Ben Aaronovitch was a TV screenwriter who'd worked on Doctor Who in the late 1980s, and by the time he started writing the Rivers of London series in 2011, he'd been living in London for decades. He knew the city the way a cab driver knows it, which is to say: not the landmarks, but the shortcuts. The alley behind the kebab shop. The particular acoustic quality of the Tube at Bank station during rush hour. The way the Thames smells different at Teddington Lock than it does at Greenwich.
When he put magic into London, it didn't arrive from the outside. It grew from the geography. The rivers became goddesses. Not metaphorically. The goddess of the Thames is an aging Nigerian woman named Mama Thames who controls everything downstream of Teddington Lock. Her daughters are the tributary rivers: the Fleet, the Tyburn, the Effra. Each one has a personality shaped by her river's history and neighborhood.
The reason this works, and the reason it's become one of the most imitated approaches to urban fantasy worldbuilding, is that Aaronovitch didn't paste supernatural elements onto London. He looked at what was already strange about the city and gave those things names.
Urban Fantasy Worldbuilding Tips: Start With What's Already Weird
Every city has layers that most fiction ignores. The old tunnels under downtown that nobody uses anymore. The building that's been empty for fifteen years and nobody can explain why. The intersection where accidents happen at a statistically improbable rate. These are the seams where your worldbuilding should start, because real places come pre-loaded with mystery if you pay attention.
Jim Butcher set his Dresden Files in Chicago, and he uses the city well, but I've always thought the most interesting detail is one of the smallest: Harry Dresden's apartment is in a basement, and electronics don't work around him because his magic interferes with technology. This means he lives in a world that looks like the 1940s from the inside. Candles. An icebox instead of a refrigerator. A rotary phone on good days. It's a worldbuilding detail that does triple duty: it establishes a rule of the magic system, it creates a visual aesthetic, and it isolates the protagonist in exactly the way the plot needs him isolated.
That's the density you're aiming for. One detail that does three jobs.
The Map Problem in Urban Fantasy
Here's where a lot of worldbuilding goes wrong. Writers pick a real city, mention a few street names, and assume the work is done. But naming the real Wrigley Field and putting a vampire underneath it doesn't make your Chicago feel like Chicago. It makes it feel like you read a Wikipedia article about Chicago.
Patricia Briggs avoids this entirely with her Mercy Thompson books. She set them in the Tri-Cities area of Washington state: Richland, Kennewick, Pasco. It's a place defined by the Columbia River, by the Hanford nuclear site, by the specific emptiness of the eastern Washington desert. Mercy is a VW mechanic who turns into a coyote. She lives in a trailer. The supernatural community is small because the human community is small, and everyone knows everyone's business in that way that only happens in cities under 100,000 people.
The magic grows from the land. The fae are connected to the river. The tensions between supernatural factions mirror real tensions between the three cities themselves, which have always had a complicated relationship despite being lumped together. Briggs didn't need London or New York. She needed a place she understood at the level of weather patterns and commute times and where the good Mexican food is.
Worldbuilding Tips for Urban Fantasy: Infrastructure Over Aesthetics
I'm not sure enough writers think about the logistics of hiding magic in a modern city, and I think that's where many urban fantasy manuscripts feel thin. Think about what a hidden magical community actually needs. Where do they get medical care when a spell goes wrong? Do they have lawyers? When a werewolf damages property during a full moon, who pays for it? Is there insurance?
Kim Harrison's Hollows series is set in an alternate Cincinnati where a genetic plague wiped out a huge portion of the human population and supernatural beings came out of hiding because they were suddenly the majority. The worldbuilding is dense with infrastructure. There's a government agency, the I.S., that polices supernatural crime. Witches have their own supply chains for spell ingredients. There are businesses that cater specifically to vampire needs. It reads as real because Harrison thought through the second and third-order consequences of her premise.
When you're building your city, ask the boring questions. The boring questions are where the texture lives. Where does a troll park? If there's a magical market, who collects the rent? How does a shapeshifter get a driver's license renewed? You won't use all of these answers. But knowing them makes every scene feel grounded in a way readers can sense even if they can't articulate why.