A building's extension is a wing. As in, "The hospital is adding a wing". Or, "They named it 'The Gonzo Wing' because Gonzo funded it".
Check with your local government for building permits...it may differ from place to place.
Skeg
An annex is an extension of or an addition to a building. A small room off of a main room is an annex, and attics are another type of annex.
A circular building is called a round building or a cylindrical building.
You need a building permit before building any kind of extension to your house. You also may need foundations for the building, but this may not be a requirement.
Cape
The Disney brand has the brand extension called the Line Extension Strategy.
side of a building called
it will help in building national unity among the people and there will be no kind of prejudice for caste, creed,religion etc
The increase in length of a spring when a force is applied is called "extension." This phenomenon occurs due to Hooke's Law, which states that the extension of a spring is directly proportional to the force applied to it.
If you mean the type of file it is called the file extension.