Meaning of “extension” - Learner’s Dictionary

extension

noun [ C ] uk us /ɪkˈstenʃən/
PART OF A BUILDING

a new room or rooms that are added to a building:

You could build an extension onto the back of the house.
EXTRA TIME

B2 extra time that you are given to do or use something:

You might be able to get an extension on your visa.
TELEPHONE

B2 a telephone that is connected to the main telephone in an office or other large building:

Call me on extension 213.

(Definition of “extension” from the Cambridge Learner’s Dictionary © Cambridge University Press)