We moved into a new house and I wanted the cats to be able to enjoy the beautiful Alabama climate but also wanted them to be safe. We had a window in the basement that could be adapted for a "pet door" although it was high and would need boxes in the basement so the cats could reach the windowsill and pet door.
Step 1: Determine Size
To fit between generator and a/c unit I decided on 10' x 14' x 8 ' high. Made list for Home Improvement store for 4x4's, 2x4's, 100' roll of 2x4 fencing. 2 x 10's for framing bottom and used a pre-hung door I found in the basement of home I bought.
Step 2: Fun in the sun
The cats can go downstairs and into Cat-io whenever they feel like it.
Step 3: Available for night use, too.
The door to outside is locked but gives me access when I am outside.
Step 4: Show and Tell
Maybe this isn't really an INSTRUCTABLE but rather a show and tell. With handy friends and some donated materials, it might cost less than you think. Garden plants can use the fencing as a trellis. Some day I hope to make a door and stairs from upstairs level and have a screened room to enjoy with them. You can add patio furniture and/or other things for pets. Since the fencing is 2 x 4 there have been times when they bring in birds, little snakes and mice, so far I caught them all and took them outside.