Five years ago, the typical Illinois resident had one state to think about: Illinois. Now we routinely see W-2s from California, Wisconsin K-1s, condos in Florida, and consulting work performed in three states during a single trip. Each one creates a state filing question, and the answers do not always agree with each other.
Here's the framework.
Step 1: Where are you a resident?
Your resident state taxes you on all your income, no matter where it was earned. Determining residency is mostly a domicile test — where do you live, where is your driver's license, where do you vote, where do your kids go to school. Most people have one resident state and don't move during the year. If that's you, this step is easy: Illinois is your resident state.
If you moved during the year, you'll be a part-year resident in two states. Each one taxes the income you earned while you were a resident there. The forms ask for an apportionment — exactly how to split the year.
Step 2: Where did you earn the income?
Your non-resident states tax you on income sourced to that state — income that was earned because you performed work there, owned property there, or operated a business there. Common non-resident situations:
- Illinois resident with a W-2 from a Wisconsin employer (Wisconsin sources the wages)
- Illinois resident with a rental property in Florida (Florida has no income tax — easy this time)
- Illinois resident with a K-1 from an Indiana partnership (Indiana sources the K-1 income)
- Illinois resident traveling to Iowa for two days of consulting work (Iowa technically sources those two days, though the practical filing threshold matters)
Step 3: The reciprocity shortcut
Illinois has reciprocity agreements with Iowa, Kentucky, Michigan, and Wisconsin. If you're an Illinois resident with W-2 wages from any of those four states, you don't actually file a non-resident return there — your wages are taxed only by Illinois. Your employer should withhold Illinois tax instead of the work state's tax.
If your Wisconsin employer mistakenly withheld Wisconsin tax on your Illinois-resident paycheck, you'd file a Wisconsin non-resident return solely to claim back what was withheld, plus the standard Illinois return.
Step 4: The credit-for-tax-paid trick
Outside of reciprocity, you'll often have income that's taxed by both your resident state and a non-resident state. To avoid double taxation, your resident state gives you a credit for taxes paid to the non-resident state. The credit is generally limited to what you would have paid your resident state on that income.
Mechanics: file the non-resident state first, calculate the tax, then take that as a credit on your resident-state return. Software gets this right when prompted. Software gets it wrong when it doesn't realize you owe a non-resident return at all.
Step 5: Remote workers and the convenience-of-employer rule
A few states (New York, Pennsylvania, Connecticut, Delaware, Nebraska) have a "convenience of employer" rule. If you work remotely from Illinois for a New York employer, New York may still claim the wages because the employer is in New York. Illinois will then tax the same income, and the credit-for-taxes-paid rules get tangled.
This trips up a lot of remote workers who moved to Illinois during 2020–2022. If your employer is in one of those five states, your filing is more complex than the generic remote-work narrative suggests.
Heuristic: if your W-2 has more than one state listed in box 15, or if you traveled to a different state to do work for more than 14 days, get a CPA's eye on it before you file.
What we ask new multi-state clients to send
- Every W-2 (with all state-line entries visible)
- Every 1099, K-1, and 1099-NEC
- A list of states you physically performed work in, with rough day counts
- Your move date(s) and old/new addresses if you moved
- Any state-level estimated payments you've made
From there we work backward through the steps above. Most situations resolve cleanly. Some require us to file in five or six states. Either way, send us your situation and we'll tell you what we see.