> you got to figure slope and elevations

I mean, you should... but there's still a lot of wiggle room on the way from doesn't meet modern code / never met code / shouldn't be done / is unsanitary or unsafe / will not work.

Residential electric is easier to do in ways that work but are either not to code or not to common best practice. The handier the previous owner was, the more new ways to do things you can learn. If a previous owner was in the trades and also heavily involved in the initial build, expect lots of weird repairs. :p