What are the requirements for extending a 30-day stay in Thailand on a visa waiver?

I'd recommend getting a 60 days tourist visa for £30 this is cheaper than getting a 30 days extension and a lot less hassle

I put additional information here about why we are strictly sticking to terminology.

There are 4 different types of entries even for "no visa entries" only.

Visa-exempt is based on the Thai government's specifications. visa-waiver(s) are based on the bilateral agreement between the Thai government and that country's government.

Those four entries are: visa-exempt, visa-waiver for 14 days, visa-waiver for 30 days, visa-waiver for 90 days.

Only visa-exempt entry of these 4 is eligible for a tourism extension.

If you are on visa-waiver for 30 days, you cannot apply for a tourism extension while those who are on visa-exempt for 30 days can apply for and get a tourism extension.

These are the common found requirements for the 30 days extension of stay at your local Thai Immigration office for 1900 Thai baht.

Most offices allow you to Apply for the 30 days extension of stay during the 30 days before your stamp will end.

However... IF you are using Jomtien Pattaya Thai Immigration office, they will accept the 30 days extension of stay application during the week PRIOR to your current ending date.

Be sure you have a TM30 on file with the Thai Immigration office.

Do you really mean Visa Waiver or Visa Exempt, what nationality is your passport?