Legacy software modernization — no big bang, no feature freeze
Your legacy system is slowing you down — but a big-bang rewrite is usually the most expensive answer. We modernize business-critical software step by step: a TypeScript layer grows alongside your Java or PHP core, module by module, while operations keep running. And if keeping the system as it is turns out cheaper, that is exactly what our assessment will say — before you spend a euro of budget.
It starts with an assessment: a code audit, a TCO comparison, and a prioritized module roadmap — a document you can take straight into a management decision. From there, a Next.js/NestJS layer grows alongside your existing system using the strangler pattern, taking over one well-scoped process at a time. Delivery is senior-first: experienced developers instead of a nearshore bench, EU hosting, identity with Keycloak or Supabase.
Not every legacy system deserves to be modernized. If your system runs stably, maintenance costs sit well below the cost of a rebuild, and no support deadline is looming, our recommendation is: keep running it and harden it. We put that in the assessment even when it costs us a project — you are paying for a decision, not a sales document.
Terms you should know
Definition
- Legacy modernization
- The step-by-step renewal of business-critical existing software — architecture, language, operations — while the system stays in production. It is the counter-model to the big-bang rewrite. The trigger is rarely the software's age but the risk behind it: at one in two companies, continued operation of legacy systems is no longer assured in the medium term (Lünendonk study, 2025).
Definition
- Strangler fig pattern
- New modules grow around the legacy system behind an integration layer (facade); requests move route by route to the new system until the old one can be switched off. The approach pays off measurably: strangler projects fail roughly 40 percent less often than big-bang rewrites (Security Boulevard, 2026).
Definition
- Two-speed IT
- The stable core system — Java or an ERP, say — keeps running on its own release cadence while a fast TypeScript layer next to it ships new features, coupled via APIs and events. You gain speed without touching or replacing the core system.
The pressure to modernize is measurable: 62% of companies consider parts of their business-critical applications so outdated they need to be renewed, 83% are raising their IT modernization budgets in 2026 — and at one in two companies, continued operation of legacy systems is no longer assured in the medium to long term (Lünendonk study 2025). At the same time, more than 50% of enterprise IT transformation programs run over budget, while projects following the strangler pattern see roughly 40% lower failure rates than big-bang rewrites (Security Boulevard, July 2026).
Security & compliance
Your customers trust you with sensitive data. We take that responsibility seriously.
Whether it's your data protection officer, your IT security team or a major customer's procurement: when security questions come up, you have solid answers. We build in data protection and security from day one and follow the principles of the Twelve-Factor App. That makes approvals and audits easier and gives you the technical foundation to meet requirements under GDPR, ISO 27001 or HIPAA.
Frameworks whose technical requirements we implement in our projects
- GDPR
- ISO 27001
- HIPAA
- NIS2
Keep running, modernize, or rebuild?
Not every legacy system deserves a project. This table is the short version of our assessment logic — including the option we don't earn a cent on: keep running it.
| Option | When it makes sense | Risk | Investment range | Time horizon |
|---|---|---|---|---|
| Keep running & harden | System runs stably, little need for change, maintenance costs well below rebuild costs, no support deadline | Technical debt and staffing risk keep growing; regular reassessment required | Ongoing maintenance as before, targeted hardening per the assessment's recommendation | Immediately — no project, just hardening measures |
| Modernize step by step (strangler / two-speed) | Maintenance costs above 50% of rebuild costs, support deadlines such as the end of commercial Spring Boot 2.7 support in December 2026, a growing feature backlog | Low: operations keep running in parallel; strangler projects fail roughly 40% less often than big-bang rewrites | First module €8,000–20,000, full modules €20,000–60,000, retainer €4,000–12,000/month | First module in production quickly, then predictable replacement module by module |
| Big-bang rewrite | Almost never — at most for small systems without critical parallel operation | High: feature freeze during the rebuild; the domain knowledge hidden in the legacy system is routinely underestimated | Hard to estimate — more than 50% of enterprise transformation programs run over budget | A long dry spell with no usable interim results |
| Rebuild (planned) | The business domain has fundamentally changed, or the legacy system barely carries any processes anymore; the 50% rule of thumb clearly triggers | Medium: a clean cut, but budget for migration and parallel operation | MVP class €8,000–20,000, full web app €20,000–60,000 — details on our web app page | Depends on scope; the legacy system keeps running until cutover |
Keep running & harden
- When it makes sense
- System runs stably, little need for change, maintenance costs well below rebuild costs, no support deadline
- Risk
- Technical debt and staffing risk keep growing; regular reassessment required
- Investment range
- Ongoing maintenance as before, targeted hardening per the assessment's recommendation
- Time horizon
- Immediately — no project, just hardening measures
Modernize step by step (strangler / two-speed)
- When it makes sense
- Maintenance costs above 50% of rebuild costs, support deadlines such as the end of commercial Spring Boot 2.7 support in December 2026, a growing feature backlog
- Risk
- Low: operations keep running in parallel; strangler projects fail roughly 40% less often than big-bang rewrites
- Investment range
- First module €8,000–20,000, full modules €20,000–60,000, retainer €4,000–12,000/month
- Time horizon
- First module in production quickly, then predictable replacement module by module
Big-bang rewrite
- When it makes sense
- Almost never — at most for small systems without critical parallel operation
- Risk
- High: feature freeze during the rebuild; the domain knowledge hidden in the legacy system is routinely underestimated
- Investment range
- Hard to estimate — more than 50% of enterprise transformation programs run over budget
- Time horizon
- A long dry spell with no usable interim results
Rebuild (planned)
- When it makes sense
- The business domain has fundamentally changed, or the legacy system barely carries any processes anymore; the 50% rule of thumb clearly triggers
- Risk
- Medium: a clean cut, but budget for migration and parallel operation
- Investment range
- MVP class €8,000–20,000, full web app €20,000–60,000 — details on our web app page
- Time horizon
- Depends on scope; the legacy system keeps running until cutover
This page assumes an existing system. If there is none because you are building greenfield, you will find our approach and price ranges for new builds on our web app page. If the legacy system should stay untouched and all you need in front of it is a self-service frontend for your customers, that is a case for our customer portal page. And once the decision to modernize has been made and the conversation turns to the technology behind it, our TypeScript agency page shows what we build the new layer with.
If you are building a new product rather than replacing a system, and it is still open whether customers will use and pay for it, start with an MVP: scope, pricing and process are on our MVP development page.
What you can buy from us
Three clearly scoped offerings — from the decision document to ongoing replacement on a retainer.
Modernization assessment
You find out whether modernization pays off before any budget flows.
Code and architecture audit
You see where your existing system stands.
TCO comparison
Keep running, modernize or rebuild.
Prioritized replacement roadmap
Which module gets replaced first.
Decision document for management
Even if it says “keep running and harden.”
Fixed price
Set once discovery is done.
Step-by-step migration
from €8,000
for the first replaceable module
A Next.js/NestJS layer takes over module by module, with no feature freeze in day-to-day business.
Facade in front of the existing system
Strangler or two-speed: your Java or PHP core keeps running.
First module at MVP size
One self-contained part moves first.
Full modules
With authentication and roles.
EU hosting
Identity management with Keycloak or Supabase.
Operations & development on retainer
from €4,000
a month for operations and replacement
A senior team keeps replacing modules and runs the new layer, without you having to hire.
Module replacement along the roadmap
In the agreed order.
Operations and monitoring
With ongoing security updates.
Paying down technical debt
Predictable, step by step.
Cancel monthly
Speed without hiring risk.
All prices exclude VAT, as of October 2026.
Packages and pricing
We publish price ranges before you have to ask. The exact quote depends on your existing system, but you should know the order of magnitude you are planning in before the first call — every range comes from real projects.
| Package | Scope | Price range |
|---|---|---|
| Modernization assessment | Audit, TCO comparison, module roadmap, management decision document | Fixed price after discovery |
| Strangler entry: first module | One well-scoped process as a TypeScript module alongside the existing system (MVP class) | €8,000–20,000 |
| Module replacement with auth & roles | Full web-app class incl. integration with the existing system | €20,000–60,000 |
| Integration layer & identity setup | Next.js/NestJS facade, initial Keycloak/Supabase setup | €5,000–20,000 |
| Ongoing modernization on retainer | Continuous replacement, operations, and further development | €4,000–12,000/month |
Modernization assessment
- Scope
- Audit, TCO comparison, module roadmap, management decision document
- Price range
- Fixed price after discovery
Strangler entry: first module
- Scope
- One well-scoped process as a TypeScript module alongside the existing system (MVP class)
- Price range
- €8,000–20,000
Module replacement with auth & roles
- Scope
- Full web-app class incl. integration with the existing system
- Price range
- €20,000–60,000
Integration layer & identity setup
- Scope
- Next.js/NestJS facade, initial Keycloak/Supabase setup
- Price range
- €5,000–20,000
Ongoing modernization on retainer
- Scope
- Continuous replacement, operations, and further development
- Price range
- €4,000–12,000/month
Who works on your project
Our senior team in Berlin, from the first draft to production. You talk directly to the people who write your code.
Meet the teamFAQ
Straight answers to the questions IT decision-makers ask us about modernization in the first call — no detours.