David and Greta loved their home, a modernist structure in Surrey that they had built themselves almost 40 years ago. It was filled with things they'd chosen over the years—design classics, mementoes, David's paintings. But that house was falling apart and had to come down. Meanwhile, they had lost their hearts to a German post-and-beam house, designed by architect Peter Huf and available as a customised kit.

In November 2001, Tom and Judy demolished a bungalow and started building an infinitely more ambitious house. Designed on different levels, with glass walls and exposed steelwork, it was to be a miraculous piece of engineering, giving the couple and their two young children a beautiful, technologically advanced home. Tom appointed himself project manager—a job that proved to be more challenging than he had expected. Grand Designs 3 followed the first 16 months of the build. As Grand Designs 4 goes on air, how close is it to completion?

When Reuben and April came across a ruined 19th-century house amid the tower blocks of Leith, Edinburgh, they saw it as their future home. They had no clear idea of how to restore it, and precious little building experience. Still, they were young and fit (they had met on Reuben's climbing wall) and they liked a challenge. So they bought the crumbling shell and set about transforming it with their own hands.

David and Anjana's tiny coach house was proving too small for them and their two children. So they decided to build a new house in their own garden, which was big by London standards. But this particular grassy plot came with problems. It was overlooked by huge blocks of flats, and in the middle of it stood a chestnut tree they weren't allowed to cut down. They came up with an inspired solution: they would build a curving house along the borders of their plot. What's more, they would do a lot of the work themselves.

Tom and Darron wanted to build a house that would be home to them, their art collection and their two large dogs. Darron's passion for surfing meant it would also have to be near a beach. They already shared a timber beach house in a hamlet on the Sussex coast, and when a neighbour's bungalow came up for sale, they decided to buy it, demolish it and build a dazzling white modernist house in its place.

Tony and Jo, musicians with Scottish Opera, had long dreamed of a home in the countryside. They found a perfect site on the Clyde estuary in Argyll and Bute. Located in a small village on a hillside, it had glorious views as far as the isle of Arran and was less than an hour's drive from Glasgow. Inspired by local oak-framed barns, they gave architect Andy McAvoy an open brief. In return, he gave them a design that fused medieval and modern and promised a beautifully simple interior. However, the construction was anything but simple...

Amid 55 acres of organic farmland in the New Forest, Lizzie and Mike set out to build an idiosyncratic home. They wanted a house that would reflect their love of travel and eastern cultures, yet blend into the very English countryside around them. Their first proposal, for a wooden Japanese house, was refused planning permission, but after three years of adapting their ideas with architect David Underhill, they were finally ready to build. Their ingenious design was in three sections: a living wing, a bedroom wing and a romantic tower.