The Missouri Department of Conservation provides free squirrel house plans and construction directions. A squirrel house builder needs scrap lumber or plywood, a saw, nails and a hinge.
A squirrel house that measures 6 inches by 6 inches by 20 inches requires four pieces of wood that are 6 inches by 20 inches and one piece that is 6 inches by 6 inches. The top of the house needs to be weatherproof and must overlap the rest of the house. A hole 3 inches in diameter should be cut 2 inches down from the top and off to one side of one of the long pieces of wood as an entrance. The bottom piece of wood needs to have a small drain hole. Adding a hinge to the bottom piece allows the house to be cleaned, which should be done every three years to prolong the life of the house.
Once the house is complete, the builder should place sawdust or leaves inside the finished box. The house should be placed in a tree 10 to 30 feet from the ground and attached using aluminum nails. The house is more likely to be used by squirrels if it is placed on the south side of a tree.