I am currently on a 30 day tourist visa and am needing to extend it by 16 days. I had an idea to cross the myanmar border at Mae sai and then come straight back into thailand but was unsure if I would be issued another 30day visa or a 15 day one.

If you are on a 30/15 day visa exempt entry, then you can extend it for 30 days (1900 Baht) at your local immigration office.

Land borders will give 15 days for most and 30 days for G7 countries.

You have a visa exempt entry, not a tourist visa.

You can get a 30 days extension at immigration. Cost Bt 1,900.

Thank you everyone for your quick responses. I knew about the visa extension but was thinking it would be a lot cheaper to get a free new visa when I crossed the border. I am a UK citizen which I'm assuming is in the g7. So it would be a lot easier and cheaper to go to Myanmar as they only charge $10 on arrival?! (Quoting the lonely planet guide) please let me know if you have other info on that.

You should not only consider the cost. Getting the extension is the recommended and best way.

Note again you don't get a visa when permitted entry visa exempt.

If you go to Mae Sai there is good chance they will not let you leave and tell you to get the extension. There have been reports of them doing this.