The King has commissioned a new exhibition telling the story of how he transformed the gardens of his country home Highgrove – including behind-the-scenes sketches of Princes William and Harry’s treehouse.

The then-Prince of Wales had the wooden hideaway, with its pointed, thatched roof, built for his young sons at his Gloucestershire retreat in the 1980s.

The Tree House design by Willie Bertram
The Tree House design by Willie Bertram (The Prince’s Foundation/PA)

Designs of the den, drawn by architect Willie Bertram, have gone on show at the Highgrove in Harmony: A Royal Vision display at the Garrison Chapel in London.

They include a notation from Bertram about the tall climbing pole designed for the daredevil brothers, which points to one of the wooden broomstick steps high in the tree and reads “Should these go up as high as this?”

A holly leaf-shaped fence surrounds the house and the drawing from 1988 is labelled “Tree house at Highgrove ‘Holyrood House’ for The Princes William and Harry” – a playful reference to the Palace of Holyroodhouse in Edinburgh which was then their grandmother the Queen’s official residence in Scotland.

Prince’s Foundation exhibition at the Garrison Chapel
Exhibition curator Rosie Alderton uses a rope swing created by George Richardson from a log from the Highgrove Estate woodpile (Victoria Jones/PA)

Charles refurbished the original treehouse for his eldest grandson Prince George in 2015.

The exhibition also includes never-before-seen graphite sketches by Charles of Highgrove’s Kitchen Garden and its Thyme Walk.

He acquired the 18th century countryside retreat and estate near Tetbury in 1980, when it had only a kitchen garden, an overgrown copse, some pastureland and a few hollow oaks.

Prince’s Foundation exhibition at the Garrison Chapel
Artwork in a personal sketchbook of the King depicting his country residence, Highgrove House, ahead of the Prince’s Foundation exhibition (Victoria Jones/PA)

A passionate gardener, he has spent more than 40 years devoting his energy into transforming the gardens around the house – which serves as his private home with the Queen Consort.

Another drawing by architect Charles Morris of a bench he designed for Charles is circled in red pen and features an annotation from the now-King saying “I like this one”.

Exhibition curator Rosie Alderton, of The Prince’s Foundation, said: “The special thing about this exhibition is the newly uncovered archival material, particularly photographs and design plans.

Prince’s Foundation exhibition at the Garrison Chapel
Curator Rosie Alderton adjusts a glazed ceramic Zillij tile panel by Layali Arabiyat. the tiles surround the fountain in the Carpet Garden at Highgrove House (Victoria Jones/PA)

“Anyone can see evidence of the amazing progress of the site by visiting Highgrove Gardens, but having the sketches, notes and correspondence, much of it by His Majesty’s hand, that informed that work is a unique selling point.

“The exhibition was His Majesty’s concept. He wanted to tell the story of Highgrove and how much work has gone into it.

Prince Of Wales Launches Coronation Meadows Initiative
The King in the meadows at Highgrove House (Chris Jackson/PA)

“Highgrove Gardens is a reflection of His Majesty’s philosophies and the exhibition gives a great insight into his thinking, including how he has tried to create a haven for wildlife as well as a garden that will inspire visitors.”

Highgrove in Harmony: A Royal Vision is now open.

Entry is free and it will run from Monday to Friday at the Garrison Chapel until the end of May.