The verdict in three sentences
Modernizing a legacy application starts with an audit at EUR 4,000-10,000, essential before any rebuild quote. The rebuild itself costs EUR 60,000-200,000 depending on code volume and chosen approach, over 6 to 14 months. Delaying modernization is not free: untreated technical debt inflates your maintenance costs by +30% per year and raises the risk of a critical outage.
Big-bang or gradual: two philosophies of risk
The central question isn't "which tech" but "how to migrate without breaking production". The big-bang rewrites everything then switches; the strangler pattern replaces module by module, old and new coexisting.
| Approach | Indicative 2026 cost | Risk | Timeline |
|---|---|---|---|
| Preliminary audit | EUR 4,000 - 10,000 | Low | 2-4 wks |
| Big-bang (rewrite) | EUR 80,000 - 200,000 | High (tunnel effect) | 8-14 months |
| Gradual (strangler) | EUR 60,000 - 160,000 | Controlled | 6-12 months |
| Keeping the legacy | EUR 0 apparent | +30%/yr maintenance | Growing |
Big-bang is faster on paper but concentrates all risk at the switch moment. Gradual spreads the effort and delivers value from the first months.
The hidden cost of doing nothing
Keeping an obsolete application seems economical, until the hidden bill appears: fixes taking ever longer, unsecured dependencies, developers scarce on the old tech.
| Item | Year 1 | Year 3 | Year 5 |
|---|---|---|---|
| Annual maintenance | EUR 20,000 | EUR 34,000 | EUR 57,000 |
| Security-flaw risk | Moderate | High | Critical |
| Cost of a dev on obsolete tech | +20% | +40% | +70% |
| Average fix turnaround | Baseline | x1.8 | x2.5 |
| Cumulative 5-yr maintenance | — | — | ~EUR 185,000 |
Over 5 years, doing nothing can cost as much as a full gradual rebuild — without the modernized asset at the end.
Mini case study
Thomas, IT director of a mid-sized manufacturer in Dublin, runs a production-management app written 12 years ago, no longer maintainable. Audit: EUR 8,000. He chooses the strangler pattern: rebuild at EUR 140,000 over 10 months, critical modules first. Meanwhile his legacy maintenance cost EUR 34,000/year and rising 30%/year. After the rebuild, maintenance drops to EUR 12,000/year. Annual saving: EUR 22,000, plus the end of critical-outage risk and access to a pool of modern developers. Payback on the rebuild in ~6 years, but the avoided risk alone justifies the decision.
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FAQ
Why pay for an audit before the rebuild?
Because a rebuild quote without an audit is a lottery. For EUR 4,000-10,000, the audit maps the code, dependencies and risks, and firms up the estimate to within plus or minus 20%.
Big-bang or gradual: which to choose?
The gradual (strangler) in 80% of cases: it controls risk and delivers value continuously. Big-bang is only justified on a small app or one impossible to split.
What does a legacy rebuild cost in 2026?
Between EUR 60,000 and 200,000 depending on code volume and approach. The range tightens sharply after the audit.
What is the real cost of doing nothing?
Untreated technical debt inflates maintenance by +30% per year and can accumulate ~EUR 185,000 over 5 years, not counting flaw and outage risk.
How long does a modernization take?
Between 6 and 14 months depending on approach. The gradual path spreads the effort and keeps production running throughout.
Let's scope your project. Describe your current application (tech, age, criticality) and target budget, and we'll start with a costed audit. Detailed quote within 48 h. WhatsApp +221 77 596 93 33.
Mohamed Bah
Fondateur, Kolonell
Passionate about digital and entrepreneurship in Africa, Mohamed has been helping Sénégalese businesses with their digital transformation since 2020. Founder of Kolonell, he believes every SME deserves a professional and accessible online présence.
