What Do I Need to Bring for a 30-Day Visa Extension in Chiang Mai?

I arrived in Thailand on March 18 and received a 30 day visa exemption stamp which expires on April 16. I’m in Chiang Mai and was planning on going to the immigration office tomorrow to apply for my normal 30-day extension. I’ve been searching for updated information on which location to go to (the office or mall) and what I need to bring with (do I need some sort of housing agreement, health form or letter for a regular extension?) but have read mixed responses. Could someone let me know what I should do?