Why

Foundational infrastructure — networking hubs, Active Directory, security tooling, logging platforms — exists because the organisation is in the cloud. No single application “uses” it directly, yet the costs sit in an unowned bucket. Nobody optimises what nobody owns, and these costs grow unchecked. A deliberate allocation strategy creates accountability and prevents mystery charges in Finance reports.

What

Define a dedicated platform cost centre for shared infrastructure, or distribute platform costs proportionally. Publish a documented allocation policy that Finance, Engineering, and FinOps agree on.

How

Identify Shared Infrastructure Accounts

List all accounts, subscriptions, or projects that contain foundational infrastructure. Common examples: networking hub, Active Directory / IAM, DNS / firewall, logging platform, security tooling, SSO / identity services.

Tag or label these with cost-allocation-type: platform to distinguish them in billing data.

Choose an Allocation Model

Recommended: Dedicated Cost Centre (simplest)

Assign all shared infrastructure to a named cost centre (e.g., CC-9000 Cloud Platform). This cost is known, accepted overhead — not split, not argued over, and reviewed quarterly for optimisation.

Dedicated Platform Cost Centre
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  Cloud Platform (CC-9000):   $48,000/mo
    Networking hub:    $12,000
    Active Directory:   $6,000
    Security tooling:  $18,000
    Logging platform:  $12,000

  Use when:
    ✓ Cost is stable month-to-month
    ✓ Every team benefits roughly equally
    ✓ Splitting creates more arguments than insight

  Don't use when:
    ✗ One BU drives 80% of the cost → proportional split

Alternative: Proportional Split — distribute by each BU’s share of total direct spend. Use when one BU dominates consumption of shared services.

Formula: Allocated Cost per BU = Total Shared Cost × (BU Direct Spend / Total Direct Spend)

Implement in Cloud Billing Tools

Step	AWS	Azure	GCP
Define shared	Cost Category → “Shared” value → route shared accounts	Cost Allocation Rule → pick source subscriptions	Label shared projects `cost-type: shared-platform`
Set targets	Other values in same Cost Category	Pick target subscriptions	Define target projects in SQL view
Choose split	Split Charge Rule: Even or Proportional	Even, Proportional, or Fixed %	SQL/dbt model in billing pipeline

Deliverable Checklist

Shared infrastructure accounts/subscriptions/projects identified and tagged
Allocation model chosen and documented (cost centre or proportional)
Cloud-native allocation rules configured
Platform cost centre visible in showback reports
Quarterly review cadence set for platform cost optimisation