Senior Back-end Developer

Nmbrs BV; Rotterdam; On-site; EUR 50,000 - 67,000; Full time

Benefits offered by this job

Training budget

Vacation days

Remote work (5 weeks)

Job summary

Nmbrs BV in Rotterdam is looking for a Senior Back-end Developer to own, build and scale the core financial engine of our accounting platform. You’ll lead by example in .NET (VB.NET/C#), mentor junior developers, and drive performance and architectural improvements across the stack.

You’ll join Team AO Ops in Nmbrs Accounting, collaborating with cross-functional peers while focusing on robust APIs, scalable services, and maintainable code that their SME clients trust daily.

Qualifications

• At least 4 years of experience as a back-end developer using .NET technologies (VB.NET/C#).
• Experience coaching and mentoring junior developers (e.g. peer reviewing, sharing best practices).
• Strong experience with relational database engines (MSSQL), and with API development (HTTP, REST, .NET Web API).
• Experience with Agile methodologies (Scrum), and with Azure DevOps and/or GitHub.
• An autonomous, analytical mindset and strong organizational skills.

Responsibilities

• Feature development & integration: Work on implementing new features, APIs, system integrations, and data interfaces (with external parties or Visma solutions), ensuring they're efficient, robust, and aligned with user/system needs.
• Code optimization: Optimize and refactor existing codebases, focusing on improving performance, readability, and maintainability.
• Performance enhancement: Optimize business-critical applications for maximum speed and scalability under heavy system load.
• Mentoring & coaching: Provide guidance, peer reviews, and support to less experienced team members, helping them improve their technical skills and grow professionally.
• Team collaboration: Work closely with cross-functional team members and technical stakeholders, fostering effective communication to achieve project goals.

Skills

Back-end development

.NET technologies

Mentoring juniors

Performance optimization

Education

4+ years back-end dev experience

Tools

Azure DevOps

GitHub

REST APIs

Job description

As Senior Back-end Developer, you own, build, and scale the core financial engine and infrastructure that form the heartbeat of our accounting platform. With your deep .NET expertise, you turn complex, business-critical systems into code that's fast, reliable, and built to last. Together with your team, you guide junior developers and help ship the features our clients depend on. And you don't just maintain what's there: you look for ways to make it faster, cleaner, and more resilient.

Time to grow. Time to thrive.

We are Nmbrs. We build smart business software that removes unnecessary complexity for SMEs and the professionals who support them. Nmbrs brings together four established product organisations -Payroll, Accounting, Invoicing and Reporting - with teams across the Netherlands, Sweden and Portugal.

We are one brand with a shared ambition, while continuing to work from the expertise and strengths of our individual organisations. We join forces where it makes sense, while keeping teams, expertise and decision-making close together.

For this role, you'll join Nmbrs Accounting in Rotterdam, working closely with the colleagues in your local team while being part of the wider Nmbrs organisation.

Department

Software Development, Back-end Developer

Location

Your role

As a senior developer, you participate in the entire application lifecycle; optimizing and refactoring existing codebases, enhancing the performance of critical applications, and building readable, maintainable, and performant code using the best software practices. You're also a technical reference for the team: you guide junior developers and help implement new features and functionality into our product.

You will join Team AO Ops. This team is responsible for the bedrock of our software: the core accounting domain. The team's mission is to ensure Nmbrs Accounting's financial engine remains rock-solid, highly performant, and exceeds our users' expectations every single day.

In practice, you will:

Feature development & integration: Work on implementing new features, APIs, system integrations, and data interfaces (with external parties or Visma solutions), ensuring they're efficient, robust, and aligned with user/system needs.

Code optimization: Optimize and refactor existing codebases, focusing on improving performance, readability, and maintainability.

Performance enhancement: Optimize business-critical applications for maximum speed and scalability under heavy system load.

Mentoring & coaching: Provide guidance, peer reviews, and support to less experienced team members, helping them improve their technical skills and grow professionally.

Team collaboration: Work closely with cross-functional team members and technical stakeholders, fostering effective communication to achieve project goals.

Your impact

Your work directly determines how reliable and performant the financial engine at the core of Yuki really is. When you optimize a critical application under heavy load, it's the reason thousands of accountants and entrepreneurs can trust their numbers are right, every single day. When you ship a clean integration instead of a brittle one, it's one less production fire for the whole team.

You're also raising the bar for the developers around you: as a senior reference point, the code reviews and mentorship you provide shape how the next generation of developers on your team writes and thinks about software.

You bring

You're a technical reference point with an autonomous, analytical mindset and strong organizational skills. You care about software craftsmanship, love tackling performance bottlenecks, and enjoy sharing your expertise with the team.

At least 4 years of experience as a back-end developer using .NET technologies (VB.NET/C#).

Experience coaching and mentoring junior developers (e.g. peer reviewing, sharing best practices).

Strong experience with relational database engines (MSSQL), and with API development (HTTP, REST, .NET Web API).

Experience with Agile methodologies (Scrum), and with Azure DevOps and/or GitHub.

An autonomous, analytical mindset and strong organizational skills.

We offer

Personal development: A generous training budget, available directly after your probation period, for specialized certifications, courses, and training to solidify your technical expertise.

Time off and flexibility: 28 vacation days (and the option to buy 5 extra days), plus the possibility of Workation: up to 5 weeks per year of remote work.

Well-being & focus: Access to lifestyle coaches, healthy lunches, and other initiatives that contribute to your well-being and focus, including the possibility to lease a bike via Lease‑a‑Bike.

Financial security: A base salary between €4.500 - €6.000, depending on seniority. Plus attractive benefits, such as full pension coverage.

Optimal workplace: The best hardware, including a home‑office budget to set up your remote workspace.

Plenty of opportunities: Enjoy the speed and autonomy of an innovative software collective, backed by the stability, vast resources, and global career network of the Visma Group.

Team & culture: Participate in (international) team‑building days and other Nmbrs events that strengthen the close team spirit in a diverse and international environment.

Talent Acquisition