It takes some intestinal fortitude to cut a hole in the side of your house. How do you know where to cut the hole for a pet door installation? It's a combination of an educated and common sense guess. Here are some ideas on how to make the project go smoothly.
First, make sure you buy the right door. The easiest to install are doors made specifically for going through a wall. When you cut a hole in an exterior wall, you will have insulation to contend with. These kits have pieces that span the opening from drywall to stucco. They also keep the insulation in place, as well as give the door a clean appearance.
Some people buy pet-door kits made for installing into a door, and then build a box to slide into the hole in the wall. This is OK so long as you use a material that will stand up to the weather and the pet. A laminated material, such as melamine, works well.
Cutting a hole in your house is scary. The idea is to place the opening in a spot that is convenient for you and your pet, yet does not run into electrical cables, plumbing, and other hazards.
Look at where outlets run and where switches are located. If you are placing a large pet door between two outlets, you will likely find an electrical cable (you may be OK if your pet door is small and is located low on the wall). Generally, a safe place is near a door, but not so close that you will run into the door frame. Use a stud sensor and mark the locations of the studs.
You also want to avoid areas under windows. There will be more lumber in the way under the window openings, so find another spot.
When you find a potential spot, you can cut a small access hole in the drywall and take a peek inside to see if there is a cable present, or you can mark the template on the wall and cut away. If you do run into a cable, you will need to route it around the pet door. This is a matter of installing junction boxes and running the cable up and over the pet door opening.
If you cut the opening in between studs, you won't need to frame the opening (unless the pet door manufacturer specifies so). If you have to cut through a wall stud (avoid this if possible) then you should frame the opening and install a header in the opening to support the stud you had to cut.
Most doors will easily fit in between studs, so just position it correctly and avoid all the extra work.
Generally, the pet door is installed so that the pet's shoulder is level with the top of the door. Make sure the template is level and trace around it.
Use a drywall saw and cut shallowly to avoid cutting through a cable. Use a utility knife and cut through the insulation so that the opening is square and clear of debris.
You want the opening on the outside of the wall to be identical to the opening on the inside of the wall. Drill holes at the corners of the exterior side of the wall to match the location of the corners on the inside of the wall. Mark lines to connect the holes. Use a circular saw with a Carborundum blade to cut through the stucco and wire, and then a saw to cut through the wood sheathing. You should be left with a nice hole in the side of your house.
Like I said earlier, if you didn't buy a wall kit, you can build a box out of melamine and fit it to the hole. If you do have a wall kit, assemble the pieces and push it into the opening in the wall. Simply connect the two trim pieces with flaps on either side of the box and screw the whole mess together. After everything is nice and tight, caulk around the perimeter of the trim pieces to hide any gaps.
You can also cut a piece of indoor/outdoor carpet and glue it to the bottom of the wall kit. Maybe Fido will learn to wipe his feet before he comes into the house.