Cost of completion describes the amount reasonably required to finish contractual works or services that have been left incomplete or defective, typically in construction, engineering and development disputes. It is used by courts in England and Wales, Scotland, Northern Ireland and Ireland as a measure of damages for breach of contract, rather than being a term defined in statute.Where a contractor fails to perform, the innocent party may claim the cost of engaging a replacement contractor to complete or rectify the works, subject to principles of remoteness, causation, mitigation and reasonableness. Courts may compare cost of completion with other measures of damages, such as diminution in value, and may refuse cost of completion where it would be disproportionate to the benefit obtained or inconsistent with the parties’ commercial expectations.In practice, cost of completion is commonly established by quantity surveyor or expert evidence, and may underpin claims for interim and final account adjustments, termination losses, and surety or performance bond calls. Usage and underlying principles are broadly consistent across the UK and Ireland, though applied within each jurisdiction’s contract and remedies framework.