Guide
The Illinois domicile test, and how a move actually gets proved
Updated
Illinois has no exit tax, so the only thing that can go wrong when you leave is that you did not, in the state's view, leave. That is a question about facts, and the facts are much easier to arrange before the move than to reconstruct after a notice.
The definition
The Illinois Department of Revenue states that you are an Illinois resident if you were domiciled in Illinois for the entire tax year, and that your domicile is the place where you reside and the place where you intend to return after temporary absences.
Both halves matter. Reside is about where you actually are. Intend to return is about what you plan, and planning is invisible, so it gets inferred from everything else.
What gets read
Where the home you actually live in is, and what happened to the previous one. Where the family is and where children are at school. Driving licence and vehicle registration. Voter registration. Professional licences. The doctor, the dentist and the vet. Safe deposit boxes and club memberships. The address on the employer's payroll record.
No single item settles it, which is exactly why the pattern is what matters. A set of small facts all pointing one way is far more persuasive than one dramatic one pointing the other.
The date
A part-year return splits the year at the change of domicile, so the date is doing real work and is almost never evidenced at the time.
Pick it deliberately and attach something contemporaneous: the closing on the old home, the lease start on the new one, the day the family actually moved. Then use it consistently everywhere, because inconsistent dates across a return, a licence and an employer record are the kind of small discrepancy that invites a longer look.
Second homes
Keeping property in Illinois does not by itself prevent a change of domicile. Keeping property and continuing to use it as a home is a different matter, and the distinction is behavioural rather than legal.
If the Illinois house is retained, the useful test is whether an outside reader looking only at how it is used would call it a second home or a home. Renting it out answers the question cleanly. Keeping it available and furnished, and staying in it regularly, does not.