Hey saracenas... looks like you're in Lithuania. Is the very cold weather impacting you there now?
By request, here's the three owlets in a row, plus a couple more pics. I have no pic of the nestbox, but look at the pic that shows the corner eave. The nestbox is mounted under the corner eave of another building, just like the eave you see in the pic. The one shown is two stories up, but where the nestbox is mounted, it's about 3 stories (probably 35 feet off the ground). As I mentioned before, there is a healthy population of rabbits (with babies) in the courtyard, which I imagine is a big plus for the owls.
I've used birdhouse plans from this site before:
Great Horned Owl (See links about midway down the page). The size of the box and hole are important. The nest here for the great horned owls is about 2 feet on a side.
[P.S. If you want to see another cool bird, look for my post from yesterday with the red-tailed hawk in a tree over our driveway.]