We've had a cat tree, which I constructed, in the our home office for a few years now. I built it from spare lumber, carpet remnants and some old carpet tubing which I picked up for free from a local big box store.
So, I broke down and bought six shelving brackets from the hardware store. I then cut, sanded and painted some old plywood. I topped the homemade shelves off with leftover carpet from the original carpet installation in the office.
It didn't take long for our cats, Hugo (tan) and Holiday (gray), to get the idea.
We have regained all of our floor space and the cats still have a place to chill when Laura and I are at our computers.