If you want a free mansion in The Sims 4, you've come to the right place. You can do this in only 5 steps! (The 6th step is optional.)
1. You MUST already have a household, It only needs 1 member. (Although you can have more.)
2. Next, go to a mansion/house SOMEONE ALREADY OWNS.
3. Then, go to the friendly category and do a lot of friendly things to one of the owners of the mansion until the friend bar (sorry if that's not what it's called) is half full.
4. Then, go into the friendly category again, and choose move in with.
5. Next move everyone in your family you want to be in the mansion, and make them move in. (KEEP THE OWNER(s) IN THE MANSION)
6.[OPTIONAL] (Do this part if you don't want the former owner(s) in the mansion) Then make only the former owner(s) go into a room, and once there in the room, put everything in that room in the household inventory, (put them back in the room when the former owner(s) die(s) and remove the door to the room. (Sorry for being captain obvious, but you must be in build mode to move things to the household inventory and delete the door.)