I'm coming to the end of my first non-imm O type visa. I have a work permit for doing voluntary work which is still valid for another year. I just wanted to check if the procedures are the same for obtaining a new visa. i.e. leaving the country and the paperwork required? Many thanks

You should be able to get a 12 month extension from Immigration 1,900 Baht.

Since you have a valid work permit, you can do a 1 year extension of stay at the immigration office for your area. Contact immigration to see which documents they will require for this. Cost is 1,900 baht.

Normally you will need some letter from the organisation, clarifying that you don't get paid and work at least 4 hours a week for them. Since you have a work permit, it shouldn't be a problem.

On my passport it says that I have to leave the country by a specified date ( I've already had the initial extension for 1 year which I got when I entered the country and took it to immigration in suan plu). Are you saying that it can be extended for a further 12 months?

With the correct paperwork you can keep on renewing your permission to stay each year without leaving the country. As said, check with immigration what exactly they require as requirements can vary from office to office.