The verdict in three sentences
Modernizing a legacy business app costs in 2026 between 60,000 and 200,000 EUR for a Toronto mid-market firm, with a 6-to-12-month timeline depending on size and data migration. The recommended method is the progressive rewrite (strangler pattern): you replace module by module, without stopping the tool, to avoid big-bang risk. The return comes from productivity (+15 to +30 %) and the end of technical debt that costs every year in bugs, slowness and security.
How much a legacy rewrite costs in 2026
The cost depends on the number of modules, data complexity and the big-bang vs progressive choice. Here are the 2026 orders of magnitude.
| Scope | Approach | Timeline | Budget EUR |
|---|---|---|---|
| Small tool (5-8 screens) | Direct rewrite | 4-6 months | 60,000 - 90,000 |
| Medium tool (15-25 screens) | Progressive strangler | 6-9 months | 90,000 - 140,000 |
| Critical tool (30+ screens) | Strangler + migration | 9-12 months | 140,000 - 200,000 |
| Multi-module system | Progressive in batches | 12-18 months | 200,000 - 350,000 |
Data migration is an underestimated item: budget 10,000 to 40,000 EUR depending on the quality and volume of historical data to clean and transfer.
Big-bang vs progressive: the cost of risk
Replacing a critical app all at once is tempting but risky. The table compares the two approaches on a 140,000 EUR project.
| Criterion | Big-bang | Progressive (strangler) |
|---|---|---|
| Time to first value | 9-12 months | 6-8 weeks |
| Service outage risk | High | Low |
| Cost of a failure | 100 % of budget | Only the current module |
| User training | All at once | Spread out, better absorbed |
| Estimated extra cost | Baseline | +8 to +12 % |
| Recommendation | Avoid | Preferred |
Progressive costs 8 to 12 % more but divides risk: if a problem occurs, only the current module is affected, not the whole system.
Mini case study
Marc runs production at an industrial mid-market firm in Toronto. His internal tracking tool, built in 2012, crashes regularly and costs the team 8 h/week (manual searches, re-entry, Excel exports). Technical debt costs him, per his IT lead, about 35,000 EUR/year in fixes and lost time.
He launches a progressive rewrite at 140,000 EUR over 9 months, with data migration. From month 3, the first delivered module removes 3 h/week of re-entry. Ultimately, the team gains +22 % productivity on the process, i.e. the equivalent of ~50,000 EUR/year, and the 35,000 EUR/year debt disappears. The project pays back in about 20 months.
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FAQ
Should we rewrite everything or keep the existing?
Rarely everything: you keep what works and replace the painful modules first. The strangler pattern precisely lets old and new coexist during the transition.
How much is data migration?
Between 10,000 and 40,000 EUR depending on volume and data cleanliness. Dirty historical data (duplicates, inconsistent formats) can alone represent 20 to 30 % of the migration budget.
How do we quantify current technical debt?
By adding lost time (bugs, slowness, re-entry), annual fixes and security risk. For many mid-market firms, this exceeds 30,000 to 60,000 EUR/year, often invisible in the accounts.
Is progressive really less risky?
Yes: each delivered module brings value immediately and a failure only touches a limited scope. The 8-12 % extra cost is profitable insurance on a critical tool.
How long before feeling a first benefit?
With the strangler, the first useful module ships in 6 to 8 weeks, versus 9 to 12 months for a full big-bang.
Let's scope your project. Tell us about your current tool, its critical modules and the volume of data to migrate, and we will frame a costed progressive 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.
