Building Chastleton House in north Oxfordshire 400 years ago nearly bankrupted its original owners. But, ironically, the family's poverty has given the building an integrity the National Trust intends to respect and preserve
Should ancient buildings be restored to perfection or merely prevented from falling apart? The restoration of Chastleton House, in north Oxfordshire, marks something of a philosophical watershed in the workings of the National Trust. In the six years since the country's largest charity acquired the Jacobean jewel it has had to give serious thought to its approach to the preservation game.

How was it to retain the sense of gentle decay and tranquillity which is the magical essence of Chastleton, without the fragile interior collapsing under the weight of sensible shoes tramping the stately home circuit? And how was it to stop physical rot turning to dereliction without an "as new" restoration?

The Trust has come in for criticism over recent years for the veneer of "sameness" that spreads insidiously across its properties. The tweedy "room-sitters" watching over the public; the tea rooms, the polished woodwork and faithfully reproduced gilding, along with the ubiquitous oak leaf and acorn badge, had become the abiding memory of too many visits. But at Chastleton, which has just opened to the public, the trust has broken the mould of sameness and successfully carried out a sensitive restoration programme.

The unchanged nature of the place is the result of poverty. When Irene Whitmore-Jones reluctantly admitted the public in the late 1940s, she would excuse the shabby state of her 400-year-old home by telling visitors the family had lost its money "in the war". They naturally assumed she meant World War II. But the mistress of Chastleton was talking of the Civil War, 300 years earlier, when her ancestors' support for the Jacobites brought fines and impoverishment.

Time has moved like a barely perceptible zephyr through the oak-panelled halls. Rain and frost have eroded the yellow-ochre Cotswold stone, and beetles have bitten and burrowed into almost every chair leg, board and joist. Yet if Walter Jones, the wealthy wool merchant who built Chastleton, were to return down the winding lane he would recognise all the salient features.

The five-storey stone house stands close by a 12th-century church. No rambling pile, it is compact and symmetrical, imposing but not over-bearing. Architects hadn't arrived in the 17th century and the designer of Chastleton is not known, though experts believe it could have been Robert Smythson, who drew up Hardwick Hall in Derbyshire.

"We all agreed that the great thing about this place was it had never been improved, restored or `done-over'," says Richard Wheeler, the Trust's Thames and Chilterns region land agent. "Nor had it ever been owned by rich people, except at the beginning, and, even then, building Chastleton nearly bankrupted them. We have tried to preserve the spirit of the place rather than the bricks and mortar, which would end up a wooden and mannerist approach."

None the less, major work was required on the fabric. The house and park were bought for pounds 1m by the National Heritage Memorial Fund from the widow of Professor Alan Clutton-Brock, an art critic and direct descendent of Walter Jones. Only a fortnight after the Trust took over, a large part of the plaster ceiling in the stunning Long Gallery collapsed. Rain was pouring in through the roof, the leaking gutters and downpipes had damaged exterior stonework, structural timbers were damp and rotting, and woodworm and beetle were rampant. Poverty had saved the classic Jacobean garden from the landscaping mania of the "Capability" Browns of the 18th century but not from a reversal to rampant nature over decades of neglect.

Long periods of relative poverty obliged the Joneses to adopt a pragmatic approach to redecorating and Trust specialists have followed their example. The oak panelling has been patched and cobbled together down the centuries, so gaps have only been filled where the missing piece of the jigsaw has been found elsewhere in the house. "Although the family made enough reasonable marriages to cover the cost of occasional repairs, they never made one good enough to really improve it," Richard Wheeler explains. Just as the Joneses might have smartened up a room when funds allowed, so the Trust has removed a skim of gloss paint from the ornate White Parlour and restored an earlier colour while leaving other corners of the house peeling and shabby.

The repairs have cost pounds 3.2m but it has been a skilful, invisible mend. Some 95 per cent of the Westmorland slates were re-used, fixed with new oak pegs. Remarkably, a colony of pipistrelle bats remained undisturbed. Elsewhere, floor joists were repaired and fabrics cleaned of layers of dirt and cat hairs. In the barrel-vaulted Long Gallery the plaster was drilled to within 6mm of the surface and stainless steel rods inserted to secure the plaster to the rafters.

Seventy-two feet long, the gallery was a 17th-century "work-out" room. Shuttlecocks and marbles were found beneath the floorboards during two years of delving and research before repairs to the house began. The plaster ceilings and friezes in the Gallery and the Great Chamber are thought to have been the work of the same craftsmen who decorated several Oxford colleges. The inverted forest of bosses on the ceiling of the Great Chamber is exuberant, to put it kindly. Great globules of plaster hang down to greater lengths than anywhere in the country. Critics Sherwood and Pevsner thought the effect barbaric - "uninhibited by any consideration of insipid good taste".

For students of textiles, Chastleton contains relics which in any wealthier home would have been stripped away. On a landing wall is a section of Dornix weave, a universal wall hanging in 17th-century houses. It looks remarkably modern, the equivalent, perhaps, of a fussy Laura Ashley wallpaper and something that any owner with money in the past would have obliterated.

Much more striking is the "flamestitch" hanging which covers the wall of a closet adjoining the ornate Fettiplace bedroom - named after Anne Fettiplace, who married Walter Jones's son, Henry, in 1609. "Flamestitch", so called simply because of its blazing geometric pattern, used 12 colours sewn to a jute backing. It was a common fabric of the 17th century and the pattern was revived by the Victorians. So fragile is the Chastleton flamestitch that the three-month task of conservation and cleaning had to be tackled in situ - cleaning each strand of woollen thread by hand with a needlepoint microvac. The gloom of the closet has probably helped preserve the freshness of the colours, particularly the deep blue, while the geometric pattern smacks more of the era of Conran than Cromwell. The tiny room is believed to be the only one in Britain still hung with flamestitch, though, in keeping with the Jones's frugality, the panels may have been re-used from bed-hangings.

Chastleton had early brushes with history - the house it replaced was bought in 1602 from Robert Catesby who later hatched the Gunpowder Plot, and in 1651 "Cavalier" Arthur Jones hid in a secret room from Cromwell's soldiers. But, for 300 years after that, nothing happened in this backwater apart from the codifying of the rules of croquet on the lawn in 1865. (Three years later, Walter Whitmore-Jones, author of the rules, won the first croquet championship.) The lawns, flower beds, and topiary will be restored as the seasons allow.

Martin Drury, the Trust's director general, has made the "light touch" approach to building conservation something of a crusade. Visitor numbers will be restricted to 25 every half-hour. There is no tea room and the car park is hidden behind trees a 200m walk from the house. Converted to the new religion, conservators even ruminated on the idea of putting the gardeners' derelict wooden "crappers" back to use as visitors' toilets.

A tricky question remains over the kitchen. Apart from the cast-iron range installed in 1828, little changed from the 17th century to the room's abandonment in 1952. There is a range decked with pewter vessels and little else. According to tradition bad luck will follow if the soot-blackened ceiling is ever repainted. The Trust is taking no chances with fate on the ceiling, but staff are still agonising over how to present the otherwise dismal basement. A brace of stuffed pheasants hanging from the rack and a whole new philosophy could be undone

Chastleton House and garden is open to the public on a pre-booked, timed ticket system: Wednesday to Saturday, between 1pm and 5pm, until the end of October. Telephone for tickets, 01608 674284

Rich tapestry (clockwise from top): cleaning of the `flamestitch' hanging took three months, by hand; the Old Kitchen has hardly changed since the 17th century; the Sheldon Room with its 16th-century bedstead and heraldic overmantel;

post-conservation detail of a bedroom tapestry