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Creating one of the biggest treehouses in the world in the garden of Hogwarts isn’t something we’re asked to do every day.

  • Thumbnail of Alnwick Garden
  • Thumbnail of Alnwick Garden
  • Thumbnail of Alnwick Garden
  • Thumbnail of Alnwick Garden
  • Thumbnail of Alnwick Garden
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General Information

We provided the engineering know-how behind one of the most exciting contemporary gardens created in Europe in the last 100 years – Alnwick Garden in Northumberland.

As well as one of the biggest treehouses in the world, the 12-acre garden wonderland features The Grand Cascade, the largest water feature of its kind in the UK.

It’s the perfect setting for the castle that features as Hogwarts in the Harry Potter films.

Fast facts

The Alnwick Garden Trust
2000 - 2008
Alnwick, Northumberland
North East
Sport & Leisure

Our in-house Design Group carried out the complex structural design and the temporary works for the garden’s massive treehouse.

The 60ft ‘castle in the air’ appears to float around 16 semi-mature lime trees. It comprises a series of buildings supported off the ground with certain elements built off the trees. Overall lateral stability comes from two internal concrete towers, the only non-timber elements of the structural frame.

The centrepiece concrete Grand Cascade is the largest in the UK. Unbeknown to visitors, beneath the attraction is an 80m long, 10m deep underground structure that houses all the plant rooms, storage tanks and access walkways required.

The Grand Cascade is designed to be as environmentally friendly as possible. Three separate systems filter and recycle the 250,000 gallons of water stored underground.

The garden’s treehouse is wheelchair and buggy-friendly. It was built using natural materials from sustainable sources. Throughout construction special measures were taken to protect the trees and we developed a foundation design that transfers structural loads away from the root zone.

The Garden Pavilion and Visitor Centre uses geothermal energy sourced from two 65m deep boreholes and features a rainwater harvesting tank.

A 700m3 rock store beneath the Visitor Centre provides a thermal store for cooling. At night cold air is passed over the rocks lowering their temperature and storing energy. When the buildings are occupied incoming fresh air is drawn over the cool rocks before being circulated.

Since completing the Grand Cascade in 2001, we have worked on a series of successful contracts at the garden including The Garden Pavilion and Visitor Centre, The Serpent Garden, The Poison Garden and The Bamboo Labyrinth.

Close co-operation with our supply chain led to the development of the use of coloured concrete and textured, contoured formwork for the Grand Cascade to replicate the appearance and character of the local sandstone.

Alnwick Garden Pavilion was honoured at the Royal Institute of British Architects Awards in 2008. The judges felt the emphasis on sustainability, which included ETFE and PTFE roofs, complemented the scheme’s careful planning. They said: “This is a complex scheme but it has a clear design vision and is innovative in its use of materials.”

In 2008 the treehouse was highly commended in the Best Access/Disability Regulations Innovation category of the Local Authority Building Control Built in Quality Awards.