HM Treasury Green Book

The HM Treasury Green Book is the UK government's definitive guidance on how public-sector interventions should be appraised, evaluated and managed through delivery. It sets out the Five Case Model for business cases (Strategic, Economic, Commercial, Financial and Management), the approach to social cost-benefit analysis, the treatment of optimism bias, and the expected rigour of quantitative risk analysis supporting investment decisions. Any programme seeking public funding of material scale will have its business case and supporting analysis tested against Green Book expectations.

For project controls practitioners, the most directly relevant sections are those covering optimism bias and risk management. The Green Book requires quantified optimism bias uplifts on capital cost estimates at early stages of project development, with the uplift reducing as the project matures and genuine uncertainty resolves. It also sets the expectation that QRA will be structured to produce defensible confidence levels (typically P50 for baseline and P80 for funding envelopes), with the methodology transparent and the assumptions documented.

The Green Book works in tandem with the IPA's project assurance framework and departmental-specific guidance. A business case that complies with Green Book methodology, paired with a QRA that meets AACE International standards and an IPA Gateway process that tests delivery confidence, is the canonical UK public-sector project assurance stack. Programmes that attempt shortcuts — an underpowered QRA, missing optimism bias, or a business case that skips Green Book discipline — will typically encounter those shortcuts at Gateway review or at National Audit Office scrutiny later in delivery.