First floor extension, you need to leave minimal 5Ft space, cannot fully extend like on ground floor.

Most people will apply permit with the above method, but instead build full. Technically is illegal.

So Marketing

Agree, also if you do that please be ready they will waste the materials, maybe sell it out. My friend experience before and he advice me not to take that path unless you really full time monitor them like site supervisor.

Another cheaper method is handling all the contractor directly (without ID), This include electrical, Wet work, plumbing, windows, tiles, and furniture. Still headache but better than handling the worker.

Less headache is get ID, let them handle all the problem. They design, and build base on your requirement + their suggestion.

All this 3 have its Pros and Cons

Everything from heaven to earth.