The verdict in three sentences
A legacy application modernization is budgeted between GBP 55,000 and 165,000 in 2026 in London, depending on size and accumulated technical debt. The big-bang rewrite is spectacular but risky: feature freeze, tunnel effect, budget overruns. The strangler-fig approach (incremental modernization over 6 to 12 months) cuts incidents by 40% without ever blocking production.
Big-bang or strangler fig: the real trade-off
Big-bang rewrites everything in parallel then switches over at once. Strangler fig replaces legacy module by module, old and new coexisting until the old code is progressively retired.
| Criterion | Big-bang rewrite | Strangler fig |
|---|---|---|
| Budget | GBP 85,000–165,000 | GBP 55,000–140,000 |
| Duration | 6–14 months | 6–12 months (in waves) |
| Feature freeze | yes (often) | no |
| Tunnel risk | high | low |
| Value return | at the end | from wave 1 |
| Incident reduction | at go-live | progressive, -40% |
For a debt-laden PHP/jQuery app, strangler fig is almost always the rational choice: you keep shipping value during migration.
What technical debt really costs
| Item | 2026 impact | Figure |
|---|---|---|
| Annual maintenance premium | technical debt | +30% |
| Senior developer day rate (rewrite) | London | GBP 550–680 |
| Upfront audit + mapping | 1–3 weeks | GBP 6,000–18,000 |
| Data migration | by volume | GBP 8,000–33,000 |
| Post-modernization incident cut | measured | -40% |
| Maintenance after modernization | annual | 12–15% of build |
Technical debt isn't an abstraction: it's paid every year in +30% maintenance cost, recurring bugs and slow delivery. A 1-to-3-week upfront audit (GBP 6,000 to 18,000) maps the debt and costs remediation before any commitment.
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Mini case study
Sophie, IT director of an industrial SME in London, maintains a 2013 PHP/jQuery production-management app. Maintenance costs GBP 40,000/year, of which about GBP 9,000 is attributable to technical debt alone (+30%). One vendor proposes a big-bang at GBP 130,000 over 10 months with an evolution freeze; another a strangler fig at GBP 112,000 over 9 months, no freeze. Sophie picks strangler fig: first wave delivered in 8 weeks, incidents down 40% by month 4, and maintenance brought to 15% of build (GBP 17,000/year) once migration completes. Maintenance saving: GBP 23,000/year.
FAQ
How much does a legacy app modernization cost in 2026? Between GBP 55,000 and 165,000 depending on size, business complexity and data volume to migrate. An upfront audit refines the range to ±15%.
Big-bang or incremental: which to choose? Incremental (strangler fig) in 8 cases out of 10: no feature freeze, value from the first wave, tunnel risk halved. Big-bang is justified mainly for small apps or a completely obsolete stack.
What incident reduction can I expect? As an order of magnitude, -40% after modernization, thanks to a modern, tested, monitored codebase.
Do I need an audit before costing? Yes. A 1-to-3-week audit (GBP 6,000 to 18,000) maps the debt, avoids surprises and makes the final quote reliable.
Does maintenance really drop afterward? Yes: from +30% caused by debt you move to healthy maintenance of 12 to 15% of build cost per year.
Let's scope your project. Send us your legacy app's scope, its current stack and your priority pain points: we'll propose a wave-by-wave strangler-fig migration plan, costed. 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.
