How can I extend my visa in Chiang Mai if I received only 28 days instead of 60 days?

For any future visa questions, it’s important to state your nationality. There are different rules for different nationalities. Visa-on-Arrival (VOA) is different from Visa-Exempt.

You probably mean the temporary measure of 45 days visa exemption that expired on April 1 last year. then it went back to 30 days.

I think you're mistaken, there never was "60 day on arrival" for any country 🙁

People from western countries got 30 days (which is really only 29 days western count 😮 ) and you can get ONE 30 day extension at the immigration office that serves the area where you stay for 1900baht

In Chiang Mai you can apply for the 30 day extension at either the main immigration office by the airport OR at the immigration office in Central Festival Mall

There is no reason to wait until your current stamp runs out as the 30 days is added to your current expiration date

This is a list of the things you are likely to require to get the extension up there.

The offices up there are pretty hard on people having proof of the TM30 being filed on you where you're staying so ask your hotel to print out the confirmation for you