Home Rule in Scotland could not have thrown up a stranger irony. An old mansion, which played a pivotal role in the dissolution of the old Scottish Parliament, now appears to be the main obstacle to the building of the new one.

Queensberry House was the target of the mob's fury at the time of the Treaty of Union in 1707. It was the home of the second Duke of Queensberry, the Act's principal supporter, who was paid #12,325 in bribes to see it through. On the day the treaty was signed the house achieved further notoriety with a grotesque murder.

But the current political nightmare concerns what to do with the mansion - the only historical component incorporated in Enric Miralles's grand design for Holyrood.

Critics, including The Herald and one of Queensberry's former directors when it was run as a geriatric hospital, say it should be pulled down forthwith.

Architect John Spencely, who was called in to review the whole project, agrees. He says spending between #10m and #11m on it is poor value, set against the cost of erecting a new building in its place.

Supporters say it has been made a scapegoat for the spiralling costs of the overall project. Spending on Queensberry House, which lies at the foot

of the High Street in Edinburgh, has doubled, but this has

nothing to the rest of the

project where costs have increased fivefold.

However, the Scottish Parliament project team dismisses any notion of the mansion being discarded. Spokesman Andrew Slorance says: ''Queensberry House will continue to be an integral part of the Parliament complex. You can rule out completely any question

of demolition.''

The original brief involved the use of an enfilade, or series of windows, as a walkway for MSPs. This was changed on the advice of conservation and other bodies. But its function as a centrepiece link with the High Street remains substantially the same. Slorance says it would still be used as an entrance for MSPs and would incorporate

committee rooms and offices.

In its current condition, Queensberry House is an unprepossessing dump. Encased by scaffolding and dwarfed by gigantic cranes, its upper walls and gutters have been reclaimed by weeds and bushes. Even its stoutest defenders do not make their case on aesthetic grounds.

Such beauty as it must have had originally - with its 58 rooms, a 70ft gallery, and elaborate gardens - is somewhat overshadowed by historical gore.

James Grant, chronicler of Old and New Edinburgh, refers to the mansion as a ''huge, dark, gloomy, and quandrangular mass, the scene of much

stately life, of low, corrupt intrigue, and, in one instance, of a horrible tragedy.''

The first duke died there and his daughter suffered a terrible death after accidentally setting herself alight. The second duke's son was ''an idiot of the most wretched kind, rabid and gluttonous as a wild animal''.

When everyone else was at Parliament House on the day the Treaty of Union was signed, he escaped from his room at Queensberry House, seized a servant boy, and roasted him on the kitchen spit. He was still eating the boy when the duke and his household returned.

Novelist Ian Rankin came across the cannibalism story and included it in the latest Rebus book, Set In Darkness. Eerily, during his research he was on a tour of the building at the very time when the probable murder site was being unearthed.

''The day I was there they were taking out an old boiler. Behind everything there was an original fireplace from the kitchen,'' he says.

Madness and debauchery seem to have dogged others connected with the house until it was sold off cheap to the government for #900 in 1803. Then the real vandalism started, with the Board of Ordnance wreaking havoc similar to that caused when it created another barracks in the Great Hall of Stirling

Castle, which has now been restored to its original grandeur.

The current plan for Queensberry is to remove the top

floor added for the barracks and put up a red pantile roof, although others argue that this should be slate.

Very little, if any, of the original interior survives. Some wooden floors will be retained, but others will be concreted.

The project team says the result will be a limited renovation, balancing a measure of historical authenticity with the needs of a modern Parliament.

''We were never in the game of restoring this building. That would be the wrong thing for the Parliament to do,'' Slorance adds.

Either way, it is a compromise. Some argue for a completely fresh start, rather than have a modern Parliament stuck on to a bit of pastiche or ersatz Queensberry House, a former B-listed structure which only achieved category A status in 1998 in a wider review of Edinburgh buildings.

But in a final ironic twist, demolition at this stage would undoubtedly cost more than retention, both in construction and further delays to the project. That would mean a new Parliament with all the technology of the 21st century taking longer to build than Queensberry House itself, which was completed within five years.

Saltire Society president Paul Scott says the seven sackings suffered by Edinburgh over the centuries have actually left standing very little of worth and a unique building such as Queensberry House should not only be saved, but carefully restored.

n New drawings, which turned up after 300 years pasted inside a chest of drawers, may help solve the riddle of what Queensberry House looked like in its heyday.

It is this question which has dogged arguments about its future. None of the original architect's drawings survive. Much has been added or taken away during its later career as a barracks, doss house, and geriatric hospital.

For this reason, John Spencely's independent review of the Holyrood project was critical of the proposals to create a ''conjectural seventeenth-century appearance built around extensively repaired external walls''.

But, remarkably, plans for a similar mansion in the French hotel particulier style in the nearby Cowgate have been discovered in a chest of drawers at Newhailes House, currently under restoration by the National Trust for Scotland.

The drawings by French scholar and military engineer Claude Comiers are dated 1680. Queensberry House took shape a year later and its main block, according to one authority, ''had a French roof, with storm windows in the style of the palace of Versailles.''

Both mansions were designed to celebrate the Restoration of the monarchy in 1660 - and with it the Scottish Parliament. Hope's mansion was never built because he was drowned on the frigate Gloucester in 1682 while escorting the Duke of York to Scotland.

Dr Joe Rock, of Edinburgh University, who published results of the Comiers find, said Queensberry House was important as a unique Scottish model of the hotel particulier style because it

had survived.

''It is an example, however imperfect, of a fascinating desire to emulate French taste and as a repository of national memory,''

he added.