The penalty for failing to report your 90-day stay in Thailand on time is a fine of 2,000 Thai baht (THB):
Late reporting: If you report after the deadline, you'll be fined 2,000 THB.
Caught by police: If you're pulled over by the police for an unrelated reason and they find out you didn't file your report, the fine increases to 5,000 THB.
Daily fine: You may also be fined up to 200 THB per day until you comply with the law.
You can report your stay up to 15 days before or 7 days after the due date. You can report in person or by registered mail.
Here are some other things to know about the 90-day reporting requirement:
The 90-day clock restarts if you leave Thailand, regardless of how long your trip is.
The 90-day reporting requirement is not the same as a visa extension.
Frequent or blatant breaches of the 90-day reporting requirement may affect future visa renewals or extensions.
Children living with you don't need to report if you do, but they should start reporting themselves when they turn 15.
To report your stay, you'll need:
Your valid passport
Photocopies of relevant passport pages
A completed and signed 90-Day Report Notification form (TM.47)
A photocopy of your arrival/departure card (TM.6)
A copy of your previous 90-day slip (if you had one)
A self-addressed envelope and 10 THB postage stamp if applying by registered mail