The whole composition commands the point between Tarbet Bay and the main loch, midway between the two tiny villages of Tarbet and Kylesmorar, looking due west to the distant Cuillin mountains on the Isle of Skye. You cannot get to this place by car, and it is pretty difficult scrambling miles over the headlands by foot. So building here is not easy. At first glance, you'd conclude that Mackintosh had bought himself some abandoned estate lodge in this remote spot, perhaps last extended by an Edwardian laird, and spared no expense in bringing it back to life. But the real story is much more interesting than that.

Before I went, Mackintosh had briefed me on the history. Of course: this was the site of his earlier, much-loved house which burnt down in November 2000. That was an old gaslit wooden fisherman's croft left to him by his aunt and uncle. He had visited the place since he was a child. Later, by then enriched by the huge success of the musicals he produced, (40 in London alone to date, including Cats and Miss Saigon), he bought up a large tract of the land round about, previously part of the estate of Lord Lovat. It stretches all the way between the sea and freshwater lochs of Nevis and Morar, and westwards to the fishing port of Mallaig where the road and railway both end.

Not that the 13,000 acres of land is much of a money-spinner. This is rocky, inhospitable, unproductive but beautiful terrain with a scattering of sheep and a few crofts. "You can't do very much with it except gaze at it and admire it," says Mackintosh. That is a bit of an understatement - all the locals tell you how much Mackintosh has brought to the area, gradually restoring tumbledown buildings, creating jobs and especially providing new community facilities in Mallaig. But this is certainly a romantic dream for him. It's where he has always escaped to, for 50 years. He had gradually extended the homely old croft, but kept its character and all its memories. Which is why, when it burned down, he was distraught.

Only two chimneys were left standing after the fire. Mackintosh was looking after a production in America. "The only good thing about it was that I wasn't there. I decided not to come back straight away, not to read any of the papers about it," he says. He could have sold up and moved out. He has another large estate in Somerset, and homes in London, New York, France and - a current project - Malta, where his mother's family comes from. But four weeks later he returned. By then the site had been cleared. He stayed with his brothers in the undamaged guest bothy on the hillside above the site of the old house. "I cried for an hour, then it was over," he says. "Then I started thinking: what would I build?"

His first decision was that the new house must contain much of the spirit of the old one. But it would be a bigger, stronger affair, better suited for guests. As the laird, he had rights to the loose stone around the shoreline, which turned out to be vital. He had liked the sequence of rooms of the old house, and the way it had been gradually extended, giving it a high centre with lower side wings. Combined with that was the thought: what might they have built round here, way back? The idea of a once-fortified tower house, with wings that could have been added by successive generations, took root.