There are three ways to extend a non-immigrant visa: marriage to a Thai citizen; retirement if over 50; and business. This can usually be done inside Thailand if you have the appropriate type of visa.This is what is normally needed for this visa extension. If you are married to a Thai citizen being for a Thai Marriage Visa then this one year extension is done in Thailand.
You need to show an income of 40,000 Baht a month or 500,000 Baht in a fixed deposit in a Thai bank account. You also need to provide photos of your family (wife, children if any and yourself). You also need to show where you live by having a map to your house, villa or condo in Thailand. You need a copy of your marriage certificate, house papers of your wife, her Thai ID card and the card/s of your Thai children. This process normally takes 1 week to complete.
The other option is that you are over the age of 50 and want to extend your visa for 1 year. for this you will need to show 800,000 Baht is a Thai bank account and income of 60,000 Baht a month or a combination of both. You also need to show that you are healthy with a medical certificate and also show on a map where you live.
The last option is a business visa and this is extended for the length of the work permit which has been granted to you in Thailand. Speak to a lawyer in Thailand for more information about your Thai visa extension for 1 year.