We have built-in cat doors in our house. We have two decks, a screened in deck and a open deck. Neither has steps going down to ground level. The screened in deck cat door is available all the time. The one to the open deck has a little swing door I made that can be locked. It is closed at night. You can see the little padlock for it in the picture, those cats are crafty (but we can leave the key in the lock :laughing

. The white foam is for insulation and to keep the wind and rain from blowing the door in (and getting things wet) when the door is locked.
There is also a pictures of that door from the outside, and a bad close up of how it is framed. Basically, we framed with 2x4's. We bought two brands of doors and make the size such that the door can be changed out without having to re-do the trim for the door. Caulk fills in the little gaps. So the door can be replaced without ripping up any trim.
The covered box by the cat door has a cat-5 that will someday go to electronics I'll be making. I'll design a 130 KHz loop antenae that is in the trim around the door, and run an RF ID so we know which cat is using the door. (quick home automation digression)
The bigger view of the door (the one that include cats that were curious as to what I was doing) shows why we did the doors separate. We wanted the full view window doors (Full Light Door). The cats like them too- they can sit and watch the outdoors. The "shelves" are cat steps. If we put a block at the bottom, when they get old they can still climb up to that one counter.
There is also a shot to the screened in deck door (you can tell the deck is covered and see the cross pieces for the screening). Again, the build in door make the people door very nice. Another box for RF ID. This approach is spousal approved and the big door - little door gets a very high "cute" rating from the same judge.
Finally, there is a shot of the people and cat door into the snoratorium. There is a similar set into the master bedroom. This lets the cats rome around at night while maintaining the sound isolation of the snoratorium (used in the unluckily event of snoring). Again, the big and little doors rate very high on the spousal approval scale (as does the snoratorium). The cost of triming the cat door is _more_ than the people door :confused2:. Exact same cuts, but the smaller pieces make it difficult and there has to be some trim wood put in (whereas the people door the trim is part of the door). The 2x4's are thinner than the wall, so the box has to framed out. Can't find a picture of a door before framing, I'll keep looking.
I would imagine that the same process could be used for a dog door. It is a big win to have the cats able to outside on their own. They are protected from the world since they can't get to the ground. The door we lock each night to the open deck keeps any deck climbing critters out. The door to the screened in porch is also a bit win since when it's 3:00 AM and the cats want out they can go listen to the frogs on the pond from that porch. Finally, the look of it all is very nice and unique compared to the critter door _in_ the half light people door.
Hope these pictures help you make a decision. I like this style better than the half light door with user added pet door. The pets can see out. Since you are in the fog of construction with a billion things to do, maybee you figure out how to do one like this. We have other cat features in the house, but I'll skip those so as to keep on todays topic.
Pete