Rewriting a legacy .NET Framework app in London: 2026 cost

The verdict in three sentences

An internal .NET Framework 4.x application still running on Windows Server 2012 or 2016 is not doomed, but every year without a decision raises the operational risk and the cost of the eventual exit. If the business process is stable and the code readable, a progressive migration to .NET 10 (25,000 to 60,000 EUR in France, 2026 ballpark) almost always beats a rewrite. If nobody understands the business rules any more and the interface slows field technicians down, a full rewrite (40,000 to 90,000 EUR, 55,000 to 120,000 EUR in London) pays off over 3 years.

Why 2026 is the year to decide

.NET Framework 4.8 stays supported as long as the Windows version hosting it is. That is where the calendar tightens: Windows Server 2012 R2 left support in October 2023 (paid Extended Security Updates run to October 2026), and Windows Server 2016 loses extended support on 12 January 2027. A 2012 application also tends to depend on dead components: WebForms, WCF, Crystal Reports, SQL Server 2012, abandoned NuGet packages. The day the server has to change, the application breaks or blocks the upgrade of the whole infrastructure.

The second risk is people. In a London or Marseille maintenance or services group, the original developer has usually left, and the IT services firm in charge of keeping the lights on bills every fix like an investigation. Here are the signals seen in audits (2026 market estimates):

The three options costed over 3 years

The audit always comes first: 3,000 to 6,000 EUR for 8 to 15 days analysing the code, dependencies, database and actual usage. It produces a map of business rules and a reasoned recommendation. Without it, a rewrite quote is a blind guess.

Market rows are estimates, not Kolonell prices. The Kolonell row reflects its 11/10/2026 grid for a complex application (7,000,000 to 15,000,000 FCFA and up, i.e. 10,670 to 22,870 EUR and up) and its maintenance plans at 75,000 to 120,000 FCFA a month.

A progressive migration follows the so-called strangler pattern: a facade sits in front of the old application, then modules are replaced one by one (scheduling, job sheets, invoicing, reporting), starting with the most painful. The application stays in production throughout. A full rewrite starts from a blank page with a modern web and mobile interface, but it demands careful data migration and a 4 to 8 week period of parallel running.

How to choose without getting it wrong

Go for migration if more than 60% of the business rules are still right, the database is healthy and the interface suits office users. Go for a rewrite if technicians need an offline mobile app, if the data schema has been bent out of shape over the years, or if the code no longer compiles without a former employee's machine. In both cases, insist on deliverable increments every 2 to 4 weeks and acceptance testing by field team leads, not just by IT.

Mini case study

Illustrative example: Laurence, IT director of a multi-trade maintenance group in London, runs a 2012 WebForms application used by 140 technicians and 12 planners. Keeping it alive costs her 22,000 EUR a year, and re-keying between the application and the invoicing tool keeps 2 planners busy half-time, around 40,000 EUR of annual payroll.

She picks a rewrite at 70,000 EUR with a mobile app for job sheets, then maintenance at 9,000 EUR a year. Over 3 years she spends 97,000 EUR. The status quo would have cost 66,000 EUR of upkeep plus 120,000 EUR of re-keying, 186,000 EUR in total. If the new application removes 75% of the re-keying, savings reach 90,000 EUR over the period and the project pays back in about 20 months.

FAQ

Can a WebForms app be migrated straight to .NET 10? No, WebForms does not exist in modern .NET: the interface must be rebuilt in Blazor, React or Razor Pages. Business logic and data access, however, often port at 50 to 70%.

How long does a modernisation audit take? Plan 8 to 15 working days for an application of 50,000 to 200,000 lines of code. The deliverable includes a dependency inventory, a business rule map and an estimate within plus or minus 20%.

Should we move to the cloud at the same time? It is not mandatory. Azure or OVHcloud hosting costs 150 to 600 EUR a month for this type of application, but an on-premise Windows Server 2025 machine remains an option if your constraints require it.

What if the source code is incomplete? It happens in roughly 1 project in 5. The audit then decompiles the binaries and observes the production database, which adds 3 to 5 days and makes a rewrite more likely.

How do we avoid the tunnel effect? Require a demo every 2 weeks and a first module in production before month three. A project with no real release after 90 days drifts in 2 cases out of 3.

Let's scope your project. Send us your module list, user count and server version: we scope the audit, then price migration or rewrite on our complex application grid, from 7,000,000 FCFA (10,670 EUR). Detailed quote within 48 h. WhatsApp +221 77 596 93 33.