I currently have offices in Phuket and want to open another one in Bangkok. I understand that the Phuket work permit that I have now will not be valid to also work in Bangkok. Does that mean I will have to get another work permit for BKK or can I just add that new office to my exisitng Phuket work permit.

Thank you for any advice

You will need to add the new location (branch) to your WP

This is indeed true , I do beleive however if you are working out of province the fine is 1000 baht , and advised to add province , no major headaches