To add to what
@T**
said Immigration really really don’t want you to extend the DTV so they will make it as difficult as possible to do so.
There have been a few that have actually managed to get an extension there but probably not more than a dozen. Of those I suspect most won’t bother to try again as bouncing out by land via Ranong or by air to KL and back the same day is just so much easier.
Of course you’re free to try. You’ll almost certainly need to visit the office 3 or 4 times to provide all the stuff they need. Speak with the volunteers in Room 101. They have a hand out that lists the exact requirements, but the main ones are that you have at least 15 days left on your stamp and have 500k in a bank for at least 3 months. There’s a dozen more that you’ll need to meet.
As mentioned you can only apply at the main Phuket Town office, Patong nor Blue Tree do long term extensions.