When building a world, based in our own, but with supernatural or sci-fi elements, you have to begin with the world we know. A completely alternate world does not need this calculation; a historical world within our own could begin with the normal historical world.
The “normal world” is quite simple—the world we know. Communication, transportation, language, social customs, business and economy, food, government, etc. These differ based on location, culture, class, and so on, but depending on where you would like to set your story, figure out the basics of the world around your main character.
Ultimately, the previous posts work to really differentiate between the minutiae of worlds. This quotient is a single threshold by which contemporary fantasy/sci-fi stories can be divided—either they are a “normal world with a fantasy element” or they are a “fantasy world.”
Where does that threshold lie?
First, you have unique elements, wherein a single person or object or place is different from the normal reality we know. Superheroes are great examples of this, even as they team up and become multiple elements—on their own, they are unique, and as a group, there are still few enough of them to not alter the world around them too much. A haunted house, a single immortal, a time machine—they do not alter the world because there is not enough of them to form a culture. The exception may be a single element with enough power to alter the world, but I can’t think of one off the top of my head.
Then, you have group non-culture elements, where there is more than one unusual person or object or place, but not enough (or not different enough) to form a culture of their own. This is the case with vampires, werewolves, mutants, monsters, etc.—they may have certain ways of life which differ from ours, but not enough to constitute an entire culture. Basically, if they still speak like us, dress like us, live in houses among us, eat like us, and form relationships like we do, they do not have a culture of their own. They may have one or a couple abilities which can alter normal life, but which is either unique to them (such as abilities in X-Men) or does not completely replace normal life. For example, in the case of vampires living among us, they may have to only come out at night—but they still live in houses we build, wear clothes we make, and there aren’t enough of them to have an entirely different way of speaking or relating to each other.
The key point with this second group is that they live in the normal world—not in their own world—but they may live a little differently. To determine this factor, ask yourself about random members of the group: how do they communicate across distances? How do they get around? Where do they live? How do they dress? What do they eat and how do they get it? How do they form families? How do they speak?
A few factors may be different (for example, werewolves might mate instead of marry), but if enough are the same (cell phones, cars, etc.), then you’re dealing with supernatural elements in the normal world.
Now we cross the threshold. Everything from here on is a fantasy world within our own, with enough differences to constitute a different world.
The lowest threshold is group culture elements, which is similar to the category above but with a substantially different culture. This is a group or species which lives among us, but has their own society—language, religion, social customs, fashion, etc. If they still have to live literally among us, they may disguise themselves as normal for part of their time, but they need to disguise to achieve this—understand? That’s the key difference. For the other group, the majority of their behaviors are normal with a small amount being unusual; in this group, the majority is unusual but they may purposefully pretend to be normal depending on the circumstances.
For example, a society of vampires could belong in either type of group. The former would be scattered and mostly individual (or living in small groups), where they would lead as normal a life as possible—since they have no alternative. They would just need to compensate for their few quirks: immortality, blood-drinking, sunlight allergy, etc. The latter type would have a complex society of their own, where they would have customs for clothing, language, and other behavior that differs from the normal world in substantial ways. If they do live among humans for a time, it would be like living in a foreign country and adopting their customs in place of your own.
Finally, you have geography elements, which involves a physically separate world combined with a complete culture as above. If it’s a single location, it may not be enough to substitute a separate culture, and thus could either be a unique element or a small part of the non-culture group. But if it’s a city, a series of separate locations, or an entire world, then it obviously is different from the normal world. The world of Harry Potter has enough geographically separate locations, as well as a culture of its own (dress, food, currency, communication, transportation, etc.), that it fits into this category.
If it isn’t already obvious, this is once again full of nuances. How many alternate factors does it take to form a separate culture? There’s no right or wrong answer; for me, it’s more of a feel. When I try to just sit back and visualize a character’s daily life in this world, how much time do I spend inventing various factors? How often do they just do the same things I would do? How much of their surroundings, the technology and tools they use, the places they go—how much is basically normal?
My mind is vastly interested in complex fantasy worlds, but more and more, I’m realizing I may not be able to actually write in them. This threshold may help me in coming up with future worlds that aren’t so complex or so different, and thus it’s helpful to me. Maybe to someone else, too.