The game is reading the ceiling as too low in your dormer. You need to put a roof on the dormer on to cut out the portion of the gable roof below. While it is true that sometimes roofs that encroach into the living space of a house won't interfere with the Sims during gameplay, as far as I know that refers to the overhangs of roofs that show up in the houses because the roofs are bigger than one tile. If you build a giant gable through the room like that, the game reads the ceiling height from the gabled roof, and your Sim can't get to the edge of the room where the "ceiling" is short. You solve it by making a true dormer -- in the sense that it has another roof on top. See (very basic) images below:
DORMER WITH NO ROOFDORMER WITH ROOF - note that you can see through the windows to the floor inside
Unfortunately, the flat roof does not work for this.
I used a very low pitched half-hipped roof for the next shots. You can put plants on top of the roof and add your decorative edging, but you will see the ends of the roof on top.
DORMER WITH ROOF, PLANTS AND EDGINGThere are two possible solutions to this -- which one is better depends on what exactly you are looking for. You can make the roof on top of the dormer one tile smaller than the actual size of the dormer, cover the rest with floor tile or flat roof, and place your plants and edging. That will keep the rain out and look better from the outside, but it still leaves a one-tile rim of low ceiling inside your house, which you can see by looking through the window. As far as I know, your Sims cannot route through that low ceiling, but you should be able to leave your TV in the corner, etc, by using moveobjects on.
DORMER WITH SMALLER ROOF AND EDGING The other possible solution is to bump the dormer back by one tile, put a half gable in front of it, a half gable next to it, and then another half gable on top of it, a few squares back from the front of the dormer so that it matches up with the other piece. That reduces the inside of your room by one tile, and your windows are either clipped by the roof or have to be much smaller, but it will give you a line of roofing under the dormer windows without putting a roof through the actual dormer, if that's what you're looking for.
DORMER WITH COMPOUND GABLE ROOF - plants and edging left off so you can see the roof structure better
Does all of that make sense, and do any of those solutions help?