30 Irreversibility occurs where implementation of a proposal might rule out later investment opportunities or alternative uses of resources. Examples of irreversibility are destruction of natural environments or historic buildings. It is particularly important to make a full assessment of the costs of any irreversible damage that may arise from a proposal.
31 Irreversibility is often associated with facilities on which people place 'option values' (the value of knowing a facility is available to enjoy, if they wish to do so). This is also linked to 'existence values' (the value of knowing that something continues to exist, even if the respondent does not expect to make any practical use of it).
32 Where lead options involve irreversible damage, assessment should include the consideration of options which involve delay, allowing more time for investigation of alternative less damaging ways to achieve stated objectives. Appraisal of different proposals should not ignore the 'option' value of avoiding or delaying irreversible actions, and the benefits of ensuring flexibility to respond to future changed conditions.