Delivering the new Green Book vision

PA supported HM Treasury’s 2025 review of the Green Book, with the main recommendations now addressed through this update. The guidance is more streamlined (with 40 percent fewer pages), places greater emphasis on using rounded evidence, adds clarity on place‑based impacts and use of private finance, and integrates more effectively with wider good practice across government (e.g. linking to the evaluation repository).

Sensibly, the fundamentals of public sector business cases have not changed; the “Five Case Model”, the longlist to shortlist process, and the use of wider departmental supplementary guidance all remain the same. For many practitioners, the value added by the updated Green Book comes through a greater emphasis on the holistic appraisal approach many were already applying, rather than introducing anything fundamentally new.

These changes will empower practitioners to deliver better quality analysis. However, our experience of developing major business cases in central and local government highlights that analysis alone is insufficient and achieving the end goal of better decision‑making requires action beyond the steps in the revised Green Book.

There are three important stakeholder groups practitioners need to engage with beyond their immediate project teams to get maximum benefit from the updated guidance and ensure the right outcomes are secured from proposed investments.

1. Wider organisation: Create clearer organisational priorities so business cases can demonstrate stronger strategic fit

The increased emphasis on policy hierarchy in the Green Book update was an area that didn’t make the public headlines but stresses HM Treasury’s expectation on departments and ALBs to set clear top-line targets that can cascade into required outputs. Helpfully the guidance now clarifies that each level of business case does not need to justify the layer above – i.e. if a programme business case has already justified “why” a new hospital is required in an area, then the project business case should focus entirely on identifying the optimal “how”.

Yet too often unclear objectives mean business cases can have existential crises and are forced to be mapped against multiple disconnected strategies, which risks effort gravitating towards what appears locally important, rather than what best supports overall strategic priorities.

Practitioners should push for greater clarity on how their proposed project would fit within the organisation’s priorities. Where such clarity does not yet exist, practitioners can build a bottom‑up view, based on the outcomes the initiative could deliver, combined with ongoing programmes and published strategies. This framework can be validated with leadership to confirm it is the reality they wish to prioritise.

For example, with Smart DCC we reviewed their investment portfolio to create an implied set of priorities to test with senior leads. This transparency led to debate on the right investment hierarchy, resulting in an agreed framework that connected ambition to required outputs and unblocked their business case pipeline. Across the public sector, we see the same pattern: once organisations articulate what really matters, appraisal becomes quicker, clearer and far more purposeful.

2. Approvers: Agree proportionate analysis requirements with decision makers

To minimise unnecessary development costs, the Green Book updates stress proportionality of analysis alongside use of holistic evidence (not just quantitative/monetised impacts) to identify a preferred option. However, for this approach to work in practice, you need buy-in from your approvers. Engage with these stakeholders early to collaboratively set the evidence threshold – in particular, the areas you must have in-depth evidence against as these drive the biggest differences in cost, benefits, and risks. This clarity both helps to accelerate your analysis and de-risks governance.

Consider how much exposure your decision makers have had to this type of options analysis. PA is an accredited provider of HM Treasury’s Better Business Case™ training, and we regularly use aversion we’ve tailored to senior approvers so they have confidence making decisions based on proportionate appraisal evidence. For example, we recently ran business case teach-ins alongside developing a Strategic Outline Case with an ALB in the nuclear sector, on how best to navigate uncertainty, work with broader evidence, manage trade-offs, and make decisions at the point where analysis is “good enough”.

3. Partners: Take people with you, as the analysis alone will not secure a decision

Presenting a polished appraisal within a business case without bringing your key external partners on the decision-making journey is one of the fastest routes to challenge and delay. While the Green Book rightly focuses on analytical rigour, teams can become overly invested in perfecting the model and under‑invest in engaging the people whose support determines whether the end investment is a success.

You may not be able to please everyone but sharing the output from your analysis with key stakeholders, how your team arrived at the conclusions, and the trade‑offs involved builds context and understanding.

In practice, this requires early consultation and more iterative, cross‑organisational working. We have found success in using dedicated business case working groups that bring policy and analyst colleagues together to grow confidence in the evidence base and build ownership of key assumptions, coupled with a senior steering group to agree the wider positioning. For example, when developing a recent Outline Business Case (OBC) for a central government department’s national security investment, our OBC Working Group secured early buy-in across centres of excellence in the appraisal approach which created a more stable, confident path to approval.

Turning the Green Book into impact

The updated Green Book offers clearer backing to practitioners to build arguments based on holistic yet proportionate evidence. Combining this approach with the right engagement across the broader project ecosystem will allow you to convert your robust analysis into the right decision.