Garter snakes are common in residential areas because our homes and garages offer warm shelter for these cold-blooded creatures. Personally, I get excited when I see garter snakes sunning themselves on my driveway or in the garden because I know they eat the insects and slugs that feed on my plants. But like any outdoor critter, I sure don't like finding them inside my home. It sounds like you've sealed up your home's foundation and siding to close off their points of entry. Next, look around the yard and remove any clutter that might offer them shelter. Piles of branches or firewood are popular hideouts. Avoid installing a pond or water feature in your garden; it won't attract garter snakes, but it will enhance the habitat. You don't want to make them any more comfortable. Consider installing a screech owl house to invite a predator into the neighborhood. I used to live in the Beaverdale area in Des Moines and successfully attracted a screech owl to a wooden home installed high in an oak tree.