SaaS vendors come to us to put their product inside Microsoft 365. We design and build Outlook, Word, Excel and PowerPoint Add-Ins that surface your software directly inside the Office applications your customers already work in — on Windows, Mac, the web and mobile — so they can use your product without ever leaving Office.

McKenna Consultants have spent more than a decade building Microsoft document integrations for SaaS platforms in legal, auditing, contract management, document management and compliance. Office Add-In development is one of our core specialisms: we take Add-Ins from initial concept through architecture, build and testing to publication on Microsoft AppSource, and we work either as a complete delivery team or alongside your own engineers.

If you are weighing up an Add-In for your product, talk to our Add-In specialists — we are happy to discuss what is feasible before you commit to anything.

Why Put Your Product Inside Microsoft 365?

Your customers spend their working day in Outlook, Word and Excel. An Add-In puts your product in front of them there — in the task pane, alongside the document or email they are already working on — rather than asking them to switch to another browser tab and copy information between the two. For the user it feels like a native part of Office; for you it deepens the place your product holds in their daily workflow.

One product, every platform. A single Add-In runs across the Office desktop clients for Windows and Mac, Office for the web, and (for Outlook) iOS and Android. You do not build or maintain separate integrations per platform.
Straightforward distribution. Add-Ins are distributed through Microsoft AppSource, where users can install them individually — or a Microsoft 365 administrator can deploy your Add-In centrally to some or all users in their organisation, which is how enterprise customers typically roll it out.
Built on your existing APIs. An Add-In is a front end onto the product you already have. We build against your existing APIs rather than duplicating business logic, so the Add-In stays in step with your core platform.

Add-Ins We Have Built

We have built Add-Ins which perform functions such as:

Managing catalogues of terms and clauses and inserting those terms and clauses into Word documents
Linking Excel and Word documents for the purposes of completing an audit
Managing "know your obligations" risk by implementing a standard library of legal templates for Word
Capturing email conversations for an enterprise content management system
Capturing emails for a specialist CRM system

Outlook Add-In Development

Outlook Add-Ins are the most common Add-In we are asked to build, and usually the most involved. Alongside the task pane front end, they typically need a back end that integrates with the Microsoft Graph API to work with the user's mail, calendar and contacts — synchronising email conversations into a CRM, filing correspondence into a document management system, or creating and organising meetings. That in turn means implementing single sign-on between your SaaS application and Microsoft Entra ID, so users authenticate once with their Microsoft 365 identity and your product recognises them.

We have built this pattern — task pane, Graph API back end, single sign-on — repeatedly for production SaaS products, and we know where the platform's sharp edges are.

Word And Excel Add-In Development

Word and Excel Add-Ins put your product next to the document itself. The Word Add-Ins we have delivered centre on document assembly and control — inserting managed clauses and terms from a central catalogue, applying standard template libraries, and keeping generated documents consistent with the source system. In Excel, the pattern is typically data-driven: reading from and writing to workbooks, and linking spreadsheet content to documents and records held elsewhere in your platform. If your customers produce documents or work with structured data as part of using your product, an Add-In shortens the distance between your software and the file they are working on.

How We Work With You

Full delivery. We design, build, test and ship the Add-In end to end, including AppSource submission and the supporting back-end services, while your team focuses on your core product.
Co-development. We work hand-in-hand with your existing development team, building the Add-In against your existing APIs and transferring the Office-specific knowledge as we go, so your engineers can own it confidently after the engagement ends.
Consultancy. If your own team is building the Add-In, we can validate the architecture, review code and stay on call for the parts of the platform that are easy to get wrong — authentication, manifest configuration and cross-platform behaviour chief among them.

In every model, all of the code and intellectual property is yours. Our team uses AI coding tools throughout our development workflow, so Add-Ins are delivered faster without compromising on quality.

Not sure which model fits? Tell us about your product and we will recommend one — most engagements start with a short scoping conversation.

Platform Support

A single Add-In codebase runs across the following platforms:

Outlook Add-Ins	Outlook for Windows (classic and new), Outlook for Mac, Outlook for iOS, Outlook for Android, Outlook on the web (Chrome, Safari, Edge, Firefox)
Word Add-Ins	Word for Windows, Word for Mac, Word for the web (Chrome, Safari, Edge, Firefox)
Excel Add-Ins	Excel for Windows, Excel for Mac, Excel for the web (Chrome, Safari, Edge, Firefox)
PowerPoint Add-Ins	PowerPoint for Windows, PowerPoint for Mac, PowerPoint for the web (Chrome, Safari, Edge, Firefox)

Cross-platform reach is the headline benefit of the Add-In model, but each platform has its own quirks in practice. We test across the platforms your customers actually use and design the Add-In so that behaviour is consistent everywhere it runs.

The Technology

Under the hood, an Office Add-In is a web application: HTML, CSS and JavaScript, plus a manifest file that tells Office where it runs and what it can do. The Add-In calls Office's JavaScript API (Office.js) to read and write the open document or email, and the Microsoft Graph API for everything beyond it — mail, calendar, contacts and files. The front end is lightweight enough to host on a CDN or a simple web server, and it can be built with whatever web stack your team already uses.

Most of the user interface lives in the task pane — the panel that opens inside the Office applications — so responsive design for a tall, narrow layout is essential. Authentication uses the Microsoft identity platform, with single sign-on against Microsoft Entra ID where the Add-In needs to know who the user is. Because the Add-In talks to your own APIs, your data flows through your own services under your existing security model.

For a longer, non-technical explainer, see our guide to what Microsoft Office Add-Ins are.