[fanf2]

It isn’t impossible, you just install a lintel at the top of the opening. There are plenty of 3D printed houses out there.

[fnordpiglet]

That’s not entirely 3d printed then. You need to manually insert the lintel once the supporting layers are in place.

The other way is to print supports and knock them out post construction. But the issue of overhangs is a serious one for 3d printed structures.