The verdict in three sentences
Modernizing a 10-year-old legacy web app is budgeted between EUR 60,000 and 150,000 for a progressive rewrite (strangler) and EUR 120,000 to 300,000 for a big-bang, over 4 to 9 months. Doing nothing is not free: legacy maintenance costs 25 to 35 % of its value per year, plus security holes and the loss of the devs who know it. Done right, modernization delivers 30 to 50 % savings on maintenance from the second year.
Cost of modernization strategies 2026
Three strategies, three cost and risk profiles. 2026 ranges (order of magnitude).
| Strategy | Description | Budget (excl. tax) | Timeline | Risk |
|---|---|---|---|---|
| Rehost (lift & shift) | Same code, modern infra | EUR 20,000-50,000 | 1-3 months | low |
| Refactor / strangler | Progressive module-by-module rewrite | EUR 60,000-150,000 | 4-9 months | controlled |
| Rebuild (big-bang) | Full rewrite at once | EUR 120,000-300,000 | 6-12 months | high |
| Data migration | Data reprise and cleanup | EUR 10,000-40,000 | 1-3 months | depends on quality |
| Upfront audit | Mapping, debt, risks | EUR 6,000-15,000 | 2-4 wks | — |
The strangler pattern (replacing module by module while keeping the app in production) is the best compromise for an SMB: it spreads the budget and avoids the tunnel of a big-bang.
The cost of staying vs modernizing
Staying on legacy looks cheap until you add up the hidden costs. Comparison over 3 years for an app valued at EUR 200,000.
| Item | Stay on legacy | Modernize (strangler EUR 100,000) |
|---|---|---|
| Annual maintenance | 25-35 % = EUR 50,000-70,000/yr | 12-18 % = EUR 24,000-36,000/yr |
| Maintenance cost over 3 yrs | EUR 150,000-210,000 | EUR 72,000-108,000 |
| Modernization investment | EUR 0 | EUR 100,000 |
| Security / breach risk | high, uncosted | strongly reduced |
| 3-year total | EUR 150,000-210,000 | EUR 172,000-208,000 |
| 5-year total (projected) | EUR 250,000-350,000 | EUR 220,000-280,000 |
Over 3 years it is close, but modernization takes the lead from year 4-5 and removes security and skills-loss risk. The 30 to 50 % maintenance savings are recurring.
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Mini case study
Sophie, CIO of a mid-market services firm in Toulouse, inherits a 2015 PHP app costing EUR 62,000/year in maintenance and blocking hiring (nobody wants to maintain that code). A EUR 220,000 big-bang is deemed too risky. She picks an 8-month strangler at EUR 105,000, module by module, keeping the app in production. New maintenance: EUR 28,000/year, i.e. EUR 34,000 saved per year. The EUR 105,000 investment pays back in roughly 3.1 years, and she re-attracts devs on a modern stack. She also closed 2 critical security holes found in the upfront audit (EUR 12,000).
FAQ
Which strategy: rehost, refactor or rebuild? Rehost if the code is healthy but the infra is old (EUR 20,000-50,000). Refactor/strangler for most SMBs (EUR 60,000-150,000). Rebuild only if the base is unrecoverable and the budget is substantial.
What does legacy maintenance really cost? Typically 25 to 35 % of the app's value per year, versus 12 to 18 % for a modern app. That is the main hidden cost of the status quo.
Is the strangler pattern slower? It spreads over 4 to 9 months but keeps the app in production throughout, with no tunnel or risky big-bang. It is the best cost/risk compromise for most cases.
Do I need an audit before deciding? Yes: an audit at EUR 6,000-15,000 maps debt, holes and dependencies, and prevents underestimating the rewrite by 30 to 50 %.
When does modernization pay off? The payback point often sits between year 3 and year 4 thanks to 30-50 % maintenance savings, not counting the avoided security risk.
Let's scope your project. Describe your legacy stack, your current maintenance cost and your budget: we'll audit, recommend the strategy (rehost, strangler, rebuild) and cost the savings. 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.
