You are going to go to Chaengwattana Immigration office, Government Complex Building B.
You will go here.
****************************
And you can book online
********************************************
*****
In Bangkok, they accept an extension application when your stamp is 45 days or less.
This is general information of 1-year extension based on raising a half-Thai child. But you NEED to go to CW to get the latest requirements before you apply.