Building a birdhouse is one of those projects that you can approach with endless creativity. There are almost no rules. Make the entrance good for the birds you want to host, put the house 2 - 4 m above the ground and keep away from cats. See a table of appropriate hole sizes here.
Once you take care for these things, there are literally thousands of different designs you can use.
This is a house for several birds made of wood with wooden tiles:
Birdhouses can also be vertical and hanged on a tree branch. Just don't forget the cats.
Some birdhouse builders make whole residential hotels of different houses.
Nothing stops you to make the houses really beautiful and colorful. This guy here made them look like people's homes.
And not all birdhouses have to be made of wood. This pottery one not only works, but looks pretty funny.
Why not build a whole fence of birdhouses like this guy did here:
Here is another bird hotel. This one seems to have sheet iron roofs:
Have old tree in the backyard? Don't remove it or prune it too harsh. It can do a great work for this:
Here's yet another whole suburb built on a pylon:
You can also follow entirely non-standard design for your birdhouses, like this:
Even if you don't have any scrap wood boards lying around, you can still build a birdhouse. Just use branches:
You can also make bird feeders and open-plan homes for birds:
Ever dreamt to start bed and breakfast business? Start with one for birds:
No joke, people really take birdhouses seriously in South Korea:
And for a final, enjoy this one: