Is a return flight required when extending a 30-day tourist visa exemption in Thailand?

An outbound flight is ALWAYS required when entering Thailand. Whether the airline will ask for it at departure or whether Thai immigration will check for it when you arrive is a different story but it is a requirement. I would suggest you book it but if you choose not to book an onward flight... that you at least have some flights and destinations ready bc if they ask you will need to book it on the spot or you will be denied. If your looking to extend a 30 day stay before arriving then you should book an outbound for 60 days out....with the intention to renew the 30 day for another 30 day. The simple answer is anytime you enter Thailand you need to have an outbound flight booked also.

You might get refused to board the flight in your departure airport by the airline if you haven’t got an onward flight out of Thailand though

Something worth bearing in mind alot of people overlook

You won't need to show a flight out of Thailand as part of the application for an extension.