Items in the bay in an attic are inaccessible to sims because of the roof so take this into account when furnishing.

Now you can decorate as you would any other room. Take note that to use the full wallspace, you will need to have joined the dormers up at the back (circled in white). If you join the dormers in the way circled in blue then you can't use that wall for any objects at all. Also take note that if you put stuff buried in the far reaches of the walls as shown in figure 9, Sims can't use them. Therefore make sure you put sim accessible objets inside the attic - ie not in any part of a dormer.

The Oldies in their new attic. Notice the floating telephone.

While this requirement about not furnishing the dormers might rule out most of the wall based objects, the move_objects cheat is your friend, and allows otherwise wall bound objects to float in midair. It is generally recommended that you put decorative objects or anything that Sims don't need to interact with in the bays and everything else within easy reach.

Our house with accessible roof and an attic. The dormers make the house stand out a bit.

And this is the final house. You don't need to use dormers just for attics, they can be used standalone or not at all.

Also note that this technique works for any house with however many stories up to the limit imposed by the game so in theory one can have a basement, three floors, an attic and an extra section on the roof. So in theory there should be no way to run out of room to build now.

You can download this house from my Files section on this site, and expand on it or whatever.

And that wraps this tutorial up so in a nutshell, the game does most of the work for you. :) Enjoy!