Proper sanitation and property maintenance will prevent most problems.
Most urban squirrels owe their existence to humans.
We supply both food, and shelter without thinking. A house is just another funny looking tree to the squirrel. Why should the squirrel spend days cutting and hauling twigs up a tree to build a nest, when there is a perfectly good attic to sleep in. Its warm, out of the weather, and they don't have to worry about predators coming in. Just add a few leaves, a bit of fluff, and call it home.