Giant stone pineapples, Martello towers, tree houses, windmills, triumphal arches - follies offer escape from the tyranny of polite family homes. But for romantics or bloody fools only. By Jonathan Glancey
Folly comes from the old French, folie or madness, which in turn derives from fou or fool; so a folly is a madhouse lived in by fools. But fools, as we know from King Lear, can be wise, while in a world of creeping suburbia follies can be the best of all places to live imaginative lives.

Follies make a romantic change from spec-built houses smothering what survives of the English countryside in a virulent rash of red brick and fake Tudor beams. They are a romantic alternative, too, to that pot-pourri of super old rectories so lovingly restored; these are the weekend goals of smart middle-class families speeding down motorways on Friday evenings, keen to toy, like latter-day Marie-Antoinettes, at ersatz country pursuits as if born to muck and hay.

Follies, however, are very different from rural Joke Oak and Neo-Geo houses ordered from home-builders' catalogues (twin-garage, double-glazed leaded-style windows, rockery, Hello! on the glass-topped coffee-table and shell-suits in the hall) or super old rectories (Aga, conservatory, kitchen garden, Country Life in the lav and Barbours in the lobby). They are bolt holes, places to hide, not the sort of second homes that require all weekend to heat before the 80mph dash home on Sunday evening.

When we were very young, we dreamED Swallows and Amazons dreams of hiding away in tree-houses in summer or retiring to houses dug in the sides of river-banks in winter. We put tents up in trees and opted out of the adult world for afternoons at a time. We wanted to live in castles drawn from Disney's Sleeping Beauty or Mervyn Peake's Gormenghast. We relished the fact that Big Ears lived in a toadstool, and were fascinated by the idea that Eskimos (the Inuit of our politically incorrect childhoods) lived in houses made of ice (the sheerest folly) and that God lived in a silver chalice in a tabernacle on the high altar draped in horrid fabrics that resembled the sort of house a

Mongolian gnome might find appealing. But God moves in mysterious ways, and his various homes have often been some of the most exquisite and barmy follies of all, from Strict Baptist chapels to Gothic Revival parish churches.

Because they are to do with escape, follies must be as unlike regular family homes as possible. They can be tree houses and seaside cabins, giant stone pineapples (the Landmark Trust has one to rent in Scotland) and Triumphal Arches that would have done the Caesars proud (I stay in such a one in Norfolk); they can be old railway carriages (as at Dungeness), windmills or even new homes built, perhaps entirely, underground - an increasingly popular trend among people who want to live as close as possible to the earth - without appearing to harm it.

The folly must be an adventure when you stay in it. It must make you smile. It's an escape from the tyranny of polite homes, the sort of place that you light with hurricane lamps and candles and where you can sleep outside in a hammock with the stars for company. It must be the sort of home that most people find a little too demanding, so that they do not invade your privacy. It is a place where you can sleep in your clothes on the floor besides the embers of a fire, or, like a friend of mine, soak in a bath-tub sitting in a field.

When I stay in Norfolk, there are no smart people to greet in the morning, only a herd of handsome brown cattle grazing around the folly, following the movement of the sun. By night, owls hoot from the grove of trees the folly stands in. Inside, church candles splutter, provoking the macabre death dance of innumerable moths. A spitting log fire raises the temperature of the water in a tank above the fireplace to a little above freezing. There is no electricity, and gas comes courtesy of heavy blue Calor cylinders kept under the spiral stairs. I sleep the sleep of a child.

By day, farm workers speed across cattle grids and through the centre of the arch at rural (ie breakneck) speed, whilst polite architecture buffs stop by on bicycles, Pevsner in hand, to check the design by Brettingham and Kent.

My friend Nicholas Hills turned the Triumphal Arch into a kind of home some years ago, and it featured spendidly in the very first issue of The World of Interiors; it required then, as it does now, effort and imagination to make it comfortable, but comfort is not really the point. This is a beautiful folly, and not for anyone brought up with central heating, light and hot water on tap. It is also a grand folly in every sense, though far from being the biggest.

Follies have been built on a truly monumental scale - Fonthill Abbey springs to mind, with its fantastic architecture and William Beckford's odd crew of animals, dwarves, whores and Regency heroes - but these transcend the womb-like, hidey-hole of our imagination.

The folly allows us to express ourselves in ways that the spec-built house and restored rectory never can; for those are conventional dreams, with agendas set by middle-class manners. In a folly, you can feast on chocolate and whisky, read to your heart's content, and not bother to see a soul until you get back to work again. You can let go and retreat into who you really are. It's where foolishness and wisdom are richly intertwined.

The original purpose of the Deer Tower - built in the 18th century amid beautiful Sussex countryside - is something of a mystery. Was the folly, as some suggest, a watchtower for the local deerkeeper, a vantage- point for taking rakish pot-shots at the animals below? Was it a comfortable base from which the gentry could enjoy a day's hunting, or stroll through the grounds? Or an "eye-catcher" - folly as feature in the orchestrated landscape?

It could have been all of these, or none. As the present owner, the modern art dealer James Kirkman, says, the name may simply have been "fanciful", based on there being "a lot of deer at the door".

Today, such ambiguity is merely a charming footnote to the building's new use - for Kirkman and his wife, Clare, have indulged in their own folie de batir to transform the Deer Tower and its grounds into a fully fledged new home. "To me it is a building for pleasure," says Kirkman, "a wonderful pavilion in which to relax and have a good time."

The couple have spent more than a decade restoring the building, creating its own magnificent gardens and, with Chichester-based architect Duncan O'Kelly, designing an extension which was completed this year and more than doubles the living space. In so doing, O'Kelly has created something more playful, more grandiose than the Deer Tower ever was.

Built in the 1780s, by the first Earl Winterton on the grounds of his Palladian house on the Sussex/Surrey border, near Haslemere, the original could never have laid claim to being the most picturesque of follies. A small octagon flanked by four turrets, in rendered brick, the Deer Tower's plainness and lack of crennellation deprived it of the essential combination of pastiche with panache. In the 1940s, a redbrick extension on the north side so displeased the owner of the house that he planted a grove of oak trees to hide the folly from view. Later, both Pevsner and the National Trust were to dismiss the Deer Tower as "squat".

By the time the Kirkmans bought it, in 1984, the already small building had been divided into two apartments (among its tenants in the Sixties was the actress Charlotte Rampling). "It was pretty uncomfortable, and there were mushrooms growing inside," remembers James Kirkman. "But we could see that it had a lot of character, and wonderful views." The couple got to work.

An early step was to buy up surrounding land: the Deer Tower now stands in the heart of its own 110 acres of gently rolling countryside, a self- contained idyll circumscribed by trees, containing grazing sheep, a distant river, and - save for the house to which it was once an accessory - not a building in sight. The approach today is via an avenue of lime trees that leads to a modern, sculpted bronze gate, on which perches the cast of a deer skull.

The Kirkmans also saw to the long-overdue crennellation of the towers, while their formal garden is complete with lily-festooned canals lined with yew trees, a gazebo that mocks the mock turrets, and finely tuned box hedging that reveals myriad vistas of the tower and the landscape. All this so elevated the folly in the eyes of English Heritage that it was promptly listed.

But O'Kelly's subsequent extension is the coup de grace. The architect has been sympathetic to the original, while lending the two-storey extension a character of its own in what he none-too-seriously refers to as "Italianate gothic". Essentially a plain box on the east side of the folly, it is brought to life by a large oriel window with a crenellated parapet, and a sandstone render in stark contrast to the drab grey of the turrets.

Inside, he has added to the octagon's three small rooms (dining, drawing and bedroom, one on each floor) and the library and second bedroom in the first extension, with a new entrance hall/dining room and a substantial new drawing room above.

The ground floor is the most pleasurable. O'Kelly wanted the space to have "a monastic feeling, to be more contemplative". Maybe so, but with the internal walls and floor rendered in the same manner as the exterior, the low ceiling strung with chunky wooden beams, a fireplace topped by a block of green oak, and minimal furniture, there is a spartan bleakness that makes one half expect Heathcliffe to stride in with a pack of dogs.

The drawing-room is straightforward, copying the original (now an anteroom) in its articulated cornice and skirting, with an added classical moulding for the architrave and bookshelves. And there is a visual momentum that takes one's gaze out of the projecting window, along the new paved pathway, via the ha-ha and out into the landscape.

O'Kelly's task has combined the practical (underfloor heating, security shutters in the window recesses) and the theatrical - fake bookshelves that reveal secret spaces within the turrets (a room with a bar, a utility room), a whimsical "spiral captain" chimney-pot erupting from the roof, and the use of rich colours in the bar. For their part, the Kirkmans have combined antique furnishings and modern art in a daring, but harmonious, way.

"I tried to be folly-like in my approach, to create some whimsical things, to have a bit of fun," says O'Kelly. "At the same time, what was rather limited is now a fuller place, a real home." The architect appears to have caught the bug. His latest project, a 20,000sq ft out-of-town furniture store has a large brick drum in the middle of the sales floor. Ostensibly a retail area, there's no escaping its cheeky incongruity; a folly in all but name.