I hired someone to finish my basement, and was kinda astonished how quickly it came together. I'm not saying anyone could do it, but it didn't look super difficult.

I would never do electrical or plumbing myself.

Years later I had a crew build out my back yard, and that was way WAY more work than finishing a basement. In particular, it's a TON of work to date a dirt lot, level it, wait for it to settle, level it again, etc. The preparation of the building site is royal pain in the ass.