Hello, I've done some modding but I'm still fairly new. My house mod that I plan on doing will be underground, I want to access it through a load door in the Hjerm home potion making area and when I pass the load door I want the stairs to go downward.... etc. But in the creation kit should I copy and paste the Hjerm home into my mod cell and just continue on from there or do I need to make my mod and then somehow put it into the game?

You have a couple of options that I see from what you're looking to do.

If I were adding another area to a vanilla house, depending on how big it was going to be, I'd probably just do my new level in its own cell with a door, add a door in the vanilla house, and link the cells. That's the least offensive way to edit the vanilla cell because all you'd really need is the door and any door frame statics and to finalize the navmesh.

The other way would be to add everything to the vanilla house by setting the vanilla objects you need to remove as initially disabled and moving them out if the way. (Never DELETE vanilla references!) Then add your new level. You may have to re-do portals and roombounds and of course the navmesh. But it can be done.