hey @sandygrl I haven't built a basement yet, but in the wall tools there is a new basement tool. If I am not mistaken you have to use it to build the walls in the basement just like you build regular walls. Does that answer your question?
I will quote from the basement article that is on theSims.com site.
You can delete their ceilings. After you build a basement (or two) you can select the basement block, navigate to ground level, click the yellow highlighted area, and completely remove the ceiling.
Comments
The link to the article is
http://www.thesims.com/en_GB/news/basements-march-2015
The Basement Tool is found in the Build section, under walls and empty rooms.
It is next to the Wall tools.
http://forums.thesims.com/en_US/discussion/817478/hello-come-and-introduce-yourself
http://tinyurl.com/OneRoomOneWeek
http://tinyurl.com/rosemow
My Showcase thread https://forums.thesims.com/en_US/discussion/948861/rosemow-s-rooms-showcase