Do I need a quantity surveyor for a self build?

The honest answer is that plenty of small, simple builds run fine without one: a straightforward design, an experienced builder, a self builder with time and a head for paperwork, and the free templates on this site may be all the structure you need. What changes the answer is scale and uncertainty. The more money is moving, the less standard the design, the tighter your funds or the shorter your time, the more the money side rewards someone who reads building costs for a living - because on a self build every mistake is yours to keep. A quantity surveyor is the construction industry's money professional: the person who checks that prices are complete, that payments match the work actually done, and that changes get priced before they are built. If you are unsure, that is itself information: one General Support session exists precisely to tell you, honestly, whether your build needs help or does not.