They don't need trees, bushes, or shrubs to climb up the house, they can climb up downspouts (seen and heard it myself!) and even climb straight up the side of the house.
Recent construction homes (last 10 years or so) are amazingly "open" to varmints and critters. Frequently there is a gap between the roof decking and the facia board around the entire perimeter of the house where the shingles drape over to keep water out, but not critters...
Like Vash and T-bird said, you have to seal up the house, and stay proactive in keeping them out. And yes, it can cost thousands.
