Let\'s take Excel for example. In Excel 2016, when I select an add-in in the STORE, after clicking on Trust It, will the code (.html,
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