People like the loft in my guinea pigs' cage. It is only 6" off of the main floor, so it doesn't require a steep ramp that is common in most second level designs. In fact, it doesn't require a ramp at all! Guinea pigs can just hop up or jump down from it.
I've had several people ask for instructions how to build one. It's not complicated, especially if you've made a C&C cage before. The loft is made out of 10 grids, a bit of coroplast and a bunch of 50lb zip ties. If you are adding this onto an existing cage, you will need to modify the base on the side you are adding the level. It requires a half-grid wall instead of the full grids.
I cut a single grid into two pieces (removing the middle row of grid-holes), so that I end up with two 4x9 hole grids. I used a pair of heavy-duty wire cutters for the inner wires, but I needed a hacksaw to get through the thicker edge wire. The two half-grids are the same height as the coroplast on the base. This is to help support the second level on the base of the cage.
The second level is basically a 1x2 grid cage - a 1x2 bottom, a 1x2 side, and two 1 grid sides. Zip tie them together. The "roof" is optional. I wanted it for extra storage space, just to be able to dump cozies and such. Plus it allows me to hang a hammock in the loft. Since I use it as a shelf, I dropped the grid roof in by one grid-hole, so I had a lip on most of the edges.
Measure and cut the coroplast following the basic directions on GuineaPigCages.com. It's only three-sided, so don't measure and cut a 4th wall. I have 14" grids, so my base measured 40x20" (a base of 28x14" plus 6" walls on three sides).
I have seen some people add this floating level without a back support. Some have it up against a wall. Others have it all supported by zip ties. I added a back support when I built mine since the second level stuck out so far from the main cage and I wanted to ensure it was sturdy. The support requires two grids, zip-tied on one end. I bent it to an L-shape, zip-tied the one side to the floating level, and let the other side run under the main cage. Do whatever works for you!
The second level overhangs into the cage by about three grid openings. It's a tight fit! I zip tied the second level to the base where the grids overlap (you can see in the picture, its a little 3x5 grid-hole overlap). I have that section zip-tied with 2 or 3 ties, on each side. I then slid part of the L-shaped piece under the main cage, and the other piece flush against the back of the second level. Zip-tied that to both the back and the base.
It's pretty sturdy. I have lifted the cage (usually a two-person job) by grabbing the base and the second level. I would not recommend moving it long distances by lifting it by the second level, but I have yet to have a cable-tie break.
I needed some sort of bedding on the second level. I used shavings on the main level, but that was not an option for this second-level design. I decided to use fleece, and it has worked very well on the upper level.
I measured a piece of fleece the same size as the coroplast bottom and sides, plus 2" to all edges (to be able to tuck under). The coroplast fits tightly against the grids, so the fleece stays put pretty nicely. I also use binder clips to help keep it in place. I have two baby binder clips on the bottom, one on each short side, and although you can't really see it in this picture, one binder clip on the front edge wall of the coroplast (for some reason, my industrious pigs pulled it out on one side, but not the other).
Ta-da! A bit of extra space without any ramps to get in the way. My girls loved this extra level so much, I eventually built a third level to the cage. Instructions for that can be found at Building A Third Level.