Oops he's not 14" wide, just 11", approximately.

I'm new to the world of dog sizes and when the size L dog door has an opening of 12x20", I don't get it. My dog is a solid medium size I think!

Dogs don't stretch and get skinny like rats do they?

The link below may help in figuring out the correct size.

Re: the step, is it lower than the floor height once your dog steps out the door? If so, you'll need to adjust the door size so that the rise is not a standard height. You don't want the dog going out the door and having to immediately step lower than the floor on the inside. It's not good for their back and hips.

We had a similar issue in our previous home and we added some pieces of lumber to raise the height of the first step. But, if people are using the steps also, then changing the rise of the first step would become a safety issue.
So.....if that's the case for you, then lowering the rise of the dog door would be the best solution.

Here is a link that might be useful: dog door sizes

Thanks Annz!

The exterior drop is about 4" to a single "step" about 2 feet deep, also about 4" high. I didn't realize how irregular it is--if it were built up to be level with the door it would be closer to normal for everyone.

I should be able to install the dog door lower. It's a paneled door so I was hoping to get it in the panel but it's also pretty trashed so going through the thick part (carpentry term!) is probably okay. It might be necessary now that I look at it--the panel is really thin. This is supposed to be a temporary arrangement as we plan on doing some major work on that part of the house but we tend to procrastinate so it could be a few years.

The dog just squeezed through a sliding door at 9" so I guess he'd be okay with an 11" wide door for now.

He's only 14 months old though. Is he done growing? Eek!