One of the most common first approaches I see for designing a magical location is determining its purpose. Sometimes, it’s to channel magic in a certain way, but that’s less common and not necessarily likely to stand on its own—we’ll get into those later. But most of the time, even on an inherently magical building, the actual purpose (at least, as far as most of the people who use it are concerned) is going to be a bit more practical.

As with anything else, the first thing to figure out is, “What is this place for?” It might be something with a direct real-world equivalent, like a library or a hospital; it might be dual-purposed, made specifically to evoke a mood in its in-world visitors as well as serve a primary purpose, like an overlord’s ominous castle or a reverent cathedral. It might have a purpose peculiar to the culture that designed it, the forces that shaped it, or the fantastic nature of the world around it—perhaps it was built to house some sort of bound creature, or designed to itself be mobile.

Once you’ve got a purpose, think about how that might affect the structure. This might be just a matter of floorplan: a building meant to keep things safe might be a lot more complicated than one that’s meant to house people who need to be able to spring into action at a moment’s notice. It might also affect materials; somewhere that’s supposed to be defensible will need to be made out of stronger stuff than one that’s just for holding parties.

Think also about what kind of stuff is going to be in there, to fit the purpose. You aren’t going to have a library without books or an equivalent thereof, or a hospital without medicines, now, are you? Even living quarters are going to tend to have a few things in common. What about decorations? Sure, they’re technically optional, but have you ever seen the posters in a classroom or on a doctor’s wall? They might even hint at another purpose; one of the best restaurants near my workplace doubles as an art gallery for its owner’s work, so its walls are covered with all sorts of interesting paintings.

Don’t forget the inherently magical aspects of it. Sure, you can make a mage’s workroom look similar to a standard one, but where’s the fun in that? There are bound to be at least a few tools that don’t match up, or particularly odd book titles—you might even have places where the flow of energies creates effects mostly impossible in its real-world counterparts for improvement of one’s work, like a section in which the directions are reversed or one in which sounds are visible.

Starting with purpose makes for a clear template on which to build other aspects of the location. Do you have something in mind?