I pmed ModHunter and asked for some more details. And found that the house they are/were trying to renovate is a house originally from Island Paradise that was renovated by the person who put together the save file.
Here's the information of the original house:
Fiesta House – 1br, 2ba
Address: 78 Civic Center
Lot size: 20x15
Occupied by: Senorita Hernandez
Lot description: Life's a party when you reside in this colorful home!
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I jumped in-game to see if I could edit the house both in Island Paradise and in a copy of the save file ModHunter is playing in.
I'm unable to edit the house. Regardless if I try pulling the walls out on the kitchen side or build by drawing out the walls on the kitchen side.
The errors that I get are - Requires Standard Level Height when trying to place a spiral stair case by the front entrance. I also get the error Covered by Floor Tile when trying to build walls on the kitchen side of the house when trying to make room for the stair case.
I searched out the errors that I got and one of the search results said to try using CFE false (constrainfloorelevation false) to place the stairs. I did try it, but it didn't help. It still gave me both errors.
It might be best to try to rebuild the house. It seems like it was built with build cheats that we players don't have access to.
The thread I'm linking to below talks about houses in Barnacle Bay, but could also be true for houses built for Island Paradise. In that they were houses built with build cheats we players don't have access to and that's why we can't fix some EA builds.
Cannot Place Stairs or Basements
https://forums.thesims.com/en_us/discussion/389408 I am sharing my notes on this just in case someone else comes across this and has the same issue of not being able to edit some EA built houses.