Natural nest holes do not come in standard sizes, so use these dimensions only as a guide. Any weatherproof timber of about 15 mm thickness is suitable.
The plan gives measurements for a small and a large box. Use only the first or the second figure throughout. For Starlings and Great Spotted Woodpeckers, use the dimensions for the large box; all the others need the small one.
The bottom of the entrance hole must be at least 125 mm from the floor of the nest-box. If it's less, young birds might fall out or be scooped out by a cat. The inside wall below the entrance hole should be rough to help the young birds to clamber up when it's time for them to leave.
Drill drainage holes in the base of the box, and use galvanised nails or screws to assemble it. It's always best to leave the box untreated with chemical wood preservatives. As it weathers, it will blend into its surroundings.
Do not nail the lid down, since you will need to clean out the box in the autumn. Attach the lid with a brass or plastic hinge that will not rust, or hinge it with a strip of leather or rubber (an old piece of bicycle inner tube will do). Fasten it down with a good catch.
The entrance hole size depends on the species you hope to attract:
25 mm for blue, coal and marsh tits
28 mm for great tits, tree sparrows and pied flycatchers
32 mm for house sparrows and nuthatches
45 mm for starlings
The small box with 100 mm high open front may attract robins or pied wagtails. A wren would need a 140 mm high front panel, while spotted flycatchers prefer a low 60 mm front to the box.