Can I extend my Thai visa if my appointment is after the expiration date?

eExtension is a waste of money. Just make a regular online booking

1 days overstay. Can do now really..

That’s weird that your appointment is so far away? I did e-extension a week ago in Bangkok so I wouldn’t have to wait in line and there were tons of appointment dates that were just a couple days after I got e-extension approval. What city are you in? You can log back in and change your appointment date. The lady at immigration told me they were only getting a few customers a day so you should definitely be able to get a timely appointment. Unless you are confusing an online appointment at immigration with the e-extension service?

You will overstay one day and you'll have to pay an extra 500 baht fine. You'll also receive an overstay stamp in your passport.

Why don't you just go do the extension yourself at immigration and avoid the overstay?

Just go line up in the regular queue.