Yes, you can sit on them...

Belsay Hall, Northumberland, sits in the heart of an area that has been nonconformist since the early 17th century. So perhaps the locals will be willing to embrace its current display of designer garden retreats - or, in the local dialect, 'sitooteries'.

The lovingly sculpted landscape that has coddled Belsay Hall in Northumberland for two centuries has become a kind of verdant Petri dish to encourage radical mutations on an old theme. Are we talking gazebos? Or summer houses? Or garden features? None of the above, because at Belsay between May and September, we're in the realm of the designer garden retreat.

The lovingly sculpted landscape that has coddled Belsay Hall in Northumberland for two centuries has become a kind of verdant Petri dish to encourage radical mutations on an old theme. Are we talking gazebos? Or summer houses? Or garden features? None of the above, because at Belsay between May and September, we're in the realm of the designer garden retreat.

Twelve structures - only nine are ready, with Norman Foster's contribution promised for June - were commissioned at just £10,000 a throw. The architects and designers involved were asked to deliver small-scale meditations on the perception of landscape to be known as (cue Billy Connolly voice) sitooteries; places to sit oot in, ya ken.

The range of the structures is remarkable, veering from a tiny Romanesque church clad with shimmering reflective discs - Elvis has entered the building, a chapel of lurve - to a cheeky, and flawlessly made, mahogany periscope straight out of a Tellytubbies merchandise set which might have been customised for Harrods.

But the course of design ne'er quite runs true, and there is, alas, one temporary blot on the copybook of what is otherwise an admirably engaging and provocative project co-curated by English Heritage's Judith King. It concerns a "hairy box" designed by Thomas Heatherwick - he of The Pretzel That Ate Harvey Nicks. His sitooterie, quite the most extreme structure in the English Heritage-organised collection, is in effect a blue plywood cube pierced with 5,500 elm wands to deliver an effect that is almost punkish.

The box has been refused its originally offered showstopper of a site on the south-west corner of the ha-ha which overlooks the swales that dip and then rise into the dense blue and pink-rinsed bouffants of Belsay's famous rhododendron garden. Heatherwick's wind-cypher - the elm wands tremble in the breeze, transmitting elemental vibrations into the box - is to be hoiked into a new temporary position which will undoubtedly lessen its impact on the event's 1 May opening day.

Perhaps this otherwise unfortunate wrinkle is in some way apt, part of the continuum at Belsay Hall which has been informed with nonconformity in matters of faith and aesthetics since the early 17th century. By 1640, the Puritan master of Belsay, Thomas Middleton, had raised hackles by entertaining "unconformable ministers"; and then had the nerve to show two leading Scottish Covenanters around the defences of Newcastle.

The development of Belsay's landscape showed an equal individuality and riskyness, beginning in 1795 when Sir Charles Monck's ideas for a lush and occasionally rocky sensurround revealed a thirst for the rugged, rather than the manners-maketh-land rhythms of Brown and Repton. It was a case of new labour, new aesthetic - and one tied to the Picturesque movement, which favoured artists such as Poussin and Claude because they gave great crag.

And crag is what Tania Kovats gives in her rocky love seat, a cracked-open white cube whose interior looks like randomly weathered rock strata. The cube sits rather politely in the utterly beguiling quarry garden, simultaneously stark and yet in some mysterious way self-effacing under the huge shoulders of sandstone, above which the medieval village that supported Belsay's original castle once stood. Kovats has obviously realised that the quarry - skeined with pelts of clinging fern and toupees of ivy, its stone clutched by the skinny-fingered rootery of precariously poised trees - cannot be trumped. Her idea was to create a place of "coquetry and conversation"; but the love seat feels much more like a plain and simple echo of the quarry, a place to sit quietly rather than to murmur to a paramour. Further along the path, the design collective Fashion Architecture Taste (Fat) has pushed its sensual envelope to the shimmering limit. Elvis was out, but the lightshow was on. The structure is set among pines, and the thousands of blue and silver sequins pinned loosely to its exterior wooden panels pick up every filigreed movement of the breeze; and on the south-facing side, a delightful surprise: the light of the afternoon sun reflected and scattered like handfuls of coloured marbles across the ground.

Fat has delivered something that may shock some of the natives (one of Belsay's ground staff was reported to have uttered dark words); but there's no doubt that the chapel is an utterly genuine and hypnotically effective mirror of its environment, a dazzling figment whose radical artifice is both up for it and yet perfectly passive.

Equally reflective, though in a very different way, is Julian Opie's smoked glass and steel sitooterie, topped with an illuminated yellow flat-topped roof. It looks like a combination of a toytown petrol station and a customs barrier. But really it's a simple maze, with panels of glass fixed to the grid of 16 steel uprights so that, once inside, the eye is constantly engaged by the actual, the reflected and the doubly reflected vistas beyond; a nicely ramifying idea carried out with a distinctly arch drollness: there must have been many other less industrial ways for Opie to achieve these overlaid transparencies.

And then, from Kraftwerk to craft. Welfare State International's charming Wishbone House on Belsay's west lawn, an arch resembling a giant willow-laced bonnet locked together by a knobbled spine at its apex and two laminated oak wishbones fore and aft. There's a charming whiff of Thor Heyerdal and Rogers & Hammerstein about this beautifully made piece and, with an entirely fitting eccentricity, the intention is to hold the occasional betrothal, or what WSI calls "rites of passage", within its finely striped shadows.

Michael Anastassiades' mahogany periscope is something of an achievement in joinery terms: solid sections of turned oak, with two curved top sections, the last of which contains a small spotlight which is triggered by sound. Only one problem: the ambient birdsong and wind noise is pretty tremendous, so its "subtle glow" will only tend to appear at night when there's nobody around to see it.

A second piece by him - a small white pillar down which a glaze of water flows - is charming. But why isn't it three times wider? The shallow scoop at the top of the pillar, from which the water cascades like melting cellophane, presents the onlooker with a delicately lucent ellipse of water which seems to hang in space. Such luscious effects can prompt an unseemly, gagging-for-it greed; this is minimalist eye-candy for quietists with a taste for controlled hallucinations.

Which takes us back to Thomas Heatherwick and the hairy box, which deserves particular attention because - despite its apparent Anarchy in the UK aspect - it poses deep questions relating to structure and texture. He came up with the idea while thinking about ways to add layers of texture to the outside of tower blocks: remove the hard edges, blur the mundane surfaces with a kind of mohican haircut.

The hairy box is remarkable because it is not mounted; it rests on its elm wands. Entered by a ramp, the sitooter is not only surrounded by thousands of nipples formed by the internal stubs of elm, but even sits on extended wands. The whole structure is therefore sprung, unified by its cubic core and the spiky halo. And this is the big idea it broaches: when does internal form become external texture? It looks like Heatherwick has set a question that is its own answer, and when it's finally on its proper site it will be a wonder. And Judith King insists that the structure will be moved to its preferred site once the ground has dried enough to allow its transport over carefully protected tracks and grass, which are currently squelchy.

There is, finally, one more rabbit that can be pulled out of the hat devised by Judith King. Simon Watkinson, a local artist, has created a brilliantly witty dumb-waiter installation in a two-storey void in Belsay Hall that would, under more Gothic circumstances, be inhabited by an ashen clutch of Steerpikes. An electrically driven system of wires and pulleys keeps two bizarre curtains in constant up-and-down motion past the fireplaces gaping open-mouthed into space. One curtain is made of hundreds of pieces of steel cutlery - wind-tines; the other is a drape of clacking Wedgewood crockery.

One wonders what the ghosts of Thomas Middleton and those Scottish Covenanters would have made of it all. Perhaps they would have approved, given that they had a taste for the radical and the unconformable.