The Glasgow School of Art on Renfrew Street was Charles Rennie Mackintosh's greatest building, designed when he was 28 years old and constructed in two phases between 1897 and 1909. The building's north facade — a massive grid of steel-framed studio windows held within a Scottish baronial stone structure — is an extraordinary synthesis of historical and modernist impulses. The library interior, destroyed by fire, was considered one of the masterpieces of European interior architecture.
The building suffered catastrophic fires in 2014 and 2018. Restoration work is ongoing; check the current status of guided tours before visiting. The exterior is fully viewable and the surrounding Garnethill neighbourhood contains several other Mackintosh buildings and connections. The Mackintosh at the Willow tea rooms on Sauchiehall Street, designed for Catherine Cranston and recently restored, is the most accessible experience of his work while the School of Art is under restoration.