Senior Full Stack Engineer

Randstad (Schweiz) AG; London; On-site; GBP 75,000 - 130,000; Full time

Job summary

A climate-tech startup in London is looking for a Senior Full Stack Engineer to lead the design and development of their SaaS platform. The role focuses on building scalable systems with a strong emphasis on business impact. Join a collaborative team dedicated to sustainability and innovation, working in an environment that values fast-paced, adaptive solutions.

Benefits

Up to £130,000 (depending on experience)

Path to generous equity

Private Healthcare

33 days holiday (including bank holidays)

Cycle to Work scheme

Advanced parental leave

Qualifications

• Experience in full-stack development with a focus on scalable systems.
• Ability to lead and mentor junior engineers.
• Strong background across the stack with a passion for learning.

Responsibilities

• Design intuitive UIs and optimize backend performance.
• Work with product teams to define problems and solutions.
• Champion high-quality code standards.

Skills

Full-stack mindset

Impact-driven development

Collaboration

Adaptability

Simplicity in solutions

Tools

SQL

Containerization

Infrastructure as Code (IaC)

Job description

About Us

We’re a mission-driven, well-funded startup on the cusp of launching a game-changing climate-tech platform, backed by three global banks . Our goal? To simplify complex challenges and help businesses build for the future. SMEs drive a significant share of global emissions but often lack the tools, resources, and support to respond. We’re delivering bold, practical solutions that enable them to operate smarter, stay ahead of regulation, and meet rising expectations from customers, investors, and partners.

Our platform empowers financial institutions and large organisation to work directly with their clients and supply chain partners to reduce operational risk, mitigate their climate impact, and drive real cost savings. We deploy through major institutions, enabling deep decarbonisation across global supply chains, fuelled by powerful data and automation. If you’re looking to build something that truly matters at the intersection of technology, sustainability, and impact, this is your chance to get in early and help shape a category-defining company.

About the Role

At FourTwoThree, our engineering culture is built for the speed, complexity, and purpose of climate-tech. We’re a team of adaptable, business-minded generalists who thrive in a fast-moving environment and are passionate about building solutions that matter. We are:

• Full-stack by default: From front-end to back-end to infrastructure, our engineers jump into any part of the platform as needed.

• Specialists who stay curious: Everyone has their strengths, but we’re all committed to learning and expanding our skills across the stack and the business.

• Business-first mindset: We build with impact in mind - understanding the “why” behind every task, designing smart solutions, and challenging anything that doesn’t add value.

• Engineered for change: We operate in a complex, rapidly evolving space, and our approach is tailored to meet that challenge head-on.

Whether you're just starting out or have spent years writing scalable code, this is your chance to build fast, learn deeply, and make a real difference.

Key Responsibilities

• Build end-to-end: Design intuitive UIs, optimise backend performance, and deploy secure, scalable infrastructure, wherever the work takes you.
• Shape the product: Work directly with product and business teams to define problems and create smart, impactful solutions.
• Raise the bar: Champion clean, secure, high-quality code and always be open to learn and improve.
• Own what matters: Lead features, projects, and initiatives that drive value, our process is built to give you real ownership and impact.

About You

We’re looking for an experienced Senior Full Stack Engineer to design, develop, and maintain the server-side logic and infrastructure for K423’s SaaS platform. As a Senior Engineer, you should be highly skilled and highly curious with a strong background across the stack, a passion for building scalable and secure systems, and the ability to lead and mentor junior team members.

But most of all, we’re looking for:

• Full-stack mindsets: You might have a specialty, but you’re excited to work across the stack and always hungry to learn something new.

• Impact-driven builders: You code with purpose, focusing on business value over hype, and delivering tech that truly moves the needle.

• True owners: You speak up, collaborate widely, and take responsibility beyond your role to help shape how we grow.

• Big-picture thinkers: You care as much about solving the right problem as writing elegant code, and you build with scale in mind.

• Simplicity champions: You cut through complexity with clean, maintainable solutions that reduce risk and speed up growth.

• Agile operators: You thrive in early-stage chaos, adapt quickly, and are ready to lean in when priorities shift.

• Technical depth: You bring solid foundations and hands-on experience, ideally with containerisation, IaC, component-based UIs, and SQL, whatever your language background.

The Practical Stuff

If you’ve read this far, hopefully you’re already very keen on this opportunity and confident that it aligns with the kind of company you’d like to be a part of. As a last step before reaching out, please take a moment to review these points to ensure they align with your own career and personal preferences.

• London based role: easily accessible near Euston Station.

• Four-day in-office rhythm: We work from the office four days a week, with one fixed WFH day to recharge and focus.

• Real-time, in-person culture: As a fast-moving startup, we value face-to-face collaboration, quick huddles, and spontaneous problem-solving.

• Collaborative energy: From coffee-fuelled brainstorms to whiteboard sessions, we’re a team that thrives on sharing ideas in the moment.

️ Compensation, Perks & Benefits

• Up to £130,000 (depending on experience)
• We offer a path to generous equity for all of our employees.
• Private Healthcare
• 33 days holiday (including bank holidays)
• Cycle to Work scheme
• Advanced parental leave
• A chance to make a genuine, practical impact on building a more sustainable future
• Large financial institution backing, and a small yet driven team of experts.