u/Hot-Position-9513

First time poster here please go easy on the noob question: I want to put a small addition on my home. Roughly 400 sq ft. Just a bedroom and closet, attached off the side. My MIL did something similar without permits, etc, for about 50K. I want to do it “right” but the quotes I’m getting are 2-300K. WTF? We’re talking about some lumber and dry wall here. What am I missing? Is there a way to do this much more affordably?

PS I’m in the Bay Area so I get that things are more expensive. But I could my an entire house in the Sierra for what these folks want to charge to bolt a box on the side of my house.