Filing in more than one state

State systems have their own residency, sourcing and part-year concepts. We explain the concepts from the official state material we have read, and we stop there.

State returns

We have read official material for CA, IL, NJ, NY. Everything below is a concept those agencies state in their own words.

• CA: California separates residents, part-year residents and nonresidents, and the category decides which return concept applies.
• CA: For a California nonresident, what matters is income sourced to California, not the reader's total income.
• NY: New York requires the resident, nonresident or part-year question to be settled before the filing question can be answered.
• NY: A New York nonresident may still owe New York tax where there is income from New York sources.
• NY: For a New York part-year resident the year is split, with one part measured by resident rules and the rest by nonresident rules.
• IL: An Illinois resident who worked in Iowa, Kentucky, Michigan or Wisconsin must still file the Illinois return and include that compensation as Illinois income.
• IL: Illinois describes narrow situations where withholding covers the liability and narrow situations where a return is still due, so withholding and the filing duty are separate questions.
• NJ: New Jersey publishes its own individual filing guidance, separate from the federal return.

What we are not saying

What is still open

• All five boundary concepts are certified from 8 established propositions across 4 jurisdictions.
• Scope of certification: Self-File can identify multistate facts and explain that state filing duties require separate state-law analysis.
• Explicitly NOT certified: any determination that a particular state return is or is not required.
• 3 propositions remain uncertified and are recorded rather than filled in.
• The bounded evidence certifies the BOUNDARY, not fifty state filing regimes, so per-state Self-File pages are not justified.

Sources

• CA — Residency status — residents, part-year and nonresidentsCalifornia Franchise Tax Board. Read for: Establishes that California distinguishes resident, part-year and nonresident status, and that nonresident taxation turns on California-sourced income. Read on 2026-08-16.
• CA — 540NR booklet — nonresident and part-year resident return instructionsCalifornia Franchise Tax Board. Read for: Confirms California publishes a separate nonresident and part-year resident return, which is the structural fact SF3 relies on. Read on 2026-08-16.
• NY — Frequently asked questions about filing requirements, residency and telecommutingNew York State Department of Taxation and Finance. Read for: Establishes that a reader must first settle resident, nonresident or part-year status, that a nonresident may still owe New York tax on New York-source income, and that a part-year year is split between resident and nonresident rules. Read on 2026-08-16.
• NY — Income tax definitions — resident, nonresident, part-year residentNew York State Department of Taxation and Finance. Read for: Establishes New York's definitional split between the three residency categories. Read on 2026-08-16.
• IL — Individual income tax filing requirementsIllinois Department of Revenue. Read for: Establishes the reciprocity contrast directly: an Illinois resident who worked in Iowa, Kentucky, Michigan or Wisconsin must still file the Illinois return and include that compensation as Illinois income. Read on 2026-08-16.
• NJ — New Jersey gross income tax — individual filing informationNew Jersey Division of Taxation. Read for: Confirms New Jersey publishes its own individual filing guidance separate from the federal return, supporting the contrast concept only. Read on 2026-08-16.

Where to go next

Educational information only. This is not tax advice, we are not your tax preparer, and nothing here is a determination about your own return. Check the linked official sources, and consider a professional for anything that turns on your specific facts.