The verdict in three sentences
Modernizing a legacy application in 2026 costs between 40,000 and 150,000 EUR (excl. VAT) depending on accumulated technical debt, over a 4 to 9 month project. The real risk is not the rebuild budget but the cost of inaction: +30 to 50 % maintenance per year, recurring outages and inability to hire on dead technologies. A phased approach (strangler pattern) spreads the investment and runs old and new side by side.
What drives the bill
The price of a modernization depends first on technical debt: a PHP 5 codebase with no tests or documentation is far costlier to take over than a recent but poorly architected app. Data migration, UI rewriting and rebuilding third-party integrations are the three line items that overrun most.
| Modernization item | 2026 range (EUR excl. VAT) | Indicative timeline |
|---|---|---|
| Technical audit + mapping | 4,000 - 9,000 | 2 - 3 weeks |
| Back-end re-architecture | 18,000 - 55,000 | 8 - 16 weeks |
| Data migration | 6,000 - 25,000 | 3 - 8 weeks |
| New UI / design system | 10,000 - 35,000 | 4 - 10 weeks |
| Third-party integration rework | 5,000 - 20,000 | 2 - 6 weeks |
| Automated tests + CI/CD | 4,000 - 15,000 | 2 - 5 weeks |
An SME with a moderately complex management app most often lands around 60,000 to 90,000 EUR excl. VAT. A large enterprise with critical flows and compliance quickly exceeds 120,000 EUR excl. VAT.
The hidden cost of inaction
Keeping an unsupported application is never free. Every year, maintenance gets more expensive, the security risk surface widens and time lost by teams piles up.
| Indicator | Legacy app (status quo) | Modernized app |
|---|---|---|
| Annual maintenance cost | 18,000 - 40,000 EUR excl. VAT | 6,000 - 15,000 EUR excl. VAT |
| Blocking incidents / year | 12 - 25 | 2 - 5 |
| Average fix time | 2 - 5 days | 2 - 8 hours |
| Time to add a feature | 4 - 10 weeks | 1 - 3 weeks |
| Unpatched vulnerability risk | High | Low |
| Developer hiring | Very hard | Standard |
The maintenance overhead alone (2026 order of magnitude) often represents 12,000 to 25,000 EUR excl. VAT per year versus an up-to-date stack: over three years, you fund almost the whole rebuild just by avoiding waste.
Mini case study
Marc, CIO of a 140-employee industrial SME in Lille, runs an in-house ERP in PHP 5.6, out of support since 2021. Corrective maintenance costs him 28,000 EUR excl. VAT/year, with 3 to 4 production stoppages per quarter. He opts for a phased modernization at 78,000 EUR excl. VAT over 7 months (strangler pattern), with no downtime. From the first post-project year, maintenance falls to 11,000 EUR excl. VAT and stoppages to a single one. Recurring annual saving: 17,000 EUR excl. VAT, plus the value of production hours saved. The project pays back in a little over four years on maintenance alone, much faster once productivity is factored in.
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FAQ
Should you rewrite everything at once or in stages?
In stages 8 times out of 10. The strangler pattern replaces modules one by one and spreads the budget over 2 to 4 quarters, keeping the app in production throughout the transition.
How much does the upfront audit cost?
Expect 4,000 to 9,000 EUR excl. VAT for a serious technical audit (code mapping, debt, dependencies, migration plan). It is the best investment to avoid budget surprises.
Can we keep the existing database?
Often yes, at least during transition. Data migration represents 6,000 to 25,000 EUR excl. VAT depending on volume and quality; a clean database sharply reduces this item.
What is the concrete gain on hiring?
Huge. Hiring on a modern stack (TypeScript, React, Node) takes on average 2 to 3 times less time than on jQuery/PHP 5, and reduces technical team turnover.
How fast does it pay back?
On maintenance alone, 3 to 4 years generally. Including production hours saved and faster delivery, often 18 to 30 months.
Let's scope your project. Tell us about your current app (tech, number of screens, integrations, data volume) and your indicative budget: we will propose a costed phased modernization plan. 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.
