The verdict in three sentences
Modernizing a legacy web application (PHP/jQuery, abandoned framework) costs between 30,000 and 90,000 EUR in 2026 for progressive refactoring, and 80,000 to 250,000 EUR for a full rewrite. The strangler pattern lets you migrate piece by piece with no downtime, spreading both risk and budget. The real question is not aesthetic: it is the cost of technical debt (bugs +40%, hard hiring, blocked evolution) versus the cost of modernizing.
Rewrite, refactor or replace: the decision grid
Three paths face the IT director, with very different risk and cost profiles. The choice depends on the business value of the existing code and its maintainability.
| Strategy | Principle | Timeline | 2026 cost (EUR) | Risk |
|---|---|---|---|---|
| Progressive refactoring | Improve module by module | 4 - 8 months | 30,000 - 90,000 | Low |
| Strangler pattern | Replace route by route | 6 - 12 months | 60,000 - 160,000 | Medium |
| Full rewrite | Start from scratch | 8 - 14 months | 80,000 - 250,000 | High |
| Replace with SaaS | Drop the custom build | 2 - 5 months | 15,000 - 60,000 | Variable |
The strangler pattern is often the best compromise: it delivers value continuously and avoids the "big bang" that fails 40 to 50% of rewrites.
The hidden cost of technical debt
Keeping a legacy app is not free. Technical debt is paid every month in productivity, incidents and hiring difficulty.
| Debt line | Estimated annual impact | Order of magnitude (EUR) |
|---|---|---|
| Bugs and incidents (+40%) | Fix-up dev time | 12,000 - 40,000 |
| Slow evolution | Time-to-market x2 | Variable lost revenue |
| Hiring premium | Rare obsolete tech | 8,000 - 20,000 |
| Security risk | Unpatched dependencies | GDPR exposure |
| Inefficient hosting | Unoptimized architecture | 3,000 - 10,000 |
Decision threshold: when annual debt exceeds 25 to 30% of a modernization's cost, the overhaul pays back in under 3 years.
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Mini case study
Julien, IT director at a services SME in Bordeaux, inherits a management app in unmaintainable PHP 5, built by a vanished vendor. Each change takes 3x too long and one incident halts production 2 days per quarter. He prices the debt at about 34,000 EUR/year (fixes, slowness, hosting). Rather than a 180,000 EUR rewrite, he chooses the strangler pattern at 95,000 EUR over 9 months. By month 4, the migrated critical modules cut incidents by 60%. The modernization pays back in about 30 months, with an app finally evolvable and secure.
FAQ
How do I know whether to rewrite or refactor? If the code carries rich business logic and mostly works, refactor. If it is unstable, untested and on a dead stack, a rewrite or strangler pattern is justified. A 3-5 day audit (2,500-4,500 EUR) settles it objectively.
What is the strangler pattern in practice? You place a router in front of the old app and replace features one by one with modern modules. Users see nothing, risk is spread, and you can stop at any time.
Can the app keep running during the overhaul? Yes, that is the point of progressive refactoring and the strangler pattern: zero downtime, unlike a big-bang rewrite that forces a risky cutover.
How long does a modernization take? From 4 months (targeted refactoring) to 14 months (full rewrite). The strangler pattern delivers value from month 3 or 4, which reassures management and funds the rest.
Isn't replacing with SaaS simpler? Sometimes, at 15,000-60,000 EUR of migration. But SaaS imposes its limits: if your app carries a specific business advantage, modernized custom stays more strategic.
Let's scope your project. Describe the current stack, the app's size and your continuity constraints so we pick the right strategy (refactor, strangler or rewrite). 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.
