• Why Legacy Migration Costs Are Hard to Pin Down

• The Four Cost Categories in Any Legacy Migration

  • 1. Assessment and Discovery

  • 2. Engineering Execution

  • 3. Testing and Quality Assurance

  • 4. Infrastructure and DevOps

• Migration Approaches and How They Affect Budget

  • Big Bang Rewrite

  • Lift and Shift

  • Incremental Modernisation (Strangler Fig Pattern)

• What UK Companies Are Actually Spending in 2026

• The Hidden Costs That Blow Budgets

• The Leadership Cost That Most Budgets Miss

• How to Build a Credible Internal Budget

• What a Structured Migration Engagement Looks Like

• FAQs

Legacy systems have a way of making their costs invisible — until they do not. The bill arrives as a missed product deadline, a failed investor due diligence, or a security incident that exposes infrastructure nobody has touched in eight years. By the time a UK scale-up is seriously pricing a migration, the question is rarely "should we do this?" It is "how much will this actually cost, and what are we budgeting for?"

This article breaks down the real cost drivers behind legacy code migration in 2026, what UK companies are typically spending, and how to think about scoping and sequencing the work before you commit.

Why Legacy Migration Costs Are Hard to Pin Down

Ask three engineering firms for a quote on migrating a legacy system and you will get three very different numbers. That is not evasion. It reflects the fact that migration cost is almost entirely a function of what you are starting with.

A ten-year-old monolith running on-premise with no test coverage and a single engineer who understands it is a fundamentally different problem from a five-year-old SaaS product that needs its data layer modernised. The underlying variables that drive cost are:

• Codebase size and complexity. File count, lines of code, and the degree of coupling between components. Some legacy codebases have individual files exceeding 30,000 lines. That is not unusual. It does mean the assessment phase alone can take three to four weeks.

• Test coverage. A codebase with no automated tests cannot be migrated safely at speed. You either write tests first or you accept the risk of breaking production. Neither is free.

• Documentation quality. Undocumented systems require reverse engineering before any migration work can begin. That adds time and cost before a single line of new code is written.

• Data complexity. Migrating application logic is one problem. Migrating years of production data — often with inconsistent schemas — is a separate and frequently underestimated one.

• Regulatory and compliance constraints. UK financial services, healthcare, and legal technology companies operate under constraints that affect how and when data can be moved, adding compliance overhead to every phase.

• Team continuity. If the engineer who built the original system has left, the cost of understanding it before migrating it increases significantly.

The Four Cost Categories in Any Legacy Migration

Regardless of scope, legacy migration spend tends to fall into four categories. Understanding them separately helps you challenge a quote and build a more accurate internal budget.

1. Assessment and Discovery

Before any migration work begins, someone needs to understand what exists. This covers codebase analysis, architecture mapping, dependency auditing, and identifying the riskiest components. For a mid-sized product, this phase typically runs two to six weeks. It is not optional — skipping it is one of the most common reasons migrations overrun.

A structured Technical Readiness Report, like the fixed-scope assessment WireApps offers, gives you a documented baseline before committing to a full engagement. That document becomes the foundation for every cost estimate that follows.

2. Engineering Execution

This is the largest cost category and the most variable. Execution costs depend on the migration approach chosen, the size of the engineering team, and the duration of the engagement. A team of three to four engineers working over six months will cost materially more than a two-engineer team working over twelve months, even if the total hours are similar — because speed has a price, and so does risk.

3. Testing and Quality Assurance

Migration work without rigorous QA is how you introduce new failures while fixing old ones. Automated test suite creation, regression testing, performance testing, and user acceptance testing all carry cost. For products with live users, this phase is non-negotiable.

4. Infrastructure and DevOps

Moving from legacy infrastructure to cloud-native or containerised environments requires CI/CD pipeline work, infrastructure-as-code setup, and often a parallel-running period where old and new systems operate simultaneously. That parallel period has a direct cost in cloud spend and engineering time.

Migration Approaches and How They Affect Budget

The approach you choose has more impact on total cost than almost any other decision. The three most common options carry very different cost profiles.

Big Bang Rewrite

A complete rewrite from scratch. High risk, high cost, and a long period before the new system reaches production. For most scale-ups, this is the wrong choice. The history of failed big bang rewrites in software is long, and the business risk of running without a working product while the new one is built is rarely acceptable.

Lift and Shift

Moving the existing system to new infrastructure with minimal code changes. Low initial cost, but it does not address the underlying technical debt. You end up with the same problems in a new location. Appropriate as a short-term measure to reduce infrastructure risk — not as a long-term modernisation strategy.

Incremental Modernisation (Strangler Fig Pattern)

The approach most experienced engineering teams recommend. New functionality is built on modern architecture while the legacy system continues running. Components are replaced one at a time. Risk is contained. The business keeps operating throughout.

This approach costs more to plan and sequence correctly, but it dramatically reduces the risk of a catastrophic failure mid-migration. The cost implication is that you are running two systems for a period, which means engineering time and infrastructure spend overlap. Budget for that explicitly.

What UK Companies Are Actually Spending in 2026

Precise benchmarks are difficult because most companies do not publish migration costs. What follows reflects realistic ranges based on scope, not guaranteed quotes.

Small-scale migration (single service or module, under 50,000 lines of code): £30,000 to £80,000. Typically three to four months with a small team.

Mid-scale migration (multi-service monolith, 50,000 to 500,000 lines of code): £100,000 to £350,000. Six to twelve months. Requires senior architectural oversight throughout — not just at the start.

Large-scale migration (enterprise system, multiple integrated platforms, data migration included): £400,000 to £1,000,000-plus. Twelve to twenty-four months. At this scale, the absence of a dedicated technical leader overseeing the programme is a significant risk factor.

These figures assume UK-based or UK-timezone engineering capacity. Offshore delivery can reduce day rates, but introduces coordination overhead that partially offsets the saving — particularly on complex legacy work where context and communication matter.

The Hidden Costs That Blow Budgets

Most migration budgets that overrun do so for predictable reasons. These are the ones to plan for explicitly.

Scope creep during discovery. The assessment phase reveals problems that were not visible from the outside. The honest response is to adjust the budget. The common response is to proceed with the original budget and absorb the overrun later.

Parallel system costs. Running old and new infrastructure simultaneously costs money. Budget for it from day one rather than treating it as a surprise.

Knowledge transfer gaps. When the engineer who built the original system is no longer available, reverse engineering adds weeks. If your lead engineer has recently departed, factor this into your timeline and cost estimate.

Regression and rollback. Even well-managed migrations encounter unexpected failures. Budget for rollback capacity and the engineering time to diagnose and fix issues that only appear in production.

Investor and compliance timelines. If you are migrating ahead of a funding round or a regulatory audit, timeline pressure increases cost. Compressed timelines require larger teams, which means higher monthly spend even if total hours are similar.

The Leadership Cost That Most Budgets Miss

Technical execution is only part of the cost. Someone needs to make the architectural decisions, manage the sequencing, communicate progress to the board, and ensure the migration does not derail the product roadmap.

For a scale-up without a CTO, that person is often the founder. The cost of that is not zero. Founder time spent on technical oversight is founder time not spent on commercial growth, fundraising, or hiring.

The alternative is bringing in senior technical leadership for the duration of the migration. A full-time CTO hire in the UK costs £170,000-plus per year in salary alone, before equity, National Insurance, and benefits. For a migration that runs six to twelve months, that is a significant commitment that outlasts the project.

A Fractional CTO engagement covers the architectural oversight, investor readiness, and technical decision-making for the duration of the migration — without the permanent headcount cost. For scale-ups at the post-seed to Series A stage, this is often the right model. You can read more about what that engagement structure looks like at WireApps' Fractional CTO service overview.

How to Build a Credible Internal Budget

If you are preparing a migration budget for board approval or investor review, the structure matters as much as the number.

Start with the assessment. Do not present a migration budget without a documented baseline. A Technical Readiness Report gives you the specific data points — file count, test coverage percentage, dependency map, risk areas — that make the rest of the estimate defensible.

Separate the phases. Present discovery, execution, QA, and infrastructure costs as distinct line items. A single blended number is harder to defend and harder to manage.

Build in a contingency of 20 to 30 percent. This is not pessimism. It reflects the reality that legacy systems contain surprises. Contingency that is budgeted and not used is a good outcome. Contingency that is needed but not budgeted is a board conversation you do not want to have mid-migration.

Model the cost of delay. Legacy systems are not static. Every month of delay accumulates more technical debt, increases security exposure, and compounds the eventual migration cost. The cost of doing nothing is real, even if it does not appear on a balance sheet.

What a Structured Migration Engagement Looks Like

The 12-week legacy modernisation framework we use provides a practical sequencing model: stabilise first, then modernise, then scale. That phase structure maps directly to the cost categories above. Stability work in the first phase reduces the risk of expensive surprises during execution.

The post-cyber-attack infrastructure recovery case study is a useful reference for what happens when legacy infrastructure fails under pressure. The migration work in that engagement had to happen under active incident conditions, which compressed timelines and increased cost considerably. Planning a migration before an incident forces your hand is always cheaper than responding to one.

FAQs

How long does a legacy code migration typically take for a UK scale-up?

For a mid-sized product, six to twelve months is a realistic range using an incremental approach. A big bang rewrite can take longer and carries higher risk. The assessment phase alone, done properly, runs two to six weeks before execution begins.

What is the biggest reason legacy migrations overrun budget?

Scope discovered during the assessment phase that was not visible beforehand. Undocumented systems, missing test coverage, and data schema inconsistencies are the most common culprits. A thorough discovery phase reduces but does not eliminate this risk.

Should we rewrite from scratch or migrate incrementally?

For most scale-ups with a live product and active users, an incremental approach is lower risk. A full rewrite requires running without a working product for an extended period. The incremental approach costs more to plan but keeps the business operating throughout.

Do we need a CTO to manage a legacy migration?

You need senior architectural oversight. Whether that is a full-time hire, a Fractional CTO, or a senior technical lead embedded in the team depends on your budget and timeline. Proceeding without that oversight risks poor sequencing decisions that increase total cost.

How does AI tooling affect legacy migration cost in 2026?

AI-assisted code analysis can accelerate the assessment phase significantly. A 57-page codebase analysis that would take a senior engineer days can be completed by an AI agent in three hours. That reduces discovery cost and compresses the time to a credible migration plan. It does not replace the architectural judgement required to act on that analysis.

What should a migration budget include that most companies forget?

Parallel infrastructure costs during the transition period, rollback capacity, and the cost of knowledge transfer when the original engineers are no longer available. These three items account for a significant proportion of budget overruns.

When is the right time to start a legacy migration?

Before a forcing event. Migrations planned ahead of investor due diligence, a product scaling milestone, or a compliance deadline are cheaper and lower risk than migrations triggered by an incident or a failed audit. The cost of delay compounds.

The honest answer to "what does legacy code migration cost?" is: it depends on what you are starting with, how fast you need to move, and whether you have the technical leadership to sequence the work correctly. What does not vary is the cost of getting it wrong. A migration that overruns, stalls, or introduces new failures costs more than one that was properly scoped and led from the start.

If you are building a business case or preparing for investor scrutiny, we work with UK scale-ups on exactly this problem. Visit wireapps.co.uk to book a strategy call.