Jenny Needham visits George Clark's Kielder Forest treehouse, which proves that accessible design can offer a whole new perspective on the great outdoors

When you rent somewhere to stay, you don't usually have to construct the chairs and tables before you can sit down for supper. But Sky Den isn't your usual sort of place. In fact, it's safe to say it's unique.

The riverside bolthole in the shadow of Kielder Forest was built in 2013 by Sunderland-born George Clarke for the second series of his Amazing Spaces programmes for Channel 4. Now it can be rented out through Canopy & Stars to anyone with a sense of adventure.

The architect and TV presenter visited Kielder as a child on family visits and school trips "It's a real playground and people don't realise how inspiring it is," he says. "I wanted to help put it on the map.

Sky Den was developed in collaboration with his sidekick, designer and problem-solver William Hardie. "We'd never designed anything like this before. It's part-tree house, part art installation, and offers a unique experience," says Clarke.

After finding a site for his dream treehouse, the original concept was that it should be composed of three different shapes balanced high on posts anchored to the riverbank below. A metal walkway leads you to the covered outside porch area where muddy boots and wet jackets can be left, before you open the door to the sitting room.

This is the Square of the building; the Triangle above is the double bedroom, and the Circle on the side is an outdoor corrugated area for sitting and contemplating nature by the little wood burning stove.

On first sight, the Square is surprisingly spartan, a simple cube with window wall and balcony to the riverside, plywood walls and ceiling. No table, no chair, no sofa; a simple sink, little fridge, microwave and hob. On a wet winter weekend there was a temptation to turn and run for home, but then you notice the walls. Like a child's puzzle, there are shapes cut into the ply; circles, rectangles, peg-like pieces. Five minutes later, I have constructed two stools and a bench. Things are looking up.

There are more surprises in this clutter-phobe's paradise of foldaway furniture: the sofa folds out of one wall, the table, with a complicated hinge system, reveals itself from the one opposite. Another section of wall folds down to make a single bed. This is a truly ingenious place. One minute we have a simple square, the next a sitting room.

In little alcoves, hidden cupboards and plywood stacking shelves, we discover melamine mugs, plates, utensils, glasses, a radio ? no reception though, and there is absolutely no phone signal here – lots of board games to play. Before long, we are sitting down waiting for supper to cook, and while it can?t be said that the seating here is for lolling around on, any discomfort is soon forgotten over a bottle of wine and a competitive game of Scrabble.

When it?s time to retire, there is a small bathroom next to the Square, with loo, small sink, and a shower head so the whole room can be used as a wetroom. Then it's up the metal steps to bed - quite a feat, even with wind-up lamps, on a star-free night in one of the darkest places on the planet. (Negotiating your way to the loo in the middle of the night is quite a challenge too!)

The Triangle is the loft space of your already lofty suite. With two (very comfy) mattresses accessed by small steps, it?s reminiscent of camping in a wooden tent. In good weather, when there is no wind, it's possible at the touch of a button to open the whole roof for an unobstructed view of the Northumberland Dark Skies. The night we stayed, there was drizzle and clouds blotted out heaven's nightlights, but to the sounds of the river running underneath and owls hooting we drifted into sleep.

The next morning was misty, autumnal. Downstairs, we drew back the glass doors of the Square and noticed that someone had scattered nuts onto the posts outside to attract birds and squirrels. There was a distinct chill in the air, but we decided to trot round to the Circle, which must make a great picnic area in summer, lit the little stove and brewed up tea the old-fashioned way on top of it. It was soon warm enough to sit and read for a while and between chapters we contemplated this versatile structure which blends together the best of outdoors and indoors.

The conclusion? It?s ingenious, surprising, peaceful, only occasionally uncomfortable and fun, a plywood palace for glamping. And George, it really is an amazing space.

  • Sky Den, Calvet Trust Kielder, Hesham NE46 1BS.
  • Website: canopyandstars.co.uk
  • Low season is £175 a night, high season £195. The den sleeps four with an option of having a double bed and two singles or four singles. There is also space for a baby at no extra charge.

The Calvert Trust

It's not just the Sky Den that makes this place amazing. The structure is set in the heart of the Calvert Trust, within the Kielder Water and Forest Park. The Trust, which has just celebrated its 30th anniversary and recently won Bronze for Access for All and Silver for Self Catering Holiday Provider of the Year at the North-East Tourism Awards, provides accessible and fully inclusive activity holidays to people of all abilities, and has chalets to rent and fantastic facilities. It also provides respite care.

From March 9-12 next year there are taster breaks at £50 per person per night (max two nights) for individuals, parents, carers and professionals, and respite care at £250 per person, and until the end of 2015, the trust is offering serving, reserve or veteran armed forces families with a disabled family member a three-night activity holiday for £50 per person.

Contact the Business Development Team on 01434-250232.

There's no end to the activities you can do here: archery, watersports, rope courses, climbing, orienteering and much more.

The lake and forest are perfect for adventure-seeking families and wildlife lovers. You can hire a bike or just set off on foot to discover England's largest forest and its many woodland trails. Around 50 per cent of England's native red squirrel population and two regular pairs of osprey call it home, and within five minutes of driving into the park on a dark Monday evening we spotted deer, an owl and a fox in our headlights.

  • Website: calvert-trust.org