I am currently living in Bangkok on a retirement visa. I have been offered a full time paid job with an NGO in Chiang Mai.

Will I need to leave the country to obtain the right visa before getting a work permit?

Can all the paperwork be done by post with photo copies before I move to Chiang Mai?

Thank you.

Highly unlikely you would get a Work Permit issued against a Retirement Extension or an 'OA' visa entry.

If this is the case then try to get your new employer in Chiang Mai to apply for your WP, using form WP3, get the 'Letter of Approval', then go to Thai Consular Facility with all the other required paperwork (see link below) to get a Single Entry 90 Non 'B' visa.

Paperwork for a non immigrant 'B' Visa: http://www.mfa.go.th/main/en/services/123/15388-Non-Immigrant-Visa-%22B%22-(for-Business-and.html

Re-enter Thailand and then Labour can start the actual WP Issue once they have sight of your 90 day non 'B' entry.

Your employer should be able to sort out your Extensions of Permission to Stay from then on.

Most of this should be doable by post with the exception of your trip outside of Thailand to get the Non 'B'

Some work permit offices will not even accept and application for a work permit with an extension based upon retirement. You might have to go out for a tourist visa in order to do the application.