I'm 99.5% sure the synopsis said something like "In a city overrun by deadites, a family/group/whatever has to figure out how to survive..."
Given the assumed budget and history of the series, being contained in a single dwelling makes sense...but sure they can do something like a person makes their way to their house (cities do have residential areas, not all high rises) and they think they're saved, but he/she/they says, "You don't understand, the whole city has been overrun by demons!" or they're watching local TV or listening to the local radio channel and "Steve", the co-anchor is talking about reports of demons attacking people and "Sue", the other other co-anchor, turns into a deadite and kills him. So its established that the city is ruled by the deadites, but this group (whatever it may be) is trying to survive.
Like the Dawn of the Dead remake saying the country is completely overrun by Zombies...but the story be 97% contained to a mall, both can be true.