I'm on a 15 day visa(crossed over from border). I need an extension.

Is this correct: I can extend for an additional 7 days at a local immigration office for 1900baht?

What paperwork is required?

You have a 15 day visa exempt entry and want to extend your permitted to stay. Immigration will normally allow a 7 day under review (rejected) application for extension in such a case which allows you to stay those 7 days upon TM.7 application/payment of 1,900 baht fee. They may require show of a ticket out within that time period in addition to normal application form and passport/arrival/entry copies and 4x6cm photo. It might be just as easy to exit/return for a new 15 day visa exempt entry from some locations.