Went to Si Racha immigration for the first time today to extend my 45 day visa exemption. I used to live in Bangkok, and the extension process there, although lengthy due to the volume of people, is extremely simple, as per the Thai immigration guidelines. I have also done it in Hua Hin and it took a matter of minutes.
I have also used the immigration office in Jomtien, and they insist on a letter from whichever hotel I am residing at for some peculiar reason.
I was told at Si Racha that I needed the rental agreement and copy of the landlady's id card for the condo I am staying at now.
I really don't understand why some immigration offices are making unilateral demands regarding accommodation paperwork, when it is not required nor stipulated by the main office in Bangkok and elsewhere.
Edit:
For those who seem to think they know more the Thai immigration in Bangkok, see below.