I live in a two story house with 2 1/2 baths. when I moved a cat in I needed to put his cat box somewhere and the 1/2 bath was the best place for it. Problems arose because I now had to keep the bathroom door open a crack so the cat could come and go. Not nice looking, or smelling.
My solution was to cut a cat door into the bathroom so I could keep the door closed. After considering where to cut the door I decided on making a door into the bathroom from the storage cupboard that is also under the stairs. From there I can cut a cat door into the storage cupboard door to allow access. I chose to cut two doors so that people in the dining room don't have a little door directly into the bathroom to look at. Also if I ever want to close it all up patching a wall in a closet is much easier to hide, not to mention a new door for the cupboard is cheaper than one for the bathroom itself.
Step 1: Cutting the Hole in Your Wall
I used a sheet rock saw that cost about $6 and it worked great. From the bathroom side I drew lines where I wanted my door to be and just started cutting. After cutting the one side I used the hole as a guide to cut the opposite side. This worked ok but the door sizes are a little different because I did not hold the blade totally straight. My cat does not mind.
Step 2: How to Keep the Cat Out of the Wall
So after you open up a hole in the sheet rock you now have an entrance into your wall. Not wanting to give my cat free reign of the interior walls I used some leftover cedar flooring to block the gaps. I cut the pieces to fit and used silicon to stick them in. Not real sturdy but it's for a cat and they walk lightly. Plus I don't think he even steps on it, he just steps through.
Step 3: How to Hide the Ugly Cuts
I originally planned to go get some 1/2 inch trim and make a frame for my cat doors but I was into this and wanted to finish. I dug around and found some old frames and decided to use those. I opened them up, removed the glass and in one removed the picture it came with.
I then used a box knife to cut a hole in one of the backs. The other had nice matting in it that would work well. To cover the backs of the frames when you looked through the hole I used hot glue and black felt. I then cut the felt out of the center using the matte as a guide.
Step 4: Hang the Frames!
I used two trim nails at the bottoms and velcro at the top to secure the frames over the holes. I might need to go back and secure them better but I was running out of steam and wanted to eat dinner.
And Presto! Two hours after starting I had my cat door and Jak-Jak approves. I approve too because I can now close the bathroom door all the way. It will also make a convenient pass through if you need more toilet paper or a magazine.
Next step is to cut a door into the half door of the cupboard. I want to make this look good so I need to pull the door off and take it to a friends house that has real tools for wood. I am thinking a round hobbit door, or a Stargate, or a sliding grate would be pretty cool. That and a potter nameplate would look good.