Can you create a setting in an existant town in your story?

Okay, I kinda feel stupid asking this question, which I didn't really know how to ask, but it's been pestering me ever since I started with my story. What I actually want to know is: my story is in a town where I live, but the setting isn't quite right because I'm writing a fantasy. Now I don't want to change the town because I know it well. Now's the question that makes me feel stupid, can I, like, add stuff to the town that isn't there? Like a building of some kind that will work for my story? At the moment I need a secret place for my protagonist and her friends to hide, and it's got to be a big place. And the town the story's based on is small and plain. Can I create such a place for my characters when there isn't such a place in reality? Sorry, this is a very confusing question, but I can't write for a reason when everytime I need a setting for my story it doesn't exist. And if any of you have a tip on what I should use as my protagonist's lair, it would be appeciated. Thanks a million if you give me answers :D