The verdict in three sentences
Migrating a legacy web application costs between 25,000 and 80,000 EUR in 2026, depending on the functional scope and accumulated technical debt. The real challenge is not rewriting everything at once but choosing between big-bang and an iterative approach, the latter sharply reducing risk. Expect 12 to 24 weeks and a 30 to 60% drop in maintenance costs after migration.
What drives migration cost
Migrating is not just re-coding the interface. It means migrating data, restoring integrations, re-testing business journeys, and training users. The old app's technical debt (undocumented code, no tests) inflates the analysis line item.
| Line item | 2026 cost (EUR) | Comment |
|---|---|---|
| Legacy audit and mapping | 3,000 - 8,000 | Understand the undocumented existing |
| Technical rebuild (front + back) | 12,000 - 40,000 | New foundation, components |
| Data migration and cleanup | 4,000 - 15,000 | Cleaning, mapping, checks |
| Re-integration (API, payment, IT) | 3,000 - 10,000 | Reconnect external building blocks |
| Acceptance and regression testing | 3,000 - 9,000 | Critical journeys, load |
| Training and cutover | 1,500 - 4,000 | Documentation, support |
Big-bang or iterative
Big-bang replaces everything at once: faster on paper, but high risk if a journey breaks on cutover day. The iterative approach migrates module by module, often via a facade pattern (strangler fig): longer, but each step is reversible and testable in production.
| Criterion | Big-bang | Iterative (strangler) |
|---|---|---|
| Total cost | 25,000 - 55,000 EUR | 35,000 - 80,000 EUR |
| Timeline | 12 - 16 weeks | 16 - 24 weeks |
| Breakage risk | High | Low |
| Rollback to old system | Difficult | Possible at each step |
| Value delivered | At project end | Progressive |
| Suited to | Small stable scope | Critical application |
Mini case study
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Julien, IT director of a 110-employee trading firm in Toronto, runs a back office in PHP 5 / jQuery that has become impossible to evolve. Every small change takes weeks and maintenance costs 1,800 EUR/month in fixes. He chooses an iterative migration at 58,000 EUR over 20 weeks. After cutover, maintenance drops to 700 EUR/month, a 13,200 EUR/year saving, and new features ship in days rather than weeks. Payback on maintenance alone in about 4.5 years, but the real value is regained agility and eliminated failure risk.
FAQ
Why is migration so expensive? Because you do not start from scratch: you must understand an often-undocumented existing system, migrate historical data, and guarantee zero regression on critical journeys. The initial audit is unavoidable.
How much do you save on maintenance? As an order of magnitude, a modernized app cuts maintenance costs by 30 to 60%, thanks to tested, documented code on supported technologies.
Big-bang or iterative for a critical app? Iterative, almost always. The extra cost (10,000 to 25,000 EUR) buys the ability to roll back at each step and avoids a production outage on cutover day.
What about historical data? You clean it, map it to the new model, and check it by sampling. This line item (4,000 to 15,000 EUR) is often underestimated in rough quotes.
How long without new features? With iterative, almost no freeze period: each migrated module goes live continuously. Big-bang often imposes a freeze of several weeks.
Let's scope your project. Describe your current app (technologies, number of screens, data volume) and your continuity constraint, and we will price the migration and its method. 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.
