206), and any Solo 401(k), SEP-IRA, or SIMPLE IRA contributions. Also evaluate the Section 199A QBI deduction (20% of qualified business income, subject to W-2 wage and UBIA limitations for higher-income taxpayers) on Form 8995 or 8995-A.

Assemble the complete federal return: Schedule 1 (above-the-line adjustments including half SE tax, SE health insurance, and retirement), Schedule 2 (SE tax and any underpayment penalty from Form 2210), and Form 1040 applying the standard or itemized deduction, QBI deduction, and computing total federal income tax. Verify all line-by-line cross-references between schedules and the main form.

Prepare the California individual return starting from federal AGI, then apply California-specific additions (QBI deduction add-back, HSA deduction add-back, bonus depreciation and Section 179 excess add-back) and subtractions on Schedule CA (540). Apply California tax rates (1%–13.3%), the Mental Health Services Tax (1% on taxable income above $1 million), and any California-specific credits. Note that California does not conform to federal bonus depreciation or the QBI deduction.

If the business is operated through a California-registered single-member LLC, prepare Form 568. This is separate from Form 540 and requires payment of the $800 annual LLC tax (due April 15), the LLC fee based on total California income (Schedule IW), and estimated LLC fee payments on Form 3536 for the following year.

Compute the 2026 California estimated tax payment schedule using the mandatory 30/40/0/30 installment allocation (R&TC section 19136.1). Determine the required annual payment under the 90%-of-current-year or 100%-of-prior-year safe harbor, then allocate: 30% due April 15, 2026; 40% due June 15, 2026; $0 due September 15, 2026; 30% due January 15, 2027. Flag the common error of paying equal quarterly amounts, which causes underpayment.

Run the complete cross-form verification: Schedule C net profit flows to Schedule 1 Line 3, Schedule SE tax to Schedule 2 Line 4, California Schedule CA adjustments to Form 540 Line 17, and Form 568 balance is confirmed separate from Form 540. Produce the reviewer brief and taxpayer action checklist with specific payment amounts, methods (IRS Direct Pay or EFTPS for federal; FTB Web Pay for California), and deadlines.