A "Grade I listed building" is a designation in the United Kingdom that identifies structures of exceptional architectural or historical interest. This classification is part of a system that categorizes buildings into three grades: Grade I, Grade II*, and Grade II, with Grade I being the highest level of protection. These buildings are legally protected, meaning any alterations or demolitions require special permission from local authorities. Grade I listed buildings can include a variety of structures, such as castles, churches, and historic homes. The listing helps preserve the nation's heritage and ensures that significant architectural features are maintained for future generations.