"Grade I listed" refers to a classification in the United Kingdom that designates a building or structure of exceptional architectural or historical interest. This status is part of the UK's system of listing, which aims to protect and preserve important sites. Only about 2.5% of all listed buildings receive this highest grade, highlighting their significance. Buildings that are Grade I listed are subject to strict regulations regarding alterations and renovations. This ensures that their unique features and historical value are maintained for future generations. Examples of Grade I listed structures include historic castles, cathedrals, and notable landmarks.