Monstrous stone gargoyles keep vigil over an abandoned Gothic mansion. Do they know the answer to the riddle of why it was never finished?

For some people it is the all-time white elephant, both useless and insatiable. For others it is a priceless architectural jewel. Without question it is unique.

I refer to Woodchester, the Gothic mansion tucked deep among the woods in a valley near Stroud. Begun in 1858, abandoned unfinished in 1871, never completed, adorned by monstrous stone gargoyles, it remains an amazing memorial to the religious enthusiasm of its creator, William Leigh.

A successful merchant and, in middle age, a convert to Roman Catholicism, Leigh set out to create a building that would be not only his own home, but also a replica of a medieval monastic community. Having quarrelled with his first architect, Augustus Pugin (who also designed the House of Commons), he put the project in the hands of a young local man, Benjamin Bucknall, who had been strongly influenced by that master of Gothic revival, Viollet-le-Duc.

The result was an immense edifice of pronouncedly Gothic character with the heavy buttresses and steeply pointed arches one would expect in a church, built almost entirely of stone, and incorporating a chapel, a bakery and a brewery. Such was the scale of it, so fine the work of the masons, that rumour claimed Leigh planned it as a sanctuary for the Pope, who at one stage had been driven from the Vatican by revolutionary pressures.

After an initial three-year spurt, construction slowed, and doddered on for another decade before coming to a halt. Why it stopped, we shall never know. Leigh, living on the rim of the valley only a few hundred yards away, must have gone down a thousand times to see how his new home was progressing, and it must have been with a heavy heart that he called a halt.

Did he run out of money? Was he worried by declining health? Why did his builders leave wooden scaffolding, a ladder and even some of their tools on site?

The roof was on, but many of the windows remained unglazed. For more than a century and a quarter the wind has whistled through the great, gaunt shell - and, paradoxically, the fact that so much air goes through has helped preserve it. Nevertheless, by 1986 rain had started to penetrate the roof, and if Stroud District Council had not stepped in to buy it from the offshore trust which owned it, parts would have collapsed.

Now the building is in the care of the Woodchester Mansion Trust, a band of dedicated volunteers who rent it from the Council for a nominal sum and open it to the public on selected weekends in summer. By raising more than pounds 20,000 a year, to supplement grants from the Council and English Heritage, they have been able to carry out preliminary repairs.They have also installed electricity and running water, and made a few rooms usable for meetings; but their aim is to restore the entire structure.

The cost will be astronomical - pounds 3 million at least - but the Trust, undaunted, recently appointed a practising architect, Robert Stiling, as appeals manager. He aims to attract sponsorship from firms keen to have their names associated with the house, and hopes that day courses will become acceptable as events for people pursuing their own Continued Professional Development.

The mansion is of burning interest to architects, for its unfinished interior displays Victorian building methods in a way that can be seen nowhere else. It is also used as a centre for masterclasses in stone working techniques.

Yet a visit leaves one astonished by the house's impracticality. Although Bucknall prescribed numerous ventilation shafts, he made no provision for central heating except in the chapel. The single lavatory is high up on the second floor. There is one bath, carved from a single block of stone. The kitchen is tiny, and a day's march from the dining room.

Luckily these deficiencies cause no inconvenience to the four rare species of bats which inhabit the brewery at the back of the house during summer and autumn. So highly thought-of are these, the only residents, that English Nature pays the Trust about pounds 1,500 a year as rent for their lodging.

Enquiries to Robert Stiling, Woodchester Mansion Trust, Old Town Hall, Stroud, Glos GL5 1AP (01453 750455).