The verdict in three sentences
Migrating from legacy software (Access, VB6, thick client) to a web app costs between €40,000 and €110,000 in 2026 for the rebuild, plus €4,500 to €12,000 for audit and data migration. The database migration alone is €6,000 to €20,000, and a 2-to-3-month parallel run prevents an operational gap. For a London director, the big-bang-vs-strangler call is about business risk, not just cost.
The 2026 migration budget, line by line
| Item | 2026 range | Timeline |
|---|---|---|
| Audit + scoping | €4,500 – 12,000 | 2-4 wk |
| Application rebuild | €40,000 – 110,000 | 12-24 wk |
| Database migration | €6,000 – 20,000 | 3-6 wk |
| Data cleanup / import | included in audit | 2-4 wk |
| Parallel run | keep legacy live | 2-3 months |
| User training | €2,500 – 6,000 | 1-2 wk |
| Developer day rate | €550 – 750 | per day |
Big-bang vs progressive migration (strangler pattern)
| Criterion | Big-bang | Strangler (progressive) |
|---|---|---|
| Disruption risk | high | low |
| Overall timeline | 14-20 wk | 20-28 wk |
| Parallel-run cost | short | longer |
| Legacy upkeep cost | until switch | declining |
| Rollback | hard | module by module |
| Fits when | simple app | critical app |
| Cash flow | single spike | spread |
The strangler pattern replaces the legacy function by function: longer but far less risky for a critical production app.
Mini case study
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James, director of a 50-person trading SME in London, has run an Access application handling quotes, stock and orders for 14 years. The audit prices it: €8,000 audit/import, €78,000 web rebuild, €12,000 database migration, €4,000 training. Total: €102,000. Keeping the legacy cost him €1,500/month in patches and a critical-failure risk estimated at €20,000 for a week-long outage. Over 3 years, keeping the legacy would have cost €54,000 while solving nothing. He opts for the strangler pattern over 24 weeks, with a 10-week parallel run.
FAQ
How much to rebuild an Access or VB6 app? Expect €40,000 to €110,000 by functional complexity. Data migration adds €6,000 to €20,000.
Should we keep the legacy during migration? Yes, a 2-to-3-month parallel run is strongly recommended to secure the cutover without downtime.
Big-bang or progressive? Big-bang is faster but risky; the strangler pattern is safer for a critical app, module by module, with rollback possible.
How long does a migration take? Between 14 and 28 weeks depending on approach and data volume, audit and training included.
What happens to historical data? It's audited, cleaned and migrated. Budget a dedicated import effort: legacy data is often inconsistent and needs cleanup.
Let's scope your project. Describe your current software (technology, data volume, criticality): we'll price audit, rebuild and migration, big-bang or progressive. 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.
