The verdict in three sentences
A legacy web app rebuild costs between 40,000 and 150,000 EUR (excl. VAT) in 2026, depending on the number of modules, accumulated debt and data migration complexity. The classic mistake is the big bang: rebuilding everything at once doubles the risk and freezes evolution for 6 to 12 months. The right question isn't "how much does the rebuild cost" but "at what point does annual maintenance exceed the amortised cost of a progressive reconstruction".
What a rebuild costs by scope
The price depends first on the number of screens, integrations and business rules to reimplement. Here are the 2026 orders of magnitude for the French market.
| Scope | Screens / modules | Duration | Cost 2026 (EUR excl. VAT) |
|---|---|---|---|
| Small line-of-business app | 10-20 screens, 1 integration | 2-3 months | 40,000 - 60,000 |
| Medium business app | 30-50 screens, 3-4 integrations | 4-6 months | 60,000 - 100,000 |
| Complex platform | 60+ screens, ERP + payment | 7-12 months | 100,000 - 150,000 |
| Heavy data migration | +millions of rows, history | +1-2 months | +15,000 - 40,000 |
| UX rebuild + design system | full mockups | +4-8 weeks | +12,000 - 25,000 |
Data migration is the most underestimated line item: cleaning, schema mapping and history replay often represent 20 to 30% of the total budget.
Big bang or strangler: which approach
The strangler fig pattern rewrites the application module by module, letting the old and new systems coexist behind a router (reverse proxy or API gateway). Each migrated brick goes to production immediately, drastically reducing risk.
| Criterion | Big bang | Strangler (progressive) |
|---|---|---|
| Failure risk | High | Low |
| Go-live | At the end (6-12 months) | Continuous, per module |
| Coordination cost | Low at first | Medium (double run) |
| Rollback possible | Hard | Yes, per brick |
| Evolution freeze | Total | Minimal |
| Typical budget | 60,000 - 150,000 | 70,000 - 160,000 |
The strangler costs 5 to 10% more in coordination but divides the overrun risk by three. For any business-critical production application, it's the approach to favour.
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Mini case study
Marc, CIO of a B2B distribution SME in Nantes, inherited an order-management application in PHP 5.6, unmaintained since 2019. Corrective maintenance costs 28,000 EUR (excl. VAT) per year (bugs, security, an on-demand external developer), and each change takes 6 weeks. A progressive rebuild quote comes in at 85,000 EUR (excl. VAT) over 6 months.
The tipping-point calculation: over 4 years, maintaining the legacy costs 28,000 x 4 = 112,000 EUR (excl. VAT), with zero new features. The 85,000 EUR rebuild, amortised over 4 years, works out to 21,250 EUR/year and unblocks evolution. The tipping point is crossed by the third year: Marc approves the strangler approach, starting with the orders module, the most expensive to maintain.
FAQ
Should everything be rewritten or can the existing app be modernised? If the code is structured and tested, targeted refactoring (10,000-30,000 EUR excl. VAT) often suffices. A full rebuild is only justified when the stack is obsolete, insecure or impossible to recruit for.
How long do the old and new systems coexist? With the strangler approach, between 3 and 9 months depending on module count. The double run adds a temporary hosting cost, typically 200 to 600 EUR/month.
How do you secure the data migration? Through dry-run migrations tested on an anonymised copy, with automatic reconciliation (counts, checksums). Budget 20 to 30% and at least two rehearsals before the final cutover.
What happens to the team that maintained the legacy? They're involved from the framing phase to transfer undocumented business rules. This is the main project risk: implicit knowledge is often worth more than the code itself.
Does a rebuild really improve performance? Yes, measurably: on our 2026 projects, response times typically drop from 2-4 s to under 500 ms, and hosting costs fall by 30 to 50% thanks to modern architecture.
Let's scope your project. Describe your current application, module count and main pain point: we'll quote a progressive rebuild with a maintenance-vs-reconstruction tipping point. 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.