Top 10 Microsoft Office 365 Add-in Development Companies

Custom Microsoft 365 add-ins can make everyday work easier, whether it’s automating Excel reports, connecting Outlook to a CRM, or adding new features to PowerPoint. But finding a development partner with the right technical expertise isn’t always straightforward.

To help you find the best Office add-in development companies for your project, we compared 10 companies by their Microsoft 365 expertise, Office add-in services, technical capabilities, and published project experience. SCAND, the publisher of this article, is also included in the comparison. Each company is presented based on its publicly available services and expertise.

Quick Answer: Which Company Is Best For What?

The best Microsoft Office add-in development company depends on your project. Some providers focus on legacy Outlook integrations and VSTO solutions, while others specialize in Office.js, Microsoft Graph, Dynamics 365, or SaaS applications.

The table below helps you identify which companies are worth considering for your specific requirements. The companies are listed in alphabetical order, not ranked by performance or overall quality.

Best Microsoft Office Add-in Development Companies by Project Type

For traditional Windows-based Office extensions, VSTO and COM experience may be important. Cross-platform projects generally require Office.js expertise, while enterprise integrations may depend more heavily on Microsoft Graph, SharePoint, or Dynamics 365.

How We Compared Office Add-in Development Companies

To make this comparison useful, we evaluated companies using 11 criteria grouped into four categories. We focused on publicly available information that businesses can check before contacting a development partner.

1. Office Add-in Expertise and Technologies

We examined whether each company has dedicated Office add-in development services and experience with relevant technologies:

• Technical capabilities: Office.js, VSTO/COM, C++/MAPI for Outlook, Microsoft Graph, Excel-DNA/XLL, SSO, and event-based activation.
• Legacy modernization: Experience migrating older Office extensions to modern technologies.

We also looked for published projects demonstrating these capabilities rather than relying on general Microsoft development experience.

2. Publishing and Deployment

We checked whether companies describe experience with Microsoft Marketplace (formerly AppSource) validation, centralized deployment through Microsoft 365 or Intune, and code signing for Windows-based extensions. These capabilities matter because developing an add-in and successfully distributing it to users require different expertise.

3. Security, Experience, and Location

We considered ISO 27001 and SOC 2 certifications, GDPR-related practices, contractual jurisdiction, and intellectual property ownership. We also reviewed company founding dates, reported team sizes, and working-hour overlap with European and US East Coast clients. Certifications and compliance claims were treated as company statements unless independently verified.

4. Engagement Models and Reviews

We compared available fixed-price, time-and-materials, dedicated team, and post-release support options. Published client reviews were also considered, with particular attention to feedback mentioning Office add-in projects.

Sources and Verification

Our research draws on company websites, Clutch, DesignRush, Microsoft Partner Center, and Microsoft Marketplace (AppSource). Information was collected as of October 2026. We report verifiable facts, distinguish company claims from independently confirmed details, and avoid subjective quality scores.

Top 10 Microsoft Office 365 Add-in Development Companies

The following Microsoft Office add-in development companies offer different approaches to Office customization, enterprise integration, and legacy modernization. Some specialize in add-in technologies, while others provide broader Microsoft ecosystem development services. Companies appear in alphabetical order, not as a ranking.

Add-in Express: Best for COM/VSTO Development Tools

Add-in Express develops frameworks for creating Microsoft Office extensions using .NET, C#, C++, and COM technologies. Its tools support Outlook, Excel, Word, PowerPoint, and other Office applications. The company also offers custom development services, including Ribbon customization, Excel XLL functions, real-time data servers, and migration from older Office technologies.

• Facts: HQ: Poland | Founded: 1999 | Team: Not publicly specified; project teams of 2–10 specialists.
• Consider if: your developers need established COM/VSTO tooling or assistance with an existing Windows-based Office extension.

Concetto Labs: Best for Outlook Workflows and Multi-App Office Extensions

Concetto Labs develops Outlook add-ins for email automation, CRM integration, scheduling, and communication workflows. Its published services include Office.js and VSTO development, along with modernization of legacy COM extensions. The company also describes Word and Excel integration capabilities, including transferring spreadsheet data into Word documents through custom add-ins.

• Facts: HQ: Ahmedabad, India | Founded: 2014 | Team: 130+ technology specialists.
• Consider if: you need an Outlook extension connected to a CRM or a relatively contained Office automation project.

iFour Technolab: Best for .NET-Based Office Add-ins

iFour Technolab develops Microsoft Office add-ins using C#, VSTO, JavaScript, and .NET technologies. Its services cover custom Office functionality, business process automation, and integration with external applications. The company also offers modernization of existing add-ins, making it relevant to organizations maintaining Windows-based Office extensions alongside newer Microsoft 365 solutions.

• Facts: HQ: Brownsburg, Indiana, USA | Founded: 2013 | Team: 120+ engineers.
• Consider if: your project involves a .NET-based Office extension, existing VSTO code, or integration with Microsoft business applications.

Inogic: Best for Microsoft Dynamics 365 and CRM Extensions

Inogic specializes in Microsoft Dynamics 365 CRM, Dataverse, and Power Platform solutions. It develops business applications and extensions for CRM data management, productivity, and workflow automation. Its products address areas such as document management, data visualization, and integration. Organizations requiring Office-connected Dynamics functionality may find this specialization relevant, although standalone Office.js experience should be verified.

• Facts: HQ: Navi Mumbai, India | Founded: 2006 | Team: Not reliably disclosed in the sources reviewed.
• Consider if: your main objective is extending Dynamics 365 functionality or connecting Microsoft productivity tools with CRM processes.

MetaDesign Solutions: Best for Office.js Development

MetaDesign Solutions develops custom Office extensions using Office.js, .NET, JavaScript, and modern frontend technologies. Its published work includes migrating a Word VSTO add-in to Office.js and Angular. The company also provides Office integration services for enterprise workflows, making its documented migration experience particularly relevant to businesses moving from Windows-only extensions to web-based add-ins.

• Facts: HQ: Gurugram, India | Founded: 2006 | Team: 201–500 employees.
• Consider if: you want to replace a legacy VSTO add-in with an Office.js solution or develop a new web-based extension.

Prakash Software Solutions: Best for Microsoft Graph and SharePoint

Prakash Software Solutions provides Microsoft 365, SharePoint, Azure, Power Platform, and AI development services. Its capabilities include connecting enterprise systems, automating document workflows, and building applications around Microsoft cloud technologies. The company also discusses Office add-in modernization, although a specific VSTO-to-Office.js migration case was not established during this review.

• Facts: HQ: Vadodara, India | Founded: 2000 | Team: 250+ engineers.
• Consider if: your Office add-in must work with SharePoint documents, Microsoft Graph data, or a broader Microsoft 365 environment.

SCAND: Best for Complex Office Add-ins and Legacy Modernization

SCAND builds add-ins for Outlook, Excel, Word, PowerPoint, and Access using Office.js, VSTO, and C#/.NET. The company states it migrates VBA macros and legacy C++ COM add-ins to C#/.NET and VSTO solutions, and its VSTO page also lists migration of VSTO and COM add-ins to Office.js for new Outlook.

• Facts: HQ: Warsaw, Poland | Founded: 2000 | Team: 250+ engineers.
• Consider if: your project involves complex Office functionality, legacy C++ or VSTO code, or collaboration with a Poland-based development company.

ScienceSoft: Best for Enterprise Solutions in Regulated Industries

ScienceSoft provides software development, Microsoft ecosystem integration, cybersecurity, and enterprise consulting. Its experience includes healthcare, insurance, manufacturing, and other industries with demanding security requirements. The company offers .NET development, business application integration, and long-term software maintenance. Its broader capabilities may suit Office-related enterprise projects, though specific add-in implementations should be confirmed during vendor evaluation.

• Facts: HQ: McKinney, Texas, USA | Founded: 1989 | Team: 750+ IT professionals.
• Consider if: your Office integration is part of a regulated enterprise system requiring security expertise and ongoing support.

SG Analytics: Best for Office.js Add-ins with SSO

SG Analytics develops custom add-ins for Excel, PowerPoint, Word, and Outlook using Office JavaScript APIs, TypeScript, React, and Node.js. Its page lists single sign-on and OAuth for corporate identity requirements and describes replacing legacy VBA macros with web add-ins that run on Windows, Mac, and the browser.

• Facts: HQ: Pune, India | Founded: Not stated | Team: 1,000+ employees company-wide.
• Consider if: SSO and enterprise authentication matter and you want a broader data and analytics partner.

XLTools: Best for Office.js Add-ins and VSTO-to-Web Migration

XLTools builds custom add-ins for Excel, Word, PowerPoint, and Outlook on the Office JavaScript API. Its services page lists migration of VSTO-based add-ins to web add-ins, marketplace listing support, CI/CD deployment automation, and code-signing services. The company states 14 years of experience in custom add-ins.

• Facts: HQ: Not stated | Founded: Not stated | Team: Not stated.
• Consider if: you need a focused Office.js add-in or a VSTO-to-web migration from a specialist add-in vendor.

What Affects Office Add-in Development Cost

Hourly rates help compare vendors, but they do not show what a project will cost. According to Clutch’s software development pricing guide, rates vary by region, company, and project requirements.

The total depends on scope and risk: the Office apps involved, integrations with existing systems, VSTO or COM features that Office.js cannot replace directly, and testing across Windows, Mac, and web. Marketplace validation fixes, maintenance, and time-zone overlap add effort too.

A higher rate can still mean a lower final cost if the team needs fewer revisions. When comparing proposals, look at estimated hours, deliverables included, deployment responsibilities, and support terms.

At SCAND, the pricing model depends on how stable the requirements are. Our engagement models include a fixed bid for projects with clear requirements and time and materials when the scope may change. For long-term products, we offer outstaffing or a dedicated team.

10 Questions to Ask Before You Choose a Vendor

Before hiring an Office add-in development company, ask questions that reveal its technical experience, security practices, and approach to delivery. Each question below includes what a useful answer should show.

1. Relevant experience: Can you show completed Office add-in projects similar to ours?
   A useful answer names the Office app, the technology, and the outcome, not only general Microsoft 365 experience.
2. Tech stack: Do you work with Office.js, VSTO/COM, C++/MAPI, Graph, and Excel-DNA/XLL?
   The stack should match your case: a legacy Outlook plug-in needs different skills than a new web add-in.
3. Legacy migration: Have you migrated VSTO or COM add-ins to Office.js, and can you show which features moved, which were redesigned, and which stayed desktop-only?
   Office.js does not cover every desktop feature, so a good answer lists the gaps.
4. Authentication and events: Can you implement Microsoft Entra ID SSO and Outlook event-based activation, and what do you do about events Office.js does not support?
   Ask for a workaround, not only a yes.
5. Deployment: Do you handle AppSource approval, centralized deployment, Intune, and code signing?
   Clarify who is responsible for each step, because publishing can add weeks after development.
6. Security: Can you verify your ISO 27001 or SOC 2 certifications and GDPR practices?
   Ask for certificate numbers or reports, not only a logo on the website.
7. Legal terms: Which jurisdiction applies, and who owns the code and IP?
   The contract should state that the code and IP transfer to you.
8. Team availability: Who will work on the project, and what time-zone overlap is available?
   Check named roles and shared working hours, since slow reviews delay delivery.
9. Pricing: Which engagement models do you offer, and what’s included in the estimate?
   Compare estimated hours, deliverables, testing, and support, not only the hourly rate.
10. Support and reviews: Do you provide maintenance and Office add-in client references?
    Look for references from add-in projects specifically, since a general software rating does not prove add-in experience.

If you maintain a VSTO or COM add-in, give question 3 the most weight. Not every desktop feature has an Office.js equivalent, and VSTO and COM add-ins do not run in the new Outlook for Windows.

When choosing a VSTO development company for a migration, ask for a feature-by-feature assessment and a testing plan covering classic and new Outlook.

Conclusion

The best Office add-in development companies aren’t necessarily those with the highest ratings or the lowest hourly rates. The right choice depends on your project requirements, existing technologies, and long-term plans.

A company experienced in VSTO and COM may be better suited to modernizing a legacy Outlook extension, while an Office.js specialist may be a better fit for a cross-platform add-in. Enterprise projects may also require expertise in Microsoft Graph, SharePoint, or Dynamics 365.

Before making a decision, compare relevant projects, technical capabilities, security practices, and support options. A strong match matters more than a company’s position on any list.

Frequently Asked Questions (FAQs)

How Much Does It Cost to Hire an Office Add-in Development Company?

The cost depends on the add-in’s features, supported Office apps, integrations, and migration scope. Vendors charge hourly or per project, so compare the total estimate, not only the rate. A simple Excel add-in takes far less work than an Outlook solution with security rules, while Office add-in development with legacy migration takes the longest.

Which Company Is Best for Outlook Add-ins?

The right company depends on your Outlook requirements. Outlook add-in development companies differ in technology: some focus on Office.js and new Outlook, others on C++ or VSTO add-ins for classic Outlook. SCAND offers Outlook add-in development, including legacy C++ COM add-ins and migration to Office.js. Concetto Labs focuses on Outlook workflows and CRM integrations. Compare relevant projects and support for classic and new Outlook before choosing.

Should I Choose VSTO or Office.js for a New Project?

Office.js is generally the better choice for new cross-platform add-ins because it supports modern Office applications on Windows, Mac, and the web. VSTO remains useful for Windows-specific functionality requiring desktop APIs. However, VSTO and COM add-ins are not supported in the new Outlook for Windows.

Can a Company Migrate a VSTO Add-in to a Web Add-in Without Losing Features?

Not always. Office.js does not support every capability available through VSTO or COM, particularly Windows-specific APIs and certain Outlook events. A development company should assess existing functionality, identify unsupported features, and suggest alternatives. Some operations may move to backend services, while others may require redesign or remain desktop-only.

How Long Does AppSource Validation Take?

Microsoft’s documentation says the validation team’s review typically takes 3–5 working days, depending on the queue. In our experience, a submission that needs fixes and resubmission can take four to six weeks. After certification, a product usually appears in Marketplace within one hour.

How Do I Check a Vendor’s Reviews on Clutch and DesignRush?

Search for the company’s profile on Clutch and DesignRush. Check the overall rating, review count, publication dates, and project descriptions. Look for feedback specifically mentioning Office add-ins, Microsoft 365 integrations, or legacy modernization. General software development reviews may not reflect the vendor’s experience with your particular requirements.