As much as everyone loves to complain about red tape, building codes have a lot of useful info. Even if you're not going to pull permits (at your own risk) it's useful to be aware of what code requires and why. Specific examples from the post:

* Plywood thickness required for floor decking

* Min roof slope for asphalt shingles

* Drip edge, where it's required and how to install it

* Roof drainage - probably doesn't matter because California but as a general rule you never want to channel water towards your foundation.

In the grand scheme these are relatively minor. They'll probably need to demolish the thing when they sell the house long before the roof slope causes water issues.