I used that lot once, and it's so cluttered it can cause routing problems, so you may want to delete a few things. The black room in the basement ... are you talking about the swimming pool? It has black walls and a window to the hot tub room. Go up a floor and you can see the water The basement should be able to be expanded with no problems, but you may have to delete the pool if you want to go that direction.

The lot has 1 level skip marker on the roof and 2 hidden room markers inside 2 small rooms used as columns. The hidden room markers are not needed and can be deleted. You can delete the level skip one as well if you like.

Yes I decluttered and fixed the issues. The place is more work than it is worth. I think I am just going to get rid of it.
The pool is not the black room. I think this lot was just destined to be an issue because it was all locked up and I used the cheat to edit it and it didn't like it. The black room actually happened when I built a basement, and I drug the basement to the back end of the lot (and the lot I placed the house on was bigger than the house required) and the back part of the basement is the part that is untouchable Since this part of the basement isnt even beneath the original lot size, it couldn't possibly be a hidden room issue... so I am thinking the lot just wont recognize that its bigger than the 30x20 the creator built it at and anything I drag into the extra area is just off limits, even though I can drag a basement through it and carpet it, I can't do anything more with that space.
It's going to be demolished at noon tomorrow. I liked the house because it was tiny and I wanted room for a horse, so I needed a tiny house to place on a larger lot. This one is just the cursed.