When you acquire a company, you’re also acquiring its ERP — complete with its 347 customizations, undocumented workflows, and that freelance consultant who’s the only one who understands half the configuration. Yet IT integration remains the poor relation of most external growth operations.
According to McKinsey, approximately 70% of mergers fail to deliver expected revenue synergies (McKinsey — Where mergers go wrong). IT integration consistently ranks among the major causes of these overruns. Even more telling: in the CIO Sentiment Survey 2024 by EY, only 32% of CIOs report significantly achieving technology synergy objectives during an acquisition.
This guide decodes the strategies, pitfalls, and realistic timeline for consolidating two ERPs after an acquisition.
Why ERP is the #1 priority in post-acquisition integration
ERP as the backbone — what happens when two backbones coexist
The ERP structures the company’s critical processes: orders, production, billing, accounting, payroll. When two companies merge, their respective ERPs embody two different business logics — two chart of accounts, two item catalogs, two approval workflows, two customer repositories.
As long as these two systems coexist without integration, the group functions like two separate companies sharing a shareholder. The promises of synergies (group purchasing, logistics pooling, cross-selling) remain theoretical.
Financial stakes: accounting consolidation and group reporting
Financial consolidation is the first deliverable expected by management and investors post-closing. The group must produce consolidated P&L and balance sheet, eliminate intercompany transactions, and report according to group standards (IFRS for listed companies, local standards for others).
Without integrated ERP, consolidation relies on manual extractions, Excel mapping files between chart of accounts, and weeks of processing. Each monthly closing becomes a project in itself.
The time factor: every month of coexistence costs money
The coexistence of two ERPs generates direct costs: double licensing, double maintenance, double support team. It also generates indirect costs often underestimated: manual reconciliations, re-entry errors, inability to manage the group in real-time, extended closing deadlines.
According to the EY report on M&A integration costs, IT costs rank among the top three integration expense items, alongside HR restructuring costs and real estate. The longer integration takes, the higher the bill.
IT due diligence — what to audit before closing
IT due diligence is as important as financial due diligence. Yet it often remains superficial, reduced to a license inventory and a hastily drawn architecture diagram.
Target IT landscape mapping
The audit must cover the entire IT landscape, not just the ERP:
- ERP: vendor, version, deployed modules, number of active users, level of customization
- CRM: Salesforce, HubSpot, ERP-integrated CRM? Clean or polluted customer database?
- Business tools: MES (manufacturing), WMS (logistics), CMMS (maintenance), BI
- Infrastructure: cloud, on-premise, hybrid? Hosting contracts, expiration dates
- Integrations: supplier EDI, banking connectors, third-party APIs, inter-system flows
Assessing technical debt and migration complexity
An ERP’s technical debt is measured by three indicators:
- Customization rate: ratio between custom code and vendor standard. Beyond 30% custom, migration becomes a development project, not configuration.
- Data quality: customer duplicates, orphaned items, unused accounting accounts. Data migration typically represents 30 to 40% of total migration effort.
- Documentation: does it exist? Is it current? Who maintains it? An undocumented ERP is a risk of dependence on key individuals.
IT red flags that should alert the acquirer
Certain signals should trigger alerts during the due diligence phase:
- End-of-life ERP (unsupported versions of SAP R/3, old Sage systems, legacy Navision) — migration cost must be factored into valuation
- Absence of technical documentation — you’ll inherit a black box
- Wild customizations not version-controlled — impossible to distinguish standard from custom
- Single ERP administrator in the company — critical person risk
- No documented business continuity plan (BCP/DRP) — in case of incident, no Plan B
- Non-compliant licensing — a vendor audit post-closing can be very expensive
The 3 ERP consolidation strategies
Strategy choice depends on three variables: the relative size of the two companies, the state of their respective ERPs, and the degree of convergence of their businesses.
Strategy 1 — Absorption: the acquirer imposes its ERP
The acquired company migrates to the acquirer’s ERP. This is the fastest strategy when conditions are met.
When to choose it:
- The acquired entity is significantly smaller
- Its ERP is obsolete or end-of-life
- The businesses are similar (same sector, same processes)
Advantages:
- Single platform to maintain — reduced costs once migration is complete
- Rapid process and reporting harmonization
- Economies of scale on licensing
Risks:
- Resistance to change from the acquired company (“they’re imposing their system on us”)
- Loss of business specificities that created value for the acquisition
- Migration project conducted parallel to business integration — team overload
Typical timeline: 6 to 12 months for an SME, 12 to 18 months for a mid-market company.
Strategy 2 — Managed coexistence: two ERPs with integration layer
Both ERPs remain in place, connected by middleware (MuleSoft, Talend, Boomi) or a common data warehouse. Group reporting is fed by both sources.
When to choose it:
- Both entities are comparable in size
- Very different businesses (industry + services, B2B + B2C)
- The acquirer wants to preserve the acquired company’s operational autonomy
Advantages:
- Less disruptive — each entity keeps its processes and teams
- Rapid deployment of consolidated reporting (3 to 6 months)
- No risk of failed migration
Risks:
- Double licensing and maintenance cost, with no end date
- Growing integration debt (every new flow = new connector)
- No deep operational synergies (purchasing, logistics)
- Difficulty attracting and retaining IT talent (nobody wants to maintain two systems)
Typical timeline: 3 to 6 months for initial integration layer. But beware: “temporary” coexistence tends to become permanent if end date isn’t set from the start.
Strategy 3 — Replatforming: a new common ERP
Both existing ERPs are replaced by a common platform, often as part of modernization (cloud migration, adoption of next-generation ERP).
When to choose it:
- Both ERPs are obsolete or inadequate for the new group’s size
- The merger is an opportunity for global digital transformation
- The group has financial and human resources for a long project
Advantages:
- Start fresh on a healthy foundation, without inherited technical debt
- Choose the best ERP for the consolidated group (not the biggest one’s)
- Opportunity to redesign business processes (best practices from both entities)
Risks:
- Long project: 18 to 36 months minimum
- High cost: double migration (two ERPs → one new)
- Risk of getting bogged down if transformation project is poorly scoped
- Both teams must learn a new tool simultaneously
Typical timeline: 18 to 36 months. Budget often exceeds the sum of two individual migrations.
Typical timeline for post-M&A ERP consolidation
Regardless of chosen strategy, post-acquisition IT integration follows three phases. According to PwC’s M&A Integration Survey, successful integrations begin integrating initial functions within three months post-closing and complete everything within 18 months maximum.
Phase 1 (D+0 to D+90) — Stabilize and connect
The objective isn’t to merge ERPs, but to produce minimum viable group reporting.
Priority actions:
- Connect both accounting systems for consolidation (even if via Excel initially)
- Implement intercompany flows (shared service charges, management fees)
- Harmonize group chart of accounts (mapping between both repositories)
- Secure access and rights (who has access to what in the acquired IT system?)
Key deliverable: first consolidated closing within 90 days.
Phase 2 (M+3 to M+12) — Harmonize priority processes
This is the phase where synergies begin to materialize.
Priority actions:
- Unify critical repositories: customers, suppliers, items, commercial conditions
- Consolidate purchasing (single supplier negotiates for the group)
- Harmonize most impactful processes: order-to-cash, procure-to-pay
- If absorption strategy: start migration of acquired entity
Key deliverable: unified purchasing and sales processes, measurable synergies.
Phase 3 (M+12 to M+36) — Consolidate on target platform
The final phase aims for complete unification — or the conscious decision to remain in coexistence.
Priority actions:
- Complete migration to target ERP (if absorption or replatforming)
- Decommission old system (or freeze coexistence with maintenance contract)
- Train teams and stabilize processes
- Measure actually captured synergies vs acquisition business plan
Key deliverable: one group, one IT system (or documented and budgeted coexistence architecture).
The 5 fatal errors in post-M&A ERP consolidation
1. Underestimating IT due diligence
Discovering after closing that the target’s ERP runs on an unsupported version, with 200 undocumented customizations, is equivalent to discovering hidden debt on the balance sheet. Except technical debt doesn’t appear in any financial report.
2. Wanting to merge everything in 6 months
The impatience of general management and investment funds is understandable — every month of coexistence erodes synergies. But forcing an ERP migration in 6 months when the project requires 18 guarantees budget overrun, data quality degradation, and massive user resistance.
3. Forgetting change management for the acquired company
Employees of the acquired company are already experiencing shock: new management, new strategy, uncertainty about their position. Imposing an ERP change without support transforms a daily work tool into a symbol of lost autonomy.
4. Ignoring local regulatory constraints
A group that acquires in Germany, Italy, and the UK inherits three different tax regimes and electronic invoicing requirements (GoBD/ZUGFeRD, SDI, Making Tax Digital). The target ERP must include localizations for each country — a criterion too often discovered during migration phase.
5. Not budgeting temporary coexistence
Even with an absorption strategy, there will be a coexistence period. Double licensing, double maintenance, double support, middleware integration costs. If this budget isn’t planned in the acquisition business plan, announced synergies are overestimated from day one.
Sector examples
Manufacturing industry: harmonizing bill of materials
When two manufacturers merge, the ERP challenge focuses on product bills of materials (BOMs). Two sites can manufacture the same product with different codifications, distinct manufacturing routes, and incompatible units of measure. BOM harmonization is a prerequisite for any purchasing consolidation or production planning.
An automotive parts manufacturer acquiring a competitor will need to map thousands of item references before being able to consolidate supplier orders. Without unified ERP, each site continues ordering separately — and purchasing synergies remain on paper.
Services and consulting: consolidating staffing and billing
In service companies (consulting, IT services, engineering), ERP manages staffing (consultant assignment to missions), time entry, and time-based billing. Two consulting firms that merge must unify their skills repository, pricing grid, and billing processes.
Main risk: losing visibility on consultant availability during transition, which directly translates to lost revenue (consultants “on the bench” because the system doesn’t make them visible to managers from the other entity).
Distribution: unifying item catalog and pricing
A distributor acquiring a competitor inherits a second product catalog with its own item codes, descriptions, categories, suppliers, and pricing conditions. Item repository unification is the most voluminous project — and most critical, as it conditions group purchasing, pooled logistics, and consistent pricing toward common customers.
Which ERP for a group in external growth?
Post-M&A target ERP choice depends on the group’s profile:
| Profile | ERP to consider | Why |
|---|---|---|
| Industrial mid-market multi-site UK | SAP S/4HANA, Unit4 Business World | Native consolidation, UK localizations, production management |
| Mid-market group multi-country | NetSuite, Microsoft Dynamics 365 F&O | Multi-entity cloud, EU localizations, native intercompany |
| SME in acquisition series | Sage Business Cloud, Access Dimensions | Modularity, controlled cost per entity, rapid deployment |
| Diversified holding (heterogeneous businesses) | Coexistence + common BI (Power BI, Tableau) | No one-size-fits-all, consolidate through reporting |
The integrator’s role is crucial. A specialized M&A integrator brings experience in time-constrained migrations and coexistence — a different skill from a classic integrator deploying ERP on greenfield.
For more on related topics, consult our guide on multi-site and multi-entity ERP which details IFRS consolidation, intercompany and multi-currency, as well as our complete ERP migration guide for system change methodology. If change management is your main concern, our dedicated practical guide covers the human aspects of transformation.
