British passport holders arriving to Thailand by air or land can enter for 30 days without a visa – this is known as a visa exemption. It’s possible to extend your stay beyond the 30 days granted by the visa exemption by an additional 30 days from the expiry date of the original visa. To extend your visa you’ll need to apply at an immigration office.
If you plan to stay in the country for longer than 30 days, or you intend to work, you must get a visa before you travel. If you have entered Thailand on a visa, it’s possible to apply for an extension of stay but you must do this before your permission to stay expires.
If you overstay, you’ll be fined approximately 500 baht per day up to a maximum of 20,000 baht. You also risk being held in detention, fined, deported at your own expense and black-listed from re-entering Thailand. The Thai authorities have stated that they will always enforce detention for overstays of more than 42 days. Since March 2016, you’ll be banned from re-entering Thailand if you overstay your visa. Please check with the Thai Immigration Authority for more details.