Is it okay to return to immigration on September 15 after my tourist visa expires?

This also happened to me before. So when I returned the day after my permission to stay expired , in accordance with immigration instructions, they still did not have my passport back to give to me ( it was for a marriage 1 year extension and they had to send it out of province for the approval). They said come back in yet another week to get my passport back and told me not to worry re an overstay. When I returned in a week they had my passport with the one year permission to stay marriage extension approved. .. Still I was uneasy for the entire week with no passport and knowing my old permission to stay had expired. But they told me not to worry. What can I say other than this is Thailand.

I had similar issues that were exacerbated when the earthquake slowed down the BKK office for a few days. All good from the Hua Hin immigration office.

Had this exact situation previously and all was well. However, made sure not to go far from Hua Hin in the interim just in case another jurisdiction takes another view.

Although you don't mention the office you clearly used one of the offices that gives you "under review" for the Non-Imm visa that is as long as your current stamp, no matter when you applied for it.. 😮

Most offices (Bangkok for instance) give you a receipt that tells you when you return, which is 2 weeks after you've paid for the Non-Imm visa.

When you go back and they ink the Non-Imm visa in along with a new 90 day stamp you lose any time you have left on your free stamp/tourist visa entry.

SOME offices have you come back the day your current free stamp/tourist visa stamp expires and then ink in the Non-Imm visa along with the new 90 day stamp (I've see that at the offices in the south mostly)

Seeing as your stamp runs out Sunday (a day the office is closed) they just will have you come in Monday which is fine.

As long as you applied, turned in your documents, paid the 2000baht your application is under review and you just show up Monday the 15th to get the Non-Imm visa/90 day stamp inked in

You can't go on overstay in this situation, nothing to worry about. Just return on the date you were given

Follow the Advice given you by the Thai Immigration Officer.