To legally work in Thailand, all foreigners must possess a valid visa and a Work Permit.
The Non-Immigrant Visa must be obtained from a Royal Thai Embassy or Consulate before entering Thailand. This is a single-entry visa and it’s valid for 90 days. Some Embassies or Consular posts, before issuing the visa, may require a Work Permit pre-approval letter from the Department of Employment.
Alternatively, the foreigner having a Tourist Visa can convert this one into a Non-Immigrant Visa without leaving Thailand.
In most cases, the Non-Immigrant Visa will be a Business Visa, but a Marriage Visa for foreigners married to a Thai national will be also sufficient.

The Department of Employment in the Ministry of Labor and Social Welfare is responsible for Work Permits and labor matters for foreigners.
Employment is forbidden until a Work Permit has been obtained.

In order to obtain a Work Permit for each of their foreign employees, the company must comply with certain requirement.
The company must be registered for VAT with the Revenue Department.
The fully paid-up registered capital of a limited company must be at least 2 million Baht to be able to hire one foreigner. For each additional 2 million Baht in paid-up capital, one more foreign employee is permitted. The paid-up capital requirement is reduced by half if the foreigner is married to a Thai national.
For a foreign company, carrying on business in Thailand that has invested money from abroad, the paid-up capital must be at least 3 million Baht per each Work Permit issued.

An exemption may be granted and a maximum of 10 foreigners may receive a Work Permit if the employer:
• Paid at least 3 million Baht in income tax in the previous year
• Employs at least 100 Thai nationals
• Working in the export business that has brought not less than 30 million Baht in foreign currency into the country in the previous year
• Working in the tour business that has brought not less than 5,000 foreign tourists to Thailand in the previous year

Furthermore, it is generally required for a company registered in Thailand that the ratio of foreign to Thai employees be 1:4. If the foreigner is married to a Thai national, only 2 Thai staff are need for him.

A Work Permit is generally issued for one year period and may be renewed every year before its expiry date or it will automatically lapse. If the duration of employment is less than one year, a Work Permit will be granted only for the period requested.
Some types of Work Permits could be approved for the duration of two years, such as a Work Permits for representative offices, BOI-promoted companies, and companies with registered capital exceeding 30 million Baht.

After the Work Permit is issued and with 30 days before the initial visa expires, the foreign worker must apply in person for a visa extension. The applicant must earn the minimum income prescribed by law, which varies according to nationality. If the foreigner wants to travel outside Thailand he must obtain a multiple re-entry permit, otherwise the visa will not be valid when he tries to re-enter in Thailand.

The foreigner must work only for the employer indicated in the Work Permit. In the event of changing the work details (job title, job description) or work location in relation to that employer, the foreigner must notify the change.
If the foreigner loses or damages his Work Permit, he must apply within 15 days from that date for a substitute and the document will be replaced by a new one.

Regarding the Work Permit extension, the officer will consider the financial statements of the employer related to the preceding year, as well as VAT payments of the employer.
If the company has no income or income lower than 1 million Baht shown on the latest financial statement or VAT documentation, they will use their own discretion and may extend the Work Permit for only 6 months. The Labor Department will be flexible if appropriate reasons are provided to explain why the company operates at a loss or has no any income at this moment. If the reason is acceptable, they will agree to extend the Work Permit for one year.