ThERE is something humbling about the thought of embarking on a building project that will take 250 years to complete. In these days of instant gratification, such monumental patience seems inconceivable.

Thankfully, our medieval forbears were made of sterner stuff. Two hundred and fifty years is almost exactly how long it was between the beginning of attempts to raise funds for to improve Archbishop Thomas of Bayeux's Norman cathedral which already existed in York, and the completion of York Minster as we know it today.

Sarah Brown of English Heritage - whose York Minster, An Architectural History 1220-1500 has just been published - says that for two and a half centuries, much of the daily business of liturgy and worship at the Minster would have taken place against a backdrop of scaffolding and half-built masonry, as the new cathedral gradually took shape out of the old.

Piece by piece, each new phase of the mammoth construction project replaced a part of the earlier building, which had itself been completed in the early 12th century.

The reason such a huge project was possible is that for generations the builders of the great cathedral were driven by a force more potent than earthly ambition. They were able to take the long view because what they were building was a representation of heaven on earth.

The medieval concept of heaven was of a heavenly Jerusalem, a place filled with buildings made of jewels and with walls of glass. The Minster was an attempt to capture that in earthly form. "It was a foreshadowing of heaven, a foretaste of the splendours that were to come," says Sarah.

It seems ironic that an important new book chronicling such an achievement should have come out at the very time it has been agreed that visitors to York Minster are to be charged for the privilege.

Sarah, an English Heritage employee to her fingertips, refuses to be drawn on that, other than to say the controversy the charges have caused indicates how close to the hearts of York people the great building remains.

In that, at least, the York citizens of today are similar to their ancestors.

The Minster was probably never seen by ordinary people as their principle place of worship. That would have been their parish church.

"But it was a very inescapable and enormous presence in their lives nevertheless," says Sarah.

"The south transept stands at one of the key points in the town, and there would have been the great processions, such as at Corpus Christi and to celebrate the cult of St William. It would have imposed itself on the minds of York people."

This process would have been gradual, given the time it took to build the Minster. It is not difficult to imagine generations of ordinary people going about their lives in the shadow of the great, unfinished Minster, its beauty slowly emerging.

Such was the scale of the project it is probable that when work began on reconstructing the transepts of Archbishop Thomas of Bayeux's existing Norman cathedral in the late 1200s, none of those involved had any intention of completely rebuilding what was already a great church.

There may have been an element of church politics behind that initial phase of rebuilding. Although already one of the great cathedrals, York suffered from the lack of a home-grown saint.

In the 1220s, the York Chapter began attempts to have York's former Archbishop William Fitz Herbert canonised. In 1223, a sweet-smelling oil was said to have flowed from William's tomb at the east end of the nave. On March 1226, William was officially declared a saint.

It is possible that work began on rebuilding the transepts of the existing cathedral as a way of glorifying St William and promoting his 'cult'. Certainly, too, the fact the former Archbishop had been canonised saw the offerings of pilgrims and income from the sale of 'indulgences' swell the fabric fund. Who says financial considerations at the Minster are a new phenomenon?

The rebuilt south transept was being brought into use by the early 1240s, while work on the north transept was completed in the 1250s. Next, towards the end of the 13th century, came the building of York's chapter house. By 1291, work had begun on rebuilding the nave - work which was to continue for 60 years, until 1350; a huge achievement in itself.

By this time, the piecemeal rebuilding of the Minster might have taken on a momentum of its own. There was possibly a sense that in comparison to the magnificent new transepts and nave the choir itself, once so widely acclaimed, was no longer splendid enough.

"There clearly was a point at which a decision was made that we have started, we might as well finish," Sarah says. "They have decided that they want the whole thing to be of a piece."

Work was delayed, however. The arrival of the Black Death brought social and economic chaos, from which not even the Minster was immune.

It was Archbishop John de Thoresby who finally kick-started the redevelopment and enlargement of the Minster's entire eastern arm, including the new Lady Chapel and the choir. Progress was checked not only by the plague, but by financial constraints, Scottish wars, political upheaval, local rebellion and even the indifference of various archbishops. Nevertheless, by 1405, the east window was ready to receive its stained glass.

Then, in 1407, disaster struck. The Minster's central tower, built in the 13th century and described as "lofty and delectable to see", came crashing down. The pope apparently attributed it to a "horrible tempest" but the King of England, perhaps having better information, blamed the carelessness of masons.

Because of a gap in the Minster's fabric rolls it is not clear when work on rebuilding the central tower began, but by 1470 the tower as it now is was nearing completion, and the great cathedral was finally consecrated on July 3 1472.

To this day, however, visitors still comment on the 'unfinished' appearance of the central tower. It is possible that the original intention may have been to include another storey, similar to the tower at Durham cathedral.

It may even be possible that there were at one time plans for a spire, probably of timber. "There is reasonably strong documentary evidence that the thirteenth century tower had some kind of timber spire," she says. "We're not talking of something like Salisbury, which is stone, but some sort of timber, lead-covered spire."

We may never know. But it is interesting to imagine how different the shadow cast across the city by the Minster at its heart could have been, if this great stone representation of heaven on earth had been capped by a finger pointing upwards towards God.

York Minster: An Architectural History c 1220-1500, by Sarah Brown, is published by English Heritage price £65.

Updated: 10:37 Monday, June 30, 2003