These are prefab houses. They are built in factory and then pieced together on site. What the poster was asking for was free/cheap plans to build a house, without support.
It seems it's only available for Hungarian citizens, as it needs Hungarian Electric Id to login. I'm not sure it qualifies for open source but it's free as in beer to Hungarian citizens with the right to adapt the plan to your demands.
Kind of like open source design of a house.