Custom fintech software built on top of a Banking-as-a-Service rail, or a specific module alongside your existing banking core: customer onboarding, KYC portal, loan application, payment app. We build the part your customer or employee works with, not the regulatory banking licence underneath it.
Let's be clear from the outset: replacing an entire core banking platform (the part that manages accounts, handles ledger postings and talks to SWIFT) is the territory of Mambu, Thought Machine, Temenos and FinXact. That is a multi-year programme, a budget running into many millions, and a dependency on decades-old legacy data models. We don't do that.
What we do do is the layer on top. A BaaS rail from Solaris, Treezor or Currencycloud provides you with accounts, cards and payments-as-a-service. We build the customer-facing UI, the business logic and the compliance flows around it, so you can launch a fintech proposition under your own brand, or bring an existing banking product up to date.
We also build standalone modules alongside your existing core: a new customer portal, a digital mortgage application process, a KYC workflow, a treasury tool for SME clients. All of these complement what you already have rather than replace it. Our sweet spot is exactly that outer layer: it's where the customer experience lives, where differentiation lies, and where most of the regulation becomes visible to the end user.
We work with established banks looking to renew a specific module, with insurers holding a banking licence who need to set up a digital channel, with fintech start-ups building their MVP or V2, with savings and cooperative banks going digital, with BaaS resellers building a white-label layer for partners, and with crypto banks operating under MiCA. The common thread: you know the regulation and the product, and we build the software that makes it work.
Three types of projects where we can put our experience in fintech software, regulated industries and BaaS integrations to good use. In the first conversation, we determine which path suits your situation.
You have a proposition, such as a savings app for young people, a payment card for a community or a wallet for a specific sector, and you choose a BaaS provider for the underlying rails. On top of that, we build the customer app, the onboarding flow, the transaction overviews and the business logic that sets your product apart. Identification via BankID or iDIN, push notifications for every payment, and integrations with your CRM so that support and marketing work from the same data. Launch within one production-ready release, then keep building on the releases that follow.
Your core system works well. Only the customer-facing layer around it feels twenty years old. We build the part that customers or staff use every day: a modern customer portal, a digital loan application, a KYC or AML compliance flow, or a digital mortgage application environment. This comes with a clean API integration to your existing core, an anti-corruption layer where the legacy system behaves unexpectedly, and, if you wish, a feature flag system so you can run new flows alongside the old ones until you are confident the migration is running smoothly.
For lending start-ups, wealth platforms, treasury tools and payments companies that need their MVP, V2 or platform layer. Open Banking APIs for PSD2, fraud detection with a rules engine and optionally an ML model for anomalous patterns, real-time payment integration (SEPA Instant), and all DORA and GDPR requirements built in. No rebuild a few years down the line, but a platform that scales with your growth, with audit trails that DNB or AFM supervisors are accustomed to seeing, and an architecture that adapts as the regulation changes.
The financial sector draws a sharp line between software development and licensing. We sit on the software side, and we are open about that.
For the regulatory side — DNB or AFM licensing, BIN sponsorship, SWIFT membership — you work with a specialist compliance adviser or a licensed BaaS provider. We connect to them on the technical side.
Banking software needs a stack that will still hold up in ten years, built on patterns supervisors recognise. For back-end work we rely on Java with Spring Boot, the working language of enterprise banking, with mature libraries for security, transactional work and messaging. For modern high-throughput services and payment engines we use Go, and for data pipelines and fraud models Python. On the front end we use React or Vue for web, and native (Swift / Kotlin) or React Native for mobile, depending on what the product requires.
For the middle layer: Kong or Tyk as the API gateway, a service mesh where microservices genuinely earn their keep, and Kafka or RabbitMQ for event streams between bounded contexts. Secrets and cryptographic keys live in an HSM or a cloud equivalent (AWS CloudHSM, Azure Key Vault Managed HSM), with automatic rotation. Observability via Prometheus, Grafana and structured logging, not because it looks good, but because DORA explicitly requires demonstrable incident detection.
Security in layers: encryption in transit and at rest, secret scanning in the CI pipeline, dependency scanning for CVEs, a penetration test before go-live, and periodic penetration tests thereafter. For regulated workloads, a dedicated tenant or your own VPC is often appropriate, so the separation of data between clients and environments can be demonstrated. We document the controls in a way your compliance or internal audit team can reuse directly for DNB or AFM reporting.
With OneDayBuild, we turn your idea into something tangible in one day for €1,150, so you can see whether further development is worth the investment. Decide to go ahead with the full build? Then we credit the full cost.
Explore OneDayBuild →Four recognisable patterns we often get involved in. If one sounds familiar, we would be glad to talk further.
You have the market, you have chosen a BaaS partner, and now the end-user application needs to be built. The BaaS rail gives you accounts and cards, but the UI, branding, onboarding and all the distinctive features are up to you. That is the difference between a generic savings pot and a product people come back to. That is exactly where we step in.
The core system works well, but the layer customers interact with has not been touched in years. We build a new client portal or a modernised onboarding flow alongside your existing core, without touching it. We build modularly, so that any future core migration does not break through your new layer.
The KYC flow was losing too many applicants. The DORA deadline is approaching, and your incident management tooling is a spreadsheet. A mortgage application takes too long because data has to move between four systems. Digitise or rebuild one specific module, modularly alongside what's already there, with verifiable audit trails for your auditor.
The prototype is live and you have your first customers, but the stack was put together on a shoestring and won't scale to 10× the volume or a second country. We'll rebuild it on an architecture that is production- and audit-ready, with multi-tenancy or multi-region where needed, without starting over on the feature set that already works.
A conversation in which we understand which proposition you're taking to market, which BaaS partner or core bank is in view, which regulation applies and what your own team can already take on. By the end we'll have a shared picture of the scope.
A workshop with your tech and compliance team. We map the regulatory requirements (PSD2, DORA, GDPR, and possibly MiFID II or the Wft) onto concrete software controls. This produces an architecture document and an initial screen flow.
A working build every two weeks. We work in a feature-flagged staging environment so compliance and security checks can run in parallel. The penetration test is scheduled early, not at the end.
A phased go-live, with a controlled rollout to a first group of customers. After that, ongoing maintenance: monitoring, security patches, regulation that changes (PSD3 is on the way), and further development based on what customers actually use.
The questions we're asked most often before a project begins.
A no-obligation introductory call of half an hour. Tell us about your proposition, your BaaS partner or core vendor, and which regulations apply. We will give you direction you can use, even if you ultimately continue with someone else. For larger fintech or compliance challenges, we also work together with our enterprise software team and colleagues who build insurance software.
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