TLDR : Answer Summary
To extend your 60-day tourist visa in Chiang Mai for an additional 30 days, you will need to bring several documents to the immigration office at Promenada Mall. These documents include: an application form, one passport-sized photo (note that in Chiang Mai, a larger 4x6 cm photo is required), copies of the ID page of your passport, the current visa page, the 'Admitted to Stay' stamp, and your departure card. If you're staying in a hotel, include a receipt; if you're living in a private residence, provide the small notification paper you received when registering your address. The fee for the extension is 1,900 Baht.