You need some expertise, but electric and pumbing are not hard to do yourself. If you don't know what you are doing a bad install can kill you (or your family/friends), so you want to do this right. However it isn't hard.

I helped my uncle build his house, the only thing he didn't do himself was dig the holes (foundation and well), pour the concrete, and tape the drywall. Everything was inspected by the county and passed. We could have done those as well, but is made sense to hire someone for those parts considering the experience/tools of family/friends, and how long it would take working only Saturdays.

I miss living where I have lots of family around. There are things I'm forced to hire out that I know how to do just because I can't make a few phone calls and get a dozen people to help next Saturday.