The most important thing to remember about the setting of The Witches is that we are not in our own world. Instead, we are in a fantasy world where witches exist. We have to suspend disbelief – that means we can't say, "But wait, that's not possible! There's no way a human can turn into a mouse!" It is possible in this world. Heck, we see it happen. With that in mind, though, the rest of the setting – the more physical aspects – are a little more familiar to us.
Norway has a special place in Grandmamma's heart, and her house is really all we see of it. We don't get a huge description, but we can say one thing for sure: it's cozy. Our narrator describes his grandma's "majestic" armchair, and says that "[t]he curtains were never drawn, and through the windows I could see huge snowflakes falling slowly on to an outside world that was as black as tar" (2.14). The feeling of being inside a nice, warm house on a cold, snowy evening is always comforting, and that's the sense that we get from this place. Leaving Norway for England, in some ways, feels like leaving a security blanket behind.
When, in the end, Grandmamma and her grandson return to this house, it just needs to be adapted to the needs of a mouse. Grandmamma comes up with some pretty nifty gadgets to help her mouse-grandson get around. Remember, cozy for a human is not necessarily cozy for a mouse. Still, it seems like they figure it out, because in the end, our narrator sits on his grandma's lap and "doze[s] comfortably in the warmth [of the fire]" (21.11).
After leaving Grandmamma's home in Norway, she and her grandson head to his old family house in England. We don't learn much about the house itself, but we do get a nice description of the tree-house in the backyard. It has a finished floor and railing, and our narrator is working on the roof (there's actually a great illustration in the book of him hammering away). The structure is in a big conker tree in their backyard and our narrator describes it as a "big green cave" (4.62), high up in the tree.
Even though our narrator doesn't spend much time up there, it's nice to have an image of the tree-house because it really reminds us that we're in a kid's world here. Grandma's house is great and all, but it's not "exciting" in the way the tree-house is (4.62). Every kid deserves to have a fun hideout – somewhere they can hide from witches, perhaps? – and this is our narrator's.
We hate to say it (actually, we love to), but the Hotel Magnificent is quite magnificent. It's described as a big, white building right on the beach with a "maze of public rooms" (5.55). A lot of our narrator's time as a mouse is spent running through corridors, adding to this mazy feeling. Because of all the running from place to place, we get an image of the hotel as one big labyrinth of spaces, and less attention is drawn to each of the separate rooms. Having said that, there are a few rooms that stand out.
The Dining Room lives up to its expected magnificence: it is a "huge room with gold decorations on the ceiling and big mirrors around the walls" (18.12) and there are two long tables set up in the center. The kitchen is also magnificent, but for different reasons. It's bustling with energy, and has lots of opportunities for mouse acrobatics. And of course, there are Grandmamma and our narrator's bedrooms, connected by a door, each with a balcony (which come in quite handy).
Finally, we have the infamous Ballroom. Imagine stumbling across something like this when all you're looking for is a private spot for mouse-training: there are double doors, "rows and rows of chairs, [...] painted gold [with] little red cushions on the seats" (5.55). Fancy shmancy. To top it all off, in the back of the room is a "large folding screen with Chinese dragons painted on it" (5.57). It turns out, though, that such a magnificent room will host some important meetings. Our narrator learns this the hard way.