There is no such thing as a “Work Visa” in Thailand. It is called a business visa or “B” visa. The visa does not give you the right to work in Thailand however it gives you the option of applying for a work permit. Here is what is needed for a work permit. A job description is often the determining factor in the approval or denial of a work permit. Unless the written job description requires skills that a Thai is not likely to posses the application will not be approved. Your lawyer should write professional and accurate job descriptions in order for you to gain quick approval.
The Labor Department often seems to want to reduce the number of permits actually issued to help demonstrate that they are protecting Thai jobs. Therefore, if your company applies for three permits, they may only approve one or possibly two. You need professional help to ensure all of your applications will have a successful outcome.
Once the application is submitted for your work permit you must not let your visa expire. The Labor Department system links the Non-immigrant Visa (“extension of stay”) you listed on your application to the Thai work permit paperwork. The work permit, once approved, allows you to work at the noted company and location.