Foreign visitors in Thailand with a short-term visit engaging in volunteer works with local schools, orphanages, and working on religious or Non-Governmental Organization (NGO) missions has to obtain a Thailand Volunteer Visa and a work permit. It is best to discuss with the organization about getting the legal documents to avoid complications.
According to Thai law, any paid or unpaid work carried out in Thailand, regardless of the duration, requires holding the correct visa and a work permit. To formally volunteer for a longer period in Thailand, you must apply for a Non-Immigrant O Volunteer Visa. Holders of this type of visa will get a permission to stay in Thailand for a period not exceeding 90 days from the date of arrival in Thailand. Extension of stay can be requested at the Immigration Bureau.
It is also an option to obtain a Non-O Visa in your home country before flying to Thailand. You may contact the Thai embassy in your country for their set of requirements, as the required documents may vary.
For foreigners who are already in Thailand and wishes to apply for a volunteer visa, here is the list of requirements. They have to be submitted more than 15 days before visa expiration:
All duly registered foundations and Non-Governmental Organizations in Thailand must have all the proper documentations. Therefore, they must be able to give you all the necessary paperworks to support your application. In case the organization cannot provide you with all the supporting papers, you have to think twice. You will then end up working with them without volunteer visa and work permit, which is against the Thai law.