You can apply if you wish, its unlikely that the builder will want to do it though he may have a professional he uses. You can do it online, you'll need to provide the required drawings though and would want to put in a good robust application rather than the bear minimum. How do your neighbours feel about it? Would they object or support it? the planners may well ignore the neighbours feelings anyway.
A leading question, did the original extension require planning permission or was it done under permitted development?