As of 2015, some free birdhouse designs include the basic birdhouse design that Birdwatching Bliss offers and the Carolina Wren nest box plan from 50birds. Both of these designs are available online.
The basic birdhouse design includes the number of pieces needed to cut and the layout to cut them out of a single board. This design includes all the measurements of each piece and how to put them together. This design also includes a video of a birdhouse build project.
The Carolina Wren nest box plan includes step-by-step instructions for putting the birdhouse together with illustrations of the steps. It includes the dimensions of each piece of wood needed to put it together.