The new series kicks off with a Grand Design on an epic scale, with the story of one man's heroic attempt to fulfil his childhood dream and bring back from the ruins an incredible castle set in one of the most beautiful landscapes of southern Ireland.

Actor Sean Simons fell in love with the ruins of Cloontykilla Castle when he played there as a boy. Years later he bought it, with the idea of turning it into a spectacular, theatrical mock 16th century home, complete with grand baronial hall, luxury bedrooms, sweeping staircases, water spouting gargoyles, Jacuzzis in the battlements and a huge recording studio in the basement.

But would it prove to be a magnificent personal masterpiece, or a garish sub-Disneyesque disaster?

After borrowing an eye-watering 1.2 million Euros, Sean launches into his project just as Ireland plunges into recession. He has a head full of fairy tales, but no help from an architect, or indeed any formal plans whatsoever. Making things up as you go along is fine if you're building a toy castle, but not when you've got a large team of builders on site, a schedule to hit and the bank looking over your shoulder.

Over two years, rooms are built, then knocked down again. Contractors walk out in frustration. Planning permissions are ignored.

But somehow Sean's vision starts to come alive... just as Ireland collapses into financial meltdown.

Kevin meets a pair of civil-partnered university professors, Celia Brackenridge and Diana Woodward.  They have enlisted the services of industrial designers who have developed a new computer-assisted method for cutting precision-engineered building blocks, however although the technique may be clever, its creators have never created a building before

In Brixton in south London design-mad couple Mary Martin and Carl Turner want to build a home that resembles a giant stack of glass cubes that's a Zen retreat from the noise and chaos of the capital.

Mary and Carl want to build something never seen before amongst Brixton's traditional Victorian terraces. Minimalism-obsessed Mary and her architect partner Carl have spent years renovating their homes to be as simple to live in as possible.

But this project is bigger and more ambitious than anything they've taken on before.

In order to save money, Carl decides he has to take on as many of the specialist build jobs as he can. The results are hair raising as the cranes and steel frames hit the narrow plot.

But Carl's biggest test lies ahead, when it comes to finishing the minimalist interior. The tiniest error in the detailing could ruin all the blood, sweat and tears he's put into building this urban sanctuary.

Kevin McCloud follows Lysette and Nigel Offley, who are knocking down an old Thames boathouse and constructing a cutting-edge home in its place. Taking inspiration from the river itself, they have designed a building of waves and curves with a huge glass porthole for a roof light. But problems begin almost from the start, as they can't find a contractor willing to build over the water, their architect leaves unexpectedly and the costs turn out much higher than expected. To make matters worse, the neighbours aren't happy about such a radical sight in their traditional community. As they begin to compromise, Kevin wonders if the couple's dreams will ever be realised.

Kevin McCloud marks the 100th episode with one of the programme's most ambitious projects to date. Leigh Osborne and Graham Voce want to convert and extend a landmark 150-year-old water tower in central London into a luxury home. Grade II-listed, derelict and with 6ft-thick walls, it's a huge challenge for the pair, who also plan to build two structures at the base - a lift shaft connected by a series of glass tunnels, and a modern living space. The result will be a four-bedroom property over nine floors, complete with a room at the top offering spectacular 360 degree views across the capital.

Kevin McCloud meets artists Indi and Rebecca, who are planning to build a modern, larch-clad home on the Isle of Skye, with a second hand-crafted building alongside to use as their studio, all on a tight budget of £150,000. They have been saving for years, but making it happen will mean taking on the savage weather, not to mention the sceptical locals - and as if that weren't enough, most of the work is being done by one man, local builder Donald.

Henning Stummel and his partner Alice Dawson attempt to convert an awkward,covered space, trapped between buildings in the crowded heart of north London into a contemporary family home and office for Henning's practice.