They make the process quite hard to get an extension. The IO himself suggested to simply leave and re-enter as its much easier.

Mine was 3 days old and it was okay with the officer. But every time you go you have to give the updated one, for example if an additional doc is requested.

I tried to get an extension at Chaeng Wathhana. It was refused and the following additional documents were requested (Soft Power) -

Updated bank statement above 500k THB

Photo printout proof of attendance at cooking classes till now

Letter from Cooking school / govt body to Immigration Officer to extend my stay in Thailand by 6 months

Letter from Govt for affiliation of School to OPEC Thailand (This one may be hard to obtain)

I had carried the following documents originally -

Passport bio page copy

DTV Visa Copy

Latest visa stamp copy

90-day report copy

TM30 printout

TM7 Visa Extension Form

Bank Statement (500K must be deposited 1 month ahead)

House rental contract

Rent transfer receipts (Your name and Renter name should be on those)

Resume / CV

Cooking class student enrollment & payment receipt

Cooking class class appointment schedule (future)

Cooking class company registration & seal

You might need to present return ticket to immigration when you enter Vietnam