HERITAGE bosses will today launch a £1.5 million appeal to save crumbling Mackintosh gem the Hill House.

Charles Rennie Mackintosh and his collaborator wife Margaret MacDonald created the one-of-a-kind residence for publisher Walter Blackie in the early 1900s.

However, the Helensburgh landmark has suffered more than 100 years of weather damage as a result of the experimental Portland cement used to render the building.

Urgent efforts are under way to raise the funds needed to allow the sandstone structure to dry out and allow experts to carry out the conservation work needed to secure both the long-term integrity of the structure and preserve its priceless interiors.

Both artists produced original pieces for the project, delivering furniture, textiles, decorative panels and more.

The National Trust for Scotland (NTS) has already secured £3m towards the cost of installing a weather-proof “box” around the property, which is expected to be in place throughout the next winter.

Construction on the cover is pencilled in for a June start, but NTS needs another £1.5m by the end of spring to allow the project to proceed as planned.

Every NTS member will receive a letter asking them for support. Clydesdale Bank has donated the printing plate used to produce the £100 note – featuring Mackintosh that it released in 2009 – for auction later this year, with proceeds going towards the Hill House Appeal.

A specimen banknote will also go under the hammer in what NTS calls one of the most “ambitious and important” fundraising drives in its history. Richard Williams of NTS said: “We have very limited time to get the box in place around the Hill House and start the process of drying the building out.

“It’s imperative that we secure the funds as soon as possible, to begin construction and protect the building from further degradation.”