Do I need a passport with 6 months validity to renew my marriage visa in Thailand?

Renew your passport first and then apply for visa extension. If you are in Thailand then it doesnt matter 6 months validity to apply visa extension. but your new visa will be issued until your passport valid. So must renew your passport at your concerned Embassy.

Yes if you have 6 months left your extension will only be for 6 months not a year get a new passport now

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at local Thai Immigration Offices inside Thailand for an Extension of Stay or even with Thai Immigration Officers at Entry into Thailand, there is NO minimum validity for passports, but any Extension of Stay or even Entry Permission Stamp will only be valid up to the passport's validity (passport's expiration date)

so, in your specific and current circumstances, if using your current British Passport and your current British Passport expires in October 2026, then an Extension of Stay issued in July 2026 will only go to October 2026 (the expiration date of your current British Passport)

However, under this situation, if later get a new British Passport AFTER that Extension in July 2026, then you need to obtain a NEW Extension, because you lost the "remaining" months of the Extension in your current (expiring) British Passport

Just Renew your current British Passport Now and get a NEW British Passport Now so you can get the full 1 Year Extension in July 2026

The extension will be issued to the date of passport running out.

There is no minimum time for an extension renewal. However you will only get the extension until the date your passport expires.

So if you extend in July it will only be valid until October.

If you get a new passport now and get the stamp moved over then you’ll get a full year on your next extension.

Some people use the passport expiry option to change the time of their yearly extension renewal.