If it is just one small family room attached I agree that wouldn't be so much! But adding a bathroom and a family room and a couple other things can add up quickly
we have a screen porch that is just mesh and three season. We got a quote for having that all screened in (with glass and wood) and I believe that alone was only $15k. It's small though.
Bathrooms are expensive
A lot of it is materials though. Wood is expensive. We wanted to do a tall wood fence all around our house and even that was going to be 7-8k. For a fence! So we aren't doing that any time soon lol