How can I extend my visa in Thailand based on my marriage to a Thai citizen?

@Sven ***

you say this in your post; 𝗜’𝗹𝗹 𝗯𝗲 𝗳𝗶𝗿𝘀𝘁 𝗴𝗲𝘁𝘁𝗶𝗻𝗴 𝗮 𝗻𝗼𝗻 𝗶𝗺𝗺𝗶𝗴𝗿𝗮𝘁𝗶𝗼𝗻 𝗢 𝗳𝗿𝗼𝗺 𝗮 𝟯𝟬 𝗱𝗮𝘆 𝘃𝗶𝘀𝗮 𝗲𝘅𝗲𝗺𝗽𝘁 <- that means in Bangkok as Ellie said you need a minimum of 15 days left on your current stamp's validity. (the application goes under review for 2 weeks before you go back to get the Non-O visa and a new 90 day stamp inked in)

ALSO

you apply for the initial 90 day Non-O visa 2000baht in Section C-1 and go back there after 2 weeks to get the visa/new stamp inked in.

You apply for the YEARLY extension for 1900baht in Section L-1 once your new 90 day stamp runs down (and you need ALL new documentation to do that?

So when you go back to get the new Non-O visa and 90 day stamp inked in stop over in section L-1 and get the hand out listing the requirements for the yearly extension that way you know what you need when you go back in a couple months

In Bangkok, you will go to counter C, Chaengwattana Government Complex Building B.

Be sure you have more than 15 days with your current stamp on the day you apply for an in-country Non-O visa.

FYI, the Bangkok Immigration office site has this, but no guarantee it is the latest.