Yes, extension is a Scrabble word!

Noun

1. A mutually agreed delay in the date set for the completion of a job or payment of a debt

2. Act of expanding in scope; making more widely available

3. The spreading of something (a belief or practice) into new regions

4. An educational opportunity provided by colleges and universities to people who are not enrolled as regular students

5. Act of stretching or straightening out a flexed limb

6. A string of characters beginning with a period and followed by one or more letters; the optional second part of a PC computer filename

7. The most direct or specific meaning of a word or expression; the class of objects that an expression refers to

8. The ability to raise the working leg high in the air

9. Amount or degree or range to which something extends

10. An additional telephone set that is connected to the same telephone line

11. An addition to the length of something

12. An addition that extends a main building