British entre thailand with 60days exemption and want to stay 20days more. Can i extend in immigarion online service? Thx

Hi thanks for the add. One simple question if I may, I am British citizen living in Spain. I am travelling to Thailand in July and August this year with in order to have a look around the country. I understand I will get the 30 day visa exception on arrival. How easy is to extend to a 60 day tourist visa after I arrive. Thanks in advance for advice.