“Costs and benefits should be calculated over the lifetime of the proposal. Proposals involving infrastructure such as roads, railways and new buildings are appraised over a 60 year period. Refurbishment of existing buildings is considered over 30 years. For proposals involving administrative changes a ten year period is used as a standard measure. For interventions likely to have significant costs or benefits beyond 60 years, such as vaccination programmes, or nuclear waste storage, a suitable appraisal period should be discussed with and formally agreed by the Treasury at the start of work on the proposal.”
The Green Book (2022, p. 9)