In legal practice, a design defect describes a flaw inherent in the way a product, structure or system is conceived, specified or engineered, as opposed to a one-off manufacturing or workmanship error. It typically gives rise to liability where the design causes unreasonable risk of injury, damage or non-compliance with required standards.In product liability, practitioners use “design defect” to identify products made unsafe by their design or by inadequate warnings/instructions. Statute defines “defect” by reference to the safety the public is entitled to expect (Consumer Protection Act 1987 in the UK; Liability for Defective Products Act 1991 in Ireland, implementing the EU Product Liability Directive). Although the legislation does not label “design defect” separately, it is a useful descriptive category. Liability is broadly strict for producers and importers, subject to statutory defences (including “development risks/state of the art”).In construction and professional negligence, a design defect is an error, omission or inadequacy in drawings, specifications or calculations causing failure or non-compliance. Liability generally turns on contractual obligations and the duty to exercise reasonable skill and care, unless a stricter fitness-for-purpose obligation has been assumed (often expressly in design and build). Usage and core principles are broadly consistent across England & Wales, Scotland, Northern Ireland and Ireland (with Scots law framed in delict).