Do Office Add-ins always require Internet access to work?

As Eric mentions, Office add-ins are indeed web based. However, to add a bit to his answer:

While you definitely need a one-time access to fetch the manifest and the original HTML/JS/CSS files, if your add-in is not using license checking (it's free) and does not require web services, you should be able to make a website that uses standard offlining techniques to load with no internet connection after the first time.

I tried it out real quick with pointing a manifest at http://html5demos.com/offlineapp. After loading it once and then disconnecting my internet, I was still able to load that page.

Hope this helps,

~ Michael

Yes, the new Office add-ins are web-based. They are all defined - and installed - by a manifest file which specifies the URL location of the web source files. These are always hosted on the provider's web server and are not cached for offline use. The licensing system used by most add-ins (the ones that aren't free) requires a connection to the provider's web server which in turns needs to verify the user's license against the Office Verification Licensing Service. If offline use was enabled users could use a trial license in perpetuity without the provider being aware.