Office add-ins development overview

The modern Office add-in model lets developers build extensions for Word, Excel, PowerPoint, Outlook, and OneNote using web technologies — HTML, JavaScript, CSS — running inside the Office apps across Windows, Mac, web, iOS, and Android. It replaces the legacy COM-based add-ins (.NET, VBA, VSTO) which only worked on Windows desktop.

The model

An Office add-in is fundamentally a web app that runs in a sandboxed iframe inside the Office app:

• HTML / CSS / JS code, hosted on any web server.
• A manifest file (XML, or newer JSON for the unified manifest) that describes the add-in — what app it extends, what features it adds, what permissions it needs.
• The Office.js JavaScript library for accessing the host app's API — reading and writing document content, listening to events, manipulating UI.

When a user installs the add-in, the manifest is registered; the add-in surfaces as a button or pane in the Office app; clicking loads the web app inside the iframe.

What add-ins can do

Depending on the host app:

Word add-ins

• Insert formatted content into the document.
• Read existing document content and structure.
• Bind to specific content controls.
• React to selection changes.
• Custom task panes alongside the document.

Excel add-ins

• Read and write cell ranges, tables, charts.
• Add custom functions (similar to user-defined functions but in JavaScript).
• Bind to specific data ranges for streaming updates.
• Custom task panes for analysis tools.

PowerPoint add-ins

• Insert content (shapes, images, slides).
• Read slide content.
• Bind to specific slides or shapes.

Outlook add-ins

• React to email selection (read pane add-ins).
• React to compose events (compose pane add-ins).
• Add custom commands in the ribbon.
• Pull data from external systems and present in the read pane.

Distribution

Add-ins can be distributed via:

• The Office Store / AppSource — public marketplace.
• Microsoft 365 admin center — sideload corporate-built add-ins to your tenant's users (similar to Teams app deployment).
• SharePoint app catalog — older but still supported for some scenarios.
• Microsoft Teams app deployment — the unified-manifest model lets add-ins ship as Teams apps that also work in Office, OS, and the web.

When add-ins are the right tool

• Embed a line-of-business tool inside Office workflows — surface a CRM, a ticketing system, a translation tool, a citation manager.
• Provide custom Excel functions for specialised industry calculations.
• Integrate a SaaS app into Outlook to enable sending or acting on data without leaving email.

When something else is better

• Server-side processing of Office files — use Graph instead of add-ins.
• Document automation at scale — use Power Automate or Graph.
• Bulk-create Office documents — use server-side approaches (OpenXML libraries) rather than add-ins.

Tooling

• Yeoman generator for Office — scaffolds a new add-in project.
• Office Toolkit in Visual Studio Code — debugging and development helpers.
• Office.js library — the JavaScript API.
• Microsoft Teams Toolkit — for the unified app model spanning Teams and Office.

What's coming

The strategic direction is the unified app model: one manifest, one codebase, deployed once, runs in Teams, Outlook, Word, Excel, PowerPoint, and Microsoft 365 Copilot. Microsoft has been progressively converging the previously separate Teams and Office app extensibility models. New add-in projects in 2026 should target this model.

For developers building line-of-business extensions to Office, add-ins are the modern path. They replace the brittle COM model with a cross-platform web-based approach that's far easier to operate.