Hi,
You need signed copies of
* Passport main page bio data
* 90 days O visa and previous Visa if any
* Last entry stamp
* TM 6 if any
* Bankbook showing required money with required history ( depends on your option 800K or 65 K/ month or mix)
* Tabien baan or rental contract with signed ID copy of landlord or DBD if purchase via a company
* If owner they ask for for title deed aka chanot
* They can ask for TM 30 copy
*. Hand made drawing of your home location( depends on IO)
Need to have original doc*
* Filled TM 86 form
* Bankbook
* Passport
* Letter from the bank stating your account balance day of Io visit
* Chanot or DBD
* Photos 4x6
And a little contribution of 1900 thb for the Immigration good Samaritan foundation ????????
It's supposingly it. Nevertheless depending on IO you visit they can have invented any other requirements.
Best to do it 30 days before your 90 days visa expiry date. 12 month extension starts the day of expiry of your original visa regardless the date of extension application.
Good luck and take care