Adding a coat of polyurethane or exterior latex on the outside only, will add years to the life of the house. Avoid metal houses such as aluminum because they can become very hot in the sun making for an uncomfortable, if not deadly, interior. Don't use wood treated with stains or preservatives as the fumes could harm the birds. Starlings, Wrens and House Sparrows will have a convenient place to perch. Starlings and House Sparrows are very aggressive and will bully and kill cavity-nesting birds.
Wrens interfere by puncturing the eggs of other birds.
If you make your house from wood you can carve out a couple of groves below the entrance for a place to land. To find out more about Starlings and House Sparrows click here . Drainage is important because rain will inevitably enter the house. You can also slant the roof to help shed water. Accessibility is important because you will need to monitor your house for unwanted visitors, cleaning and to see how your house guests are faring.