I share my home with two cats. Living alone with my cats has been very satisfactory with a conventional indoor litter box. But, my girl friend and I have been talking about her moving in with me recently. She has two cats of her own. We would then have a house hold of four cats. My home is not big enough for the litter boxes needed to satisfy these four cats. I have no interest in having a home with mulitple litter boxes all over the house so, this is my solution. It removes the litter box out of the equation as far as the interior of the home and since I'm putting it on our screened in porch I've added a door in the front to let them go out onto the porch too.

Step 1: The Box

I actually got the whole idea when I saw this Suncast box (Model: VDB19500 Backyard Oasis™ Storage and Entertaining Station) at BJ's Wholesale Club but Home Depot also sells them and thought this would be perfect. It has two doors that open in front and a lid that opens on top making it very easy to get into it to clean the litter boxes.

Step 2: The Bins/Litter Boxes

I originally wanted to do one big bin to use as one big litter box but I couldn't find a bin with the right dimensions. So I chose to do two bins/litter boxes instead. They are still rather large since each one holds 40# of litter to get a 4 inch fill of litter. I found these at Walmart (Sterilite Stacker Box, 19 gal, Gray, Model#14765906). The great thing is that turned cross ways in the box makes a nice pathway to put the planned exterior door in the middle of the box.
I then used 1x2, liquid nails and screws to attach a couple of stops in the bottom of the box to hold the bins in place but with a simple lift and pull toward the center and they can be removed for cleaning.

Step 3: The Gangway

I used 1x10 for the gangway, 1x2, liquid nails and screws at either end to hold it up. I used Velcro on the ends to hold the gangway in place because I wanted it to be easy to remove when the bins needed to come out for cleaning. The Velcro seems to hold it in place very well. I covered the gangway with that 3M cat carpet to help keep the litter being brought back into the house to a minimum and help the cats with traction. I used 3M spray contact cement and staples to hold the cat carpet in place.

Step 4: The Passthrough From the House

I was very, very lucky here. Next to my back door there is a window so the studding below the window was less that the standard 16" apart. They measured 10 5/8" apart, which worked out perfectly for what I had planned. I used a large cat door from Cat Mate (model#221 white) on the inside of the house. I cut the hole, for the door, in the center between the two studs under the window and installed the door. Next I used a reciprocating saw to cut the hole on the outside of the house all the way out to the studs on the top, bottom and sides. I used 1/2" plywood and screws to anchor it to the walls, floor and ceiling. I made the tunnel go out to where the box would be located, go through the box and fit slightly inside the box. Trimmed out, caulked and painted the tunnel to protect it from the weather.

Step 5: Connecting the Box to the Tunnel

Next I moved the box back up against the tunnel and marked where the hole needed to be for the tunnel to go into the box. I used the reciprocating saw again to cut the hole in the box. Next I moved the box into its intended spot so the tunnel was just protruding through the box and gave me enough to caulk around then screwed the box down to the porch floor. I caulked around the tunnel to keep the weather out because I may need to heat it later in the winter. Not sure about that yet.

Step 6: The Exterior Cat Door

Next I installed a Cat Mate (model#221 brown) cat door in the front of the litter box so my cats can out onto the screened in porch when we're home and enjoy the great outdoors watching all the birds at the bird feeders.

Step 7: The Final Touch

I installed two battery powered LED lights that work on a motion sensor so the cats don't have to negotiate the box in total darkness. There are several makes and models out there. You are welcome to choose your own.

Thank you very much for viewing. I hope this is helpful and inspiring. Questions and comments are welcome.

Enjoy.