At Chiang Mai immigration can I wait until the last day of 45 day visa exempt?
If extend before last day, is 30 day added from that day or added from end of 45 day stay?

Yes to both questions 

Extension to visa exempt is done at IT Mall Laksi.

Appointment is possible.

 

Note leaving extension till last day possible is not a smart idea. 

Added edit: my post referring to Bangkok immigration not necessary 

5 minutes ago, DrJack54 said:

Extension to visa exempt is done at IT Mall Laksi.

question is about Chiang Mai immigration

2 minutes ago, orchis said:

question is about Chiang Mai immigration

Missed that.

Yes he obtains extension where he is living. 

If 45 day ends 1/27 and on 1/11 I extend 30 days, new end date is 2/26? Not 2/10.

3 minutes ago, WROBE said:

If 45 day ends 1/27 and on 1/11 I extend 30 days, new end date is 2/26? Not 2/10.

Yes February 26. 

Thank you

I extended my current 45 days (1 December 2022 to 14 January 2023) yesterday at Chanthaburi City Immigration. 

 

The 30 days starts from that expiry date of 14 January 2023 (not yesterday's date) and now expires 13 February 2023.

 

As an aside, I read about queues at other offices. I arrived just after 9am - three counter staff available, just one customer ahead of me. Handed over my form, passport, 1900b, 2 baht for a photocopy of the page showing my current 14 January stamp, I was out in less than 15 minutes. Magic!

 

Stylish old building, grounds are being improved, appear to be removing much of the concrete to replace with grass/gardens.