First you NEED a "proper address".
Not a "room 13 at Om's short time bar and by the hour guesthouse" address.
Banks want an address they can actually send mail to and Immigration will expect to see PROOF that you have a proper residence before they will issue you a Certificate of Residence.
And you NEED that certificate to open a bank account.
So, rent a place, have a copy of the rental/lease contract and (likely) photocopies of the landlord/owner's ID card and House Book (to prove they own/manage the place in the contract).
Set up your electricity/phone/cable/water or other utility bills right away. You don't need "bills" from them now (as it could take 5-7 weeks to get them anyways) but they will be useful when you go to do your Extension in 3 months.
Go to Immigration and get a Certificate of Residence. If you want to purchase a motorcycle and/or get a Thai driver's license (highly recommended) - get 2 extra copies made at the same time.
(You can use a Thai drivers licence as your "proof of residence" when you go to do your Extension as it has your address on the back of it.)
Pay the fee to get the Certificates done right away and don't whine about it. They are (technically) free but unless you want to wait 7-10 business days to get them done, pay the fee to "expedite them".
Then go to the bank and open TWO accounts. A regular savings account that comes with an ATM card and bank book and a Fixed Term Savings account, which comes with just a bank book.
And the only way ANYONE can take money from a Fixed Term account is if YOU go to a branch with your passport and bank book. Even your "wife you met last week in a bar" can't touch that money as YOU have to go to the bank yourself to withdraw it.
Put your 800k in the Fixed Term account and forget about it. As in, don't touch it unless you absolutely have no choice.
If you are really broke, you can have the yearly interest (which won't be much) deposited into your Savings account. Or leave it in the Fixed Term account for a few years. Mine's been in a Fixed Term for over 16 years now.
Then a WEEK before the date your Visa expires, go to Immigration and apply for an Extension of Temporary Stay in the Kingdom (form TM.7 -
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You will need lots of paperwork depending on which office you are dealing with.
The TM.7 form. Photocopies (each one signed) of the face page of your passport, the page with your most recent Entry Stamp, the page with your Visa. If you've had stamps transferred from a previous passport - that page as well.
Photocopy/print out of your TM.6 Arrival Card.
Photocopy of the front and back of your Drivers License - or a copy of your rental/lease agreement (with photocopy of the landlord/owner's ID card and house book).
A "Bank Letter" from whichever bank your "Extension" money is in. It's a standard form letter EVERY bank has on their computers. Depending on your account balance, they MAY charge you 100 baht for the letter which verifies you are a customer and "own" the account.
I've NEVER had a problem getting that letter from Bangkok Bank in the 15 years I've been doing Extension Applications.
Photocopy the front (inside) page of your bank book that shows your name and account number.
UPDATE the bank book either the day before you go to Immigration or the morning you go.
Then photocopy the page that shows your updated account balance.
IF your updated balance is on a different page from your previous balance - photocopy the previous page as well. And take your bank book with you "just in case" the IO want's to see it to verify it's really YOUR bankbook.
A "passport sized" recent photograph.
1,900 baht fee for the Extension.
And you go a WEEK before your due date in case YOUR Immigration office has DIFFERENT paperwork requirements (like for proof of residence for example) and you have to do some running around. (And watch out for weekends and holidays.)
At Jomtien they have 2 additional "local" forms they will have you sign. One is a notification of the penalties for over-staying your Visa/Extension and the other is an Acknowledgement of the Terms and Conditions of your temporary Extension of Stay.
I have most of my paperwork for my next Extension application already filled out and in a folder ready to go.
A week before I plan on going I'll update/print/photocopy the rest. The day before I go to the bank for the Letter and Bank Book update.
I generally go early the next morning (take a book). Often they may open the doors at 8 but don't start working until 8:30.
When my number is called, it usually takes about 3-4 minutes for the Immigration Officer to go through my paperwork and highlight the important parts.
Then he attaches a number tag to the paperwork and passport and gives you the matching tag and takes your 1,900 baht fee and tells you to come back the next day (after 10am) to pick up your passport.
And that's it. Barely 5 minutes and I'm on my way. At that point you're pretty sure your Extension will be approved otherwise the IO would have flagged any problems and sent you away to get them fixed.
(Had that happen last year because I forgot to include the photocopy of my driver's license. Zipped out, got the photocopy, back in and he put it with the rest of the paperwork and that was it. Went back the next day and got my passport.)
Pro tips:
* SHOWER and wear CLEAN CLOTHES before you go to Immigration. Trust me - a LOT of people DON'T and YOU don't want to be standing in front of an Immigration Officer when the guy BEHIND you smells like he just woke up on the floor of a bar and staggered into the building.
Because the IO will smell him and think it's YOU that stinks - and Thais really don't like smelly people.
* Have ALL your paperwork completed (including signing ALL the photocopies) BEFORE you even go to Immigration. Frikken annoying when the person ahead of you gets called to the desk and hasn't even started doing his/her paperwork.
I saw one old fart sit down and pull a blank TM.7 out of his pouch along with some other paperwork and just handed them to the IO like he expected the guy to fill it all out for him.
*I generally go first thing in the morning, 30-40 minutes before they even open the doors. I like to be in the first 1-3 people for the Extension desk.
Why ? Because hopefully at that point they won't already be upset by all the twats that come in without the proper paperwork and stink like they live in a dumpster.
As well, going early means if there is a change in what they want (for paperwork) or I've forgotten something, I can zip out, get (whatever) and get back so that my application won't be delayed and I can still get my passport back the next day.
*Some people go around 10:00 by which time the "morning rush" has usually died down. Or right after lunch.
*Watch out for public holidays ! Don't go on a Friday or Monday morning as they are usually the busiest times even if it isn't a holiday weekend.
Watch out for holidays that happen on Thursdays or Tuesdays as they MAY give their staff the (Friday or Monday) off as well.
One year there was a holiday on a Friday and my 90 day report was due on the Saturday. I went in on Monday but they were closed - because Tuesday was also a holiday !
So they were actually closed for 5 straight days.
You should have seen the line-ups on Wednesday morning !! I arrived over an hour before they normally opened the doors and there was a dozen people ahead of me.
By the time they opened the doors, the line was out the parking lot and up the street.
I got my report done and left about 08:40 and despite all the people that were in the building, there was still a line up the street with over 100 people waiting.
Note - at Bangkok Bank they would NOT give me "internet" access to my accounts until AFTER I got my first "annual extension".