What steps should I take for transitioning to a Non-O visa in Thailand after using a K1 visa in the USA?

The 90-day single entry Non-Imm-O "family" visa and how to obtain a one-year extension of your stay permit based on being married to a Thai wife.

There is the "90-day single entry Non-Imm-O Family visa" option, which you can apply by the e-visa online system before you fly to Thailand.

You must provide a financial proof for both the 90-day visa and later at immigration for the application for a "one-year extension of the temporary stay permit based on being married to a Thai wife."

For the application to the initial visa, you either

have at least 400,000 THB (approximately $ 12,000) in a bank account in your sole name. There is no seasoning of the deposit required at this point.

(It is irrelevant for the visa application where the account is located – as long as there is a proof that the account is in your sole name)

However, when applying for the "1-year extension" from out of the 90-days stay permit you got stamped in upon entering Thailand, the 400,000 THB must have been in your Thai bank account for 2 months, and this “seasoning” must be proven with a "bank letter of guarantee" (in Thai: rab roong thanakan).

Or you use the income affidavit of your embassy in Thailand (only if applicable - as many embassies do not issue it any more)

You need to provide evidence of an income/pension of at least 40,000 THB (approximately $ 1200).

For the application to the initial visa, you can use your original marriage documents.

However, for the application to the “1-year extension”, Immigration requires a Thai marriage registry printout - the Kor Ror 22 or 2 (whichever applies) This printout must be current, printed freshly by an Amphur office. This is possible only if your marriage has already been officially acknowledged in Thailand. If you married outside of Thailand, it will be the Kor Ror 22.

The requirements may vary slightly between every one of the 85 immigration offices in Thailand, so it is recommended to visit them right after you entered and ask for the list of requirements for the "extension of the temporary stay permit based on being married to a Thai wife."

What almost all immigration offices have in common is that they will only serve you, if you have been properly registered at your place of residence via a TM30 registration.

Immigration normally will issue the "1-year extension of the stay permit" for 4 weeks "under consideration." They will ask for photos of the two of you together in front of your house, with the house number visible, a photo of you in the living room, and in the bedroom, etc.

They normally ask for a hand-drawn map of the route from your house to the Immigration office, or the Google Maps coordinates.

Immigration will pay a visit at your home during these four weeks (with prior notice) and will question your neighbours about whether your marriage is "real."

After this visit, you visit Immigration and collect the stamp with your one-year extension of the temporary stay permit. (EOS)

NOTE: keep the 400,000 THB deposit in your bank account at least until you have gathered up the 1-year Marriage Extension stamp. You are free to spend the deposit, however the 400,000 THB need to be back in your account 2 months prior (some Immigrations ask for 3 months) to your next application for the 1-year extended stay permit.

NOTE: Always purchase a re-entry permit for your EOS.

A single re-entry costs 1000 THB, a multiple re-entry will be 3,800 THB.

The re-entry permit keeps your stay permit valid in case you leave the country. If you leave without a re-entry permit, your EOS will become invalid.

PLEASE use the correct terminology:

Technically spoken, there is no "visa extension", and the 1-year extension is not a “marriage visa”. Immigration does not extend any visa. That's technically not possible.

The use of this wording is improper English. You use the “90-days marriage visa” for entering Thailand, the visa becomes invalid. You will get stamped in for a 90-days stay permit. You then apply for a one-year extension of this stay permit. A stay permit is not a visa.