The Green Book and the Five Case Model: A Plain-English Explainer

If you work on projects, you will hear people talk about a business case. In UK government, business cases are built around the Five Case Model, which sits alongside the Green Book, HM Treasury''s guidance on appraisal. This explainer sets out the idea in plain English, using the current Green Book on GOV.UK.

What the Green Book is

The Green Book describes itself as the UK government''s guidance on appraisal, the process of assessing the costs, benefits and risks of different options for achieving government objectives. It does not set those objectives or make policy decisions. It gives decision makers a framework for the advice they receive. GOV.UK shows the current edition as The Green Book (2026), updated on 28 October 2026 to reflect changes to the approach to discounting, and notes that previous editions are withdrawn.

The five cases

The Green Book''s text says the cases are not five separate business cases but closely interconnected perspectives on the same proposal. In summary:

• Strategic case: sets out the rationale and objectives for the intervention. Why do this at all?
• Economic case: deals with the options for achieving those objectives, and which offers the best value overall.
• Commercial case: describes how the inputs needed to deliver the preferred option will be sourced.
• Financial case: is concerned with affordability for the public bodies involved.
• Management case: sets out the practical arrangements for implementation.

Three stages

The guidance describes a project business case being developed in stages: the Strategic Outline Case first, the Outline Business Case second and the Full Business Case third. The detail of what each stage contains is in the separate business case guidance, which is worth reading if you work on public-sector projects.

Why this is useful outside government

You do not need to be a civil servant to borrow the structure. A project proposal that answers five questions tends to be easier to challenge and easier to approve:

1. Why are we doing this, and what are we trying to achieve?
2. Which options did we consider, and why this one?
3. How will we buy or build what we need?
4. Can we afford it?
5. Who will deliver it, and how will we manage the work?

A common trap

The Green Book notes that its primary purpose is guidance on developing the strategic case and the economic case, and that meaningful options appraisal is generally not possible without some understanding of the commercial, financial and management cases. In other words, the five cases need to be developed together, not left until the end.

Where to read more

The Green Book and the business case guidance for projects and programmes are both on GOV.UK. Guidance changes, so read the current edition rather than relying on a summary, including this one.

This article is general information about published guidance and is not professional advice.