Terraform does not support project-level Billing Budgets (only billing-account API is available)

[mygosha]

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Description

GCP supports creating project-level budgets using the billing.resourcebudgets.* permissions.
In the Google Cloud Console UI, a user with Project Owner (but without Billing Account roles) can successfully create a budget scoped only to a project
However, the Terraform resource google_billing_budget only supports the Cloud Billing Budget API, which exposes only billing account–scoped endpoints: POST /v1/{parent=billingAccounts/*}/budgets.
This means Terraform always requires billing.budgets.create on the Billing Account even when the budget is supposed to apply to single project.

This requires Billing Account-level permissions such as billing.budgets.create, even when the intention is to create a budget for only a single project. As a result:

• UI -> works with only project-level permissions such as billing.resourcebudgets.write
• terraform -> fails with 403 unless the caller has Billing Account IAM roles

This creates a mismatch between the capabilities of the UI and terraform. Many organizations restrict billing Account IAM roles for security reasons so they cannot use terraform to manage budgets in the same way that UI allows.

Feature request: add Terraform support for project-level budget creation, using the same API path the UI uses.

New or Affected Resource(s)

google_billing_budget
(potential new resource) google_project_billing_budget

Potential Terraform Configuration

resource "google_project_billing_budget" "project_budget" {
  project = var.project_id
  display_name = "Project Budget Example"

  amount {
    specified_amount {
      currency_code = "EUR"
      units         = 100
    }
  }

  threshold_rules {
    threshold_percent = 0.5
  }
  threshold_rules {
    threshold_percent = 1.0
  }

  all_updates_rule {
    monitoring_notification_channels = [
      google_monitoring_notification_channel.alerts.id,
    ]
  }
}

This resource should use the project-level API and respect permissions billing.resourcebudgets.read, billing.resourcebudgets.write, resourcemanager.projects.get instead of requiring Billing Account IAM roles.

References

https://docs.cloud.google.com/billing/docs/how-to/budgets#create-budget
https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/billing_budget
https://docs.cloud.google.com/billing/docs/reference/budget/rest/v1/billingAccounts.budgets