How can I extend my 45-day visa exempt period in Chiang Mai, and when will the extension start?

A UK family in Chiang Mai is seeking guidance on extending their 45-day visa exempt period which ends on December 15. They intend to extend it for an additional 30 days and inquire about pre-filling forms before their visit to the immigration office. They also ask whether visiting early affects the start date of the extension. Community responses suggest that it's best to get the form directly at the immigration office, which varies by location, and that the extension will be added to the existing visa without losing any days if they go early. Additionally, it is advised to obtain proof of the TM30 filing from their landlord.