Carpenter Bees make holes around the size of the tip of your pinky finger. The female drills a round hole through the side, then drills a perpendicular hole along the grain of the wood. She will lay her eggs inside with an insect to serve as a food source, then she will close off the cell. She will lay multiple eggs per chamber and expand the chamber over the season. This site under a bay window is typical. It is protected and unfinished/unpainted wood. Paint (not stain) is good way to minimize the attractiveness of an area.