The answer is easy enough go to the roof tool and remove all roof pieces and turn off the auto roof and then add the roof when you are finished.
How to add a floor on sims 3.
Make sure the mod you are downloading is for the sims 3 and not the sims 4.
Also make sure it is compatible with the latest version of the game.
To build up a level either on a new build or a pre existing house is quite easy.
You should be able to click on the bottom story and a yellow outline will appear around the top of the walls click on the outline and there should be an option to create a ceiling if not it looks like you don t have the bottom story completely closed off which makes it where the game doesn t recognize it as a room.
To make things easier add floor tiles on the upper level over the carport.
Roof is added automatically over any area enclosed with walls.
When you find a mod you want to download click the download link on the page to download the package file as a zip file.
Go down to floor level and flatten across the garage with the level terrain tool and then build the upper walls around the carport.
Download a sims 3 mod.
Press lt and up to show the second floor.
Go up a level in view and add floors and walls where you want them.
Remove the roof all remaining walls and stat to build your second floor.
Then build walls were the flooring.
We now need to rerove some of the lower wall at the front but not all.
Btw i bought this house so i don t know much about building.
The sims 3 allow you to build up to 6 levels 6th level cannot hold walls but only roof or floor tiles and railings.
How do you add another floor on the sims 3.
Skip the piece closest to the house and remove the next three pieces.
The sims 1 allow you to build 1 or 2 levels only lower and upper level and no foundation.