Build stairs through foundations.
Sims 4 delete floor under stairs.
From there delete the 3x2 tiles on top of the basement.
As i can recall this have been really easy to achieve in previous versions of the sims.
The game will remove any extra flooring for the wider stairs or add flooring if you make wide stairs narrower.
The floor on floor one needs to be deleted select the sledgehammer tool at the top and hover over the floor on floor one.
I had to build fences round the stairs which is really strange looking when there s nothing holding up the fence.
That looks like the ground floor you re trying to delete so simply make a basement room sized 3x2 under the staircase select the room then go up a floor where you will see the yellow outline of the basement room.
From there click delete ceiling.
Delete the floor and then redraw the new room size for the balcony space in door or whatever you call it.
Then remove the wall that holds that room together.
Select the yellow outline and choose remove ceiling.
In the sims 4 you re able to adjust staircases dynamically.
You should then have the floor from the new space you drew as well as the deleted floor so you can see to the first floor below.
Once the basement is built and your stairs are in place placing a foundation and adjusting the staircase is a simple experience that works.
If you want wider stairs select the placed stairs and grab the side arrow and pull out or to the desired width.
Gone are the days of having to demolish and rebuild your house if you forget to lay down a foundation.
Now that you have an open pit on your lot that you can fill with grass place some planter boxes and you re all set.