Economic costing

Annual nominal costs from the financial case were summed and discounted (by 3.5%) in accordance with HMT Green Book methodology and principles, to provide a Present Value of Costs over a 10-year horizon (2024/25 to 2033/34). These were also presented as Additional Present Value of Costs relative to business as usual. Each of the non-BAU options will incur additional costs. Option 2 resulted in the lowest additional cost and Option 3 (NDPB model) resulted in the highest additional cost – £5.2 million and £16.0 million, respectively, across a 10-year period at 2024/25 prices.

The economic costings presented above reflect the position at the time the Outline Business Case was completed. Since that point, more detailed financial modelling has been undertaken and costs have been updated as part of the Financial Case in this Full Business Case.

The Financial Case updates the cost estimates for both the business‑as‑usual and executive agency options. While the executive agency option continues to have higher costs than business as usual, the level of additionality has reduced compared to the position set out in the OBC.

Although these updated figures would change the precise numerical outputs of the economic costings, they would not alter the overall conclusions of the Economic Case, including the relative ranking of options or the preferred way forward.