Detrimental reliance describes, in practice, a party changing their position on the faith of another’s representation, promise or assurance and, as a result, suffering prejudice or disadvantage. It is not a statutory term; it is a case‑law and practitioner expression used across multiple contexts.Key legal roles:- Estoppel: In proprietary estoppel (England & Wales, Northern Ireland and Ireland), detriment is an essential element alongside assurance and reliance; relief is fashioned to do justice. In promissory estoppel, the promisee’s reliance must make it inequitable for the promisor to resile; detriment commonly evidences that inequity. In Scotland, the closest concept is personal bar, which requires reliance to the representee’s prejudice.- Misrepresentation and negligent misstatement: the claimant must prove reliance causing loss; “detrimental reliance” is often used as shorthand for this causal and loss requirement.Typical detriment includes expenditure, improving property, foregoing legal rights, or otherwise acting to one’s disadvantage. Reliance must be causative and reasonable (or foreseeable) in context. The concept is used consistently across England & Wales, Scotland (as prejudice for personal bar), Northern Ireland and Ireland, though doctrinal labels differ. Practically, proving detrimental reliance can unlock equitable relief (e.g., an equity under proprietary estoppel or restraint on enforcement) or damages where a tort or statutory cause of action is made out.