Birdhouse or wooden house model inside aquarium???

As long as it is JUST wood, with no paint or primers on the wood, I don't see why it should be a problem...

Or you can look for the coating stuff they use to make decorations aquarium safe... Not sure where you'd find it though.

This is really not a good idea at all. A wooden birdhouse or anything like that is going to rot and make a mess, none of the hobby woods are meant to be submerged. Even driftwood that is not meant for aquariums generally has a long curing period where fuzz and mold grow, unless it is dry and cured to begin with. Slow rot is common in many types of wood.

A resin decoration would be your best bet. Do keep in mind any kind of rasping fish like a pleco or Chinese algae eater, should not be included in a tank with plastic painted decorations, as they will deteriorate it quickly as soon as it starts to grow algae and biofilm.

You'll probably have the same problem with this as you'd have on un-cured driftwood. Tanins leaching from the wood for a long time. Not necessarily bad for the fish, but the water may change color. If you want a birdhouse in the tank, I'd look for one that is made of resin and preferrably one made for aquarium use. You never know what might be in a "real" birdhouse.

Ahh, I had forgotten about the different wood used in hobby stores...

People use all kinds of toys inside aquariums, from plastic farm animals to army men. Plastic is generally safe, although the paint is more tricky to determine (although if it is safe for children to put in their mouth, it's safe for aquariums for the most part). Bricks and other stones can be used as well. However wood is usually unsuitable - wood rots quickly and only very few kinds of wood are suitable for aquariums. Most of the soft woods will not last long at all in aquariums, and as they rot their pitch will feed a fungus that is bound to envelop them. So stay away from any kind of wooden decorations, go with ceramic or plastic...there is a ton to choose from.