The verdict in three sentences
A legacy application nobody dares touch already costs you more in maintenance than a phased rebuild would. In 2026, budget 40,000 to 150,000 EUR for a full rebuild, or 8,000 to 20,000 EUR per increment with a strangler pattern that keeps production alive. The real question isn't *how much* but *how fast technical debt is eating your IT budget*.
Big-bang or progressive migration: the real trade-off
Big-bang rewrites everything then switches over at once: cheaper on paper, but maximum risk on go-live day. Progressive migration (the strangler pattern) replaces modules one by one behind a facade: slower, but each increment ships and is tested in isolation.
| Criterion | Big-bang | Progressive migration (strangler) |
|---|---|---|
| Total 2026 budget | 40,000 - 90,000 EUR | 60,000 - 150,000 EUR |
| Cost per increment | N/A | 8,000 - 20,000 EUR |
| Timeline | 4 - 6 months | 6 - 9 months |
| Go-live risk | High (full cutover) | Low (increment by increment) |
| Production during work | Frozen | Continuously running |
| Rollback | Hard | Per module |
For a business-critical application used daily, the strangler pattern is the default choice: you never shut down the tool that runs the company.
The cost of inaction: technical debt compounds
Doing nothing isn't free. An unmaintained application sees operating costs climb around 30% per year: slower fixes, obsolete dependencies, unpatched security holes, developers who refuse the project.
| Line item | Year 1 (status quo) | Year 3 without rebuild | After rebuild |
|---|---|---|---|
| Annual maintenance | 18,000 EUR | ~30,000 EUR | 9,000 EUR |
| Average time per fix | 2 days | 4 - 5 days | 0.5 day |
| Major incident risk | Moderate | High | Low |
| Data migration | — | — | 3,000 - 12,000 EUR |
| User training | — | — | 1,500 - 5,000 EUR |
Data migration (cleanup, mapping, transfer) and user training are too often missing from the quote: they represent 10 to 20% of the total budget and determine adoption.
Mini case study
Thomas, IT director at a 90-person industrial SME in Berlin, is stuck with an in-house ERP written twelve years ago. Maintenance: 26,000 EUR/year and two days for the smallest change. He opts for a 6-increment strangler pattern averaging 14,000 EUR each, for 84,000 EUR over 7 months. From increment 3, his annual maintenance drops to 11,000 EUR. Recurring saving: 15,000 EUR/year, rebuild paid back in just over five years, before counting avoided incidents and restored security.
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FAQ
Should we rewrite everything or keep some existing code?
Keep what's stable and documented; rewrite what blocks you. An upfront audit of 2,500 to 6,000 EUR settles this module by module and avoids needlessly rewriting 100% of the code.
How long before we see a first result?
With the strangler pattern, the first usable increment ships in 4 to 8 weeks. Big-bang shows nothing before the final cutover, at 4 months minimum.
What happens to production during the rebuild?
In progressive migration it runs without interruption: new modules replace old ones behind a facade. That's the decisive argument for a critical application.
How do we avoid recreating the same technical debt?
Automated tests, living documentation and a maintenance contract at 15% of build cost per year. Without planned upkeep, the new app becomes legacy again in three to four years.
Does the budget include data migration?
Not always: demand a dedicated line (3,000 to 12,000 EUR depending on volume and data quality). A botched migration sabotages adoption more surely than a bug.
Let's scope your project. Tell us about your legacy application, its code volume and your uptime constraint, and we'll propose a costed big-bang or strangler scenario. 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.
