When you are in "Create a family" you can see a very simple room, but you can change it...

The CAS is a lot file, but it isn't available on Lot Catalog. Its package isn't in the folder in My Documents, but in general directories of the game under Programs directory.
Search the CAS!.package file in GlobalLots folder, C:\Programs\EA GAMES\The Sims 2\TSData\Res\GlobalLots\, and copy it on LotCatalog folder, c:\My Documents\EA GAMES\The Sims 2\LotCatalog\. Now you must rename the file, give to it the first name avalaible.

For example if you have cx00000012.package al latest file in Name order, the CAS become cx00000013.package. Attention to the name you give to it, you'll use this file again so you must be able to identify it by the name. But if you don't remember the number of the CAS file, or you place some lots on your neighborhoods, you can identify CAS by the date of latest modify.

Now launch the game, enter in a neighborhoods, and open the lot catalog. You can find a lot with a strange name, something as jjlghjjk. Put it somewhere and open it.
You will find some objects, they are default's objects and you can't move them. If you move them the CAS will not appear. You can add some objects as you want and cover all default objects and modify the house's structure.

You can create a CAS in a garden too!! You can choose your favourite way to make a CAS as your desire.

When you finish to modify it, come back to neighborhoods after saving the changes. Now you must place the CAS lot on the Lot Catalog.
Exit the game and go to the LotCatalog folder, c:\My Documents\EA GAMES\The Sims 2\LotCatalog\.
Take the file and copy it in the GlobalLots folder, C:\Programs\EA GAMES\The Sims 2\TSData\Res\GlobalLots\. You can move it too, but if you copy it, the cas will remain on the Lot Catalog too so you can modify it when you want.

Now attention please, you must replace the old CAS!.package files with this, but is right to make a backup copy of the original CAS file before the replace. So rename the old CAS file in CAS!.package.old, for example.
Rename the file made by you in CAS!.package, then start the game, go in Create a family mode and ... look at your creation!

Warning!!! The custom CAS aren't compatible with the EPs, so, before installing EPs remove it and replace with the original CAS.

Once you install the EP you can replace the original CAS file with yours. Attention, now you must replace not only the CAS on C:\Programs\EA GAMES\The Sims 2\TSData\Res\GlobalLots\ directory but the CAS!.package file in the EP's directory too:
C:\Programs\EA GAMES\The Sims 2 University\TSData\Res\GlobalLots\
C:\Programs\EA GAMES\The Sims 2 Nightlife\TSData\Res\GlobalLots\
C:\Programs\EA GAMES\The Sims 2 Open for Business\TSData\Res\GlobalLots\

You can now create CAS without the default items too. And in addiction you can put CAS files (only once) on the Downloads folder. To do so, you require to download some additional files created by Windkeeper.
You can find these files on Modthesims2, exactly here. There are so many files, download in order to obtain what you want (remove only mirror, remove all objects and keep mirror and podium and so...).

SOME EXAMPLE
...You can download here...