FYi - for those who live in Chiang Mai, 30-day extensions are now being done back at the immigration office near the airport. Come early (officer told me 8:00am) for they only ptocess 150 people per day.. I arrived at 11am and was told to come back tomorrow
TLDR : Answer Summary
Residents in Chiang Mai should note that 30-day visa extensions are now processed at the immigration office near the airport. It's advisable to arrive early, as the office only processes a limited number of applications (150 per day).