Can the 1 year multiple B still be extended to 15mnths? Mine expires Aug 31st, must leave on that day and will receive another 3 months is that correct?

AFAIK you will get on every entry 90 days within the validity of the visa.

Agree with the above, you aren't extending anything, you are just making another entry which would be 90 days. Not sure I would wait until the last day to do it though.

If your visa is multiple entry and it was obtained from a Thai Consulate outside of Thaiand, then you will get a 90 day stamp every time you enter Thailand up to the 'Use Before' Date.

If however you are on a One Year Extension of Permission to Stay obtained at Immigration here in Thailand, then your original visa is probably long since expired. If you leave Thailand with a Re-entry Permit, then you will simply be stamped in up to the same One Year 'Pemitted to Stay' extension date, leave without a Re-entry Permit then your extension is terminated as and when you are stamped out of Thailand.

You must re-enter on that day ( or before), not leave on that day, and yes you will get another 90 days. Of course, you could leave and re-enter on the same day but the important thing is to enter Thailand on or before your visa expires.

Edited March 30, 201214 yr by inthepink