I want to do something like that too.

Cat Mate has an extension kit that works with their cat flaps. I found a number of other companies also offer this option. Try searching using the word tunnel.

I'm not sure why you need a flap. If you decide you don't need the flap, here is an article about an interior cat hole http://www.apartmenttherapy.com/tips-building-a-cat-tunnel-in-62280

Have your cats used flaps before? We have two cat doors in our current house, and neither of our cats would go through the flaps. We had to take off the flaps. Our cats were about 9 and 6 years old when we installed the cat doors. Maybe you can't teach an old cat new tricks? ;-)

dekeoboe: My husband wants a flap. I was originally assuming it would just be a tunnel, but he's worried about "acoustic privacy" (even though it just leads to the laundry room). Thanks for the link and the search hint!

Mom23Es: No, our cat has never used a flap, and she's 10. I read a lot of reviews of various cat doors and nearly everyone said their cats got the hang of it pretty quickly, but all cats are different! Ours is pretty spritely and not easily fazed (phased?) -- when we moved into our rental for the renovation, she barely missed a beat.

I'll investigate the "no flap" option -- maybe I can convince hubby.

Box in the opening with 2x lumber between the drywall layers on each wall and install the flap on one side.

2x is easier to hit with the drywall screws to fasten it in place.

It does not need to be attached to the framing.

What about this:

[Contemporary Bathroom design[(https://www.houzz.com/photos/contemporary-bathroom-ideas-phbr1-bp~t_712~s_2103)

I think the problem will be that unless you built it yourself, you are going to end up with overkill. Every "through the wall" cat flap I found is secured via microchip or otherwise to prevent unwanted critters from coming in the house. On the website I've added, the through the wall flaps are $100+, but the interior door cut out is about $30.

Here is a link that might be useful: All kinds of cat flaps/doors

We decided to just have the contractor box out a tunnel and finish it off simply (probably not as beautifully as in the photo Laura12 posted). I looked up a bunch of different flaps online and took sort of the average of the rough cut dimensions, and told him to build it 6.5w by 7.5h. That should be big enough for the cat to get through, and small enough that if we decide to add a flap in the future, there will be something to attach it to.

Knowing our track record, it's quite likely that once the house is finished, we will never get around to installing the flap, which would be fine with me!

I had to teach my ten year old cat to go through the cat flap. I pushed her head gently through the flap and encouraged her to follow through. I did that from both sides quite a few times. It took a while, but she eventually got the idea and then started using the flap on her own. They say to use treats to reward them when they go through (at your insistence) but my cat does not like treats. I got frustrated, but after I did it a bunch of times over several weeks, I gave up. Then she started using it on her own. Just like a cat! Had to be her idea. Now she loves it.