@j1z1 you can actually download builds from the gallery for standard apartments, but they come under rooms rather than lots. What people do is knock out a tile of adjoining wall so that the entire apartment is classed as one room. You download it then fill in the wall gaps and - hey presto - you have a ready-made apartment :)
But yeah, basically penthouse apartments are just normal lots but up in the sky.
https://dl.dropbox.com/s/dwqakr1qtc68q8n/05-10-20_1-49-26%C2%A0PM.png?dl=0
You can't add grass terrain paint, but there is grass flooring so you can even have a garden up there.