When fire spread through Glasgow School of Art in May, it was obvious from the tears of the students and staff who stood on the street watching the flames how important the building was to them.

In the days that followed, it also became clear how important Charles Rennie Mackintosh's masterpiece was to the country and to the world of architecture and art; in 2009, the Royal Institute of British Architects named it as the most important building of the last 175 years.

Within days of the fire, however, the despair had been turned into positive action and a campaign to restore and rebuild had begun. The Mackintosh Appeal was launched by the Glasgow School of Art Development Trust in June, just a matter of days after the fire, and it promptly attracted trustees including the actors Brad Pitt and Peter Capaldi.

The aim of the appeal is to raise the money that will be needed to restore the building, including the precious Mackintosh library, and the good news is that it has already been boosted by some large and generous donations.

One of the big donors is The Hugh Fraser Foundation, which has been a long-time supporter of the art school, and has promised £300,000 towards the restoration work.

Another £250,000 has been given to the appeal by the musical theatre producer Sir Cameron Mackintosh, who joked that he had always taken great pleasure in having a namesake as famed and illustrious as Charles Rennie Mackintosh. His more serious intention is to make a considerable contribution to the fund needed to progress the work. Sir Cameron said he was doing so because of his interest in architecture but also his personal admiration for the GSA's designer.

"His legacy as an artist and designer," he said, "and his unique style and invention lived on through the amazing building he designed for the Glasgow School of Art."

Sir Cameron's honest tribute, and his donation, are welcome, and will bring the fund close to achieving its target. Although £20 million will be needed in all to restore the building, both the Scottish Government and the UK Government have promised £5m each towards the total.

But it is not just the big donations that matter. As well as the millions from Sir Cameron Mackintosh, the Hugh Fraser Foundation and government, £150,000 has also been received in individual donations.

These donations, while not as big as others, have come from the network of former students, fans of architecture and admirers of Mackintosh who were shocked by the fire but are now determined to help the building recover. Their contribution proves how important the building at the top of the hill is; it is a work of art in itself and those who love it are well on the way to restoring its greatness.