A "Grade I Listed" building is a designation in the United Kingdom that indicates a structure of exceptional architectural or historical interest. This classification is part of a system that categorizes buildings into three grades, with Grade I being the highest. These buildings are protected by law, meaning any alterations or demolitions require special permission from local authorities. Grade I Listed buildings can include a variety of structures, such as historic homes, churches, and monuments. Examples include famous sites like Westminster Abbey and St. Paul's Cathedral. The listing helps preserve the nation's heritage and ensures that these important sites are maintained for future generations.