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“Microsoft 365 add-ins” usually means Microsoft’s web-based Office Add-ins. “Office extensions” is a looser umbrella term: it may mean those web add-ins, or older Windows technologies such as COM and VSTO add-ins. They are not two formally parallel Microsoft products, so the important comparison is between web-based Office Add-ins and COM/VSTO add-ins.
What is a Microsoft 365 or Office Add-in?
Microsoft’s documentation calls the web-based model Office Add-ins and sometimes Office web add-ins. An add-in has a manifest or app package that describes how it integrates with Office, plus a web application that supplies its interface and behavior. It can use Office.js APIs to work with Office documents and services. Microsoft describes the distinction this way: “Unlike COM and VSTO add-ins, Office Add-ins are web add-ins: the application (for example, Excel), reads the add-in manifest and connects the add-in’s custom ribbon buttons and menu commands in the UI.” (Microsoft Learn: Office Add-ins platform overview)
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The app package includes the manifest and icons, while the application code and data are hosted separately and accessed over HTTPS. Administrators can deploy Office Add-ins centrally, and developers can submit them to Microsoft Marketplace. See Microsoft’s platform overview for the model and beginner learning path for an introduction.
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How do Office Add-ins differ from COM and VSTO add-ins?
COM and VSTO add-ins are older desktop-oriented approaches, not another name for the web-based Office Add-in model. In a VSTO add-in, Office loads a managed-code assembly. The add-in communicates with Office COM components through the application’s primary interop assembly; installation and discovery use Windows mechanisms such as registry entries. Microsoft explains this design in its VSTO architecture documentation.
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| Aspect | Office Add-ins (web add-ins) | COM/VSTO add-ins |
|---|---|---|
| Architecture | Manifest or app package plus a separately hosted web application; Office.js provides APIs for Office content and services. | COM add-ins use the Windows Office extensibility model; VSTO add-ins use managed-code assemblies and Office COM components. |
| Typical implementation | Web technologies and Office.js. | For VSTO, managed code; specific languages for COM add-ins are not stated in the cited overview. |
| Platform reach | Microsoft describes support in Office on the web, Windows, Mac, and iPad, subject to host and API availability. | Windows-only in Microsoft’s general comparison. |
| Deployment | Can be deployed centrally by administrators or submitted to Microsoft Marketplace. The web application and data are hosted separately and reached over HTTPS. | VSTO installation and discovery rely on Windows mechanisms such as registry entries and a managed assembly. |
| Network and hosting | Requires separately hosted application code; Outlook add-ins require network access. | Not stated in the cited overview as a general requirement; requirements depend on the add-in. |
| Outlook client support | Outlook web add-ins load from a manifest and web code in a sandbox; compatibility depends on the client and add-in requirements. | Not supported in new Outlook for Windows; supported in classic Outlook for Windows. |
The platform and deployment descriptions are from Microsoft’s Office Add-ins overview and VSTO architecture guide. Outlook details are in Microsoft’s Outlook add-ins overview.
Do Office Add-ins work on Mac and the web?
Microsoft lists Office on the web, Windows, Mac, and iPad as platforms for Office Add-ins. That does not guarantee that every add-in feature is available in every Office app or client. The host, client version, and APIs an add-in needs can affect compatibility. Check Microsoft’s compatibility guidance for the specific host and APIs before choosing or deploying an add-in; the general platform list is not a feature-by-feature support matrix.
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Do COM add-ins work in the new Outlook?
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Which type should you choose?
Choose an Office Add-in when
- You need an approach that can reach Office on multiple platforms, subject to the specific host and API support.
- You want to build the interface and behavior with web technologies and Office.js.
- Central administrator deployment or Marketplace distribution fits your rollout.
Consider VSTO when
- Your solution is Windows-only and depends on existing managed code.
- Its design relies on close integration with the Office object model.
- The exact Office client you target supports the add-in, and the cost of keeping or migrating the solution is acceptable.
These are starting points, not blanket recommendations. Confirm required APIs, target client versions, offline needs, deployment constraints, and migration effort for the particular add-in. In particular, do not assume a web add-in is offline-capable or that its features behave identically across all Office platforms. Microsoft’s VSTO-to-Office-web-add-in transition learning path introduces the migration context.
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