A round building is commonly referred to as a "circular building" or "cylindrical structure." In architectural terms, it may also be called a "rotunda," especially when it has a dome-like roof. Examples include the Pantheon in Rome and various modern designs that utilize circular forms.
Rotunda
A circular building is called a round building or a cylindrical building.
round room
Capitol records.
A building with a round roof, like the Capitol building in Washington DC or Saint Paul's cathedral in London.
Rondavel
Silo
A circular room in commercial buildings or out buildings is called a round house, in a private house it can be called among other things a turret. A rotunda is a round building or a room in a building, the central hall in the Capitol Building in Washington DC is an excellent example of a rotunda ( round room).
round pizza
Hearthome City, the round building.
The scientific name for round worms is "nematodes."
A roof that overhangs its supporting wall could be called Eaves or a Gable, depending on which aspect of the building it is on. On the ends of a building it is called a Gable, whereas on the sides it is called the Eaves. "Eaves" also applies to buildings with no perceivable ends or sides; a round building for instance.