Custom functions
Your own worksheet functions (=YOURCO.VALUE(...)), including async and streaming functions that fetch live data into cells.
Excel add-in development creates custom functions, task panes, worksheet automation, and API-connected data workflows using Office.js. It is intended for organizations that need calculations, reporting, imports, or two-way data synchronization inside supported Excel clients, with availability determined by platform and requirement set.
What I build
What is included in Excel add-in development?
Task panes & ribbon tools
UI to import data, configure settings, and push updates back — beyond what native Excel offers.
Ranges, bindings & events
Reading and writing ranges, persistent bindings, and reacting to onChanged / onSelectionChanged worksheet events.
Live data sync via API
Pulling from your database or API into a workbook and pushing edited rows back — two-way sync, not a static export.
Problems this solves
What problems can Excel add-in development solve?
- Analysts pasting the same report data into Excel from another system every morning.
- Workbooks that go stale the moment they're saved because nothing refreshes them.
- A bespoke calculation copied between dozens of files with no single source of truth.
- Needing a function Excel simply doesn't have built in.
Example use cases
What does Excel add-in development look like in practice?
Live CRM figures
A streaming custom function =CRM.DEALVALUE("id") that pulls live deal values into a sales workbook.
Templated import
A task pane that fetches yesterday's sales from an API and drops them into a formatted reporting template.
Two-way sync
An add-in that validates edited rows and writes them back to a Postgres database, keeping Excel and the system in sync.
Tech stack
What technologies power Excel add-in development?
Core
- Office.js (Excel API)
- Custom Functions runtime
- TypeScript
Data model
- Ranges
- Bindings
- Worksheet events
Integration
- REST API
- Your database
Delivery
- Manifest
- AppSource
- Sideloading
Process
How does Excel add-in development work?
01
Define the workflow
What data comes in, what functions are needed, what goes back.
02
Build with Office.js
Custom functions, ranges, and events in a typed add-in.
03
Wire the data
Connect your API/database for live, two-way sync.
04
Ship
AppSource, sideload, or central deployment.
Excel is part of my broader Microsoft 365 development work. For Word and generated documents, see document automation software.
Related services
Microsoft 365 Developer
Microsoft 365 development creates add-ins and integrations for supported Outlook, Word, Excel, and PowerPoint clients using Office.js, Microsoft Graph, and Entra ID. It is intended for organizations that need to place business workflows inside Microsoft 365 or connect Microsoft data to another system, with compatibility determined by each host and requirement set.
Office Plugin Development
Office add-in development creates web-based task panes, ribbon commands, and document or workbook tools for supported Word, Excel, Outlook, and PowerPoint clients. It is intended for organizations replacing manual Office workflows or assessing whether a legacy VSTO or COM plugin can move to Office.js, subject to the APIs available in each host.
Document Automation Software
Document automation uses templates, structured data, add-ins, and workflow integrations to generate or process documents with less manual assembly. It is intended for teams that create recurring contracts, proposals, reports, or approval packages in Word or server-side systems.
FAQ
Questions, answered
- Can you build custom Excel functions like =MYFUNC()?
- Yes. Office.js custom functions let me add your own worksheet functions, including asynchronous and streaming ones that pull live data into cells and update automatically.
- Can the add-in pull live data from our API or database?
- Yes — I connect the add-in to your REST API or database so cells show current data, with optional refresh on a schedule or on demand.
- Will custom Excel add-in development work on Excel for the web and Mac?
- Yes. Office.js add-ins run on Excel for Windows, Mac, iPad, and the web; I add capability checks where an API differs by platform.
- Can it write data back, or only read?
- Both. Using ranges and bindings the add-in can validate and push edits back to your system, not just display data.
- How is the add-in delivered to our team?
- Via AppSource, sideloading for testing, or centralized deployment through the Microsoft 365 admin center so everyone gets it automatically.
Let's build it together
Email me with what you need — I'll reply with a clear plan and next steps. Prefer chat? WhatsApp works too.