Can I enter Thailand on a 30-day visa exemption and extend for 60 days to visit my Thai wife?

The sisaket immigration wants to see a marriage certificate in thai script , stamped by the consulate in your home country if you dont have a thai marriage certificate if you want a 60 day extension

LHR yea but emorates at Gatwick dont care, well did not before not sure if do now.

You can get the 60 day visit family (wife, kidz) extension on ANY entry. It doesn't matter if it's a 30 day visa exempt, a 60 day tourist visa, or a 90 day Non-Immigrant visa.

As

@Biff ******

, correctly pointed out it's NOT the passport control officer who stamps you into the country that cares about onward travel it's the airline when you check in.

Flying in visa exempt they can make you have proof of onward travel within 30 day (seeing as that's all you get stamped in for to begin with). They don't care that you might extend or might leave by a land border. They can deny you boarding if you don't have proof of onward travel within 30 days flyin' in here on a visa exempt entry.

Coming here with a valid visa exempts them from that liability.

The issue I can see would be boarding the flight in the first place. If the return flight is more than 30 days after departure, check in staff would want to see either a visa, or a ticket out of Thailand before 30 days.