Watch Grand Designs Season 12 in Australia

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.

Leigh Osborne and Graham Voce want to convert and extend a landmark 150-year-old, 100-foot-high water tower right in the heart of central London. Grade II listed, but utterly derelict, with six foot thick walls and crowned with a huge steel tank - it is an almighty task. As if this isn't challenging enough, Graham and Leigh also plan to build two additional contemporary structures at the base.

Kevin McCloud meets Audrey and Jeff, who dream of living in one of west London's most exclusive areas. However, the way they can afford it is to go underground. Their plan is to transform a listed Edwardian artist's studio - along with 5,000 sq ft of derelict basement - into a bespoke subterranean home, complete with supersize kitchen, mezzanine sitting room, four en-suite bedrooms, a gym, cinema, steam room and wine cellar. As if the task weren't tricky enough, they are doing it without proper architect's drawings and Audrey is project managing - and the difficulties begin almost from the start.

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.

Architectural designer Lincoln Miles and his wife found an uninspiring 70s bungalow on the Isle of Wight on a prime piece of land surrounded by unspoilt ancient woodland. Did their unconvential techniques and alternative materials pay off?

In 2010 Claire Farrow and Ian Hogarth embarked on a wild journey to build the ultimate fun family home, right in the heart of London, including a dance floor, DJ booth and sauna.

Freddie Robins and Ben Coode-Adams left their cramped two bedroom London flat and took on the conversion of a huge Grade II listed timber framed barn in the Essex countryside. Seven times the size of an average three bedroom house, turning this 500-year-old building into a home was a big project. A year after they finished the house, Kevin returns to discover how Ben and Freddy are adjusting to rural life.

On the first series of `Grand Designs', a younger Kevin McCloud witnessed as a co-operative of ten families built their own homes on the edge of Brighton. Unpaid, they slogged away for over two years on this pioneering venture, based on designs from architect Walter Segal. A decade later, Kevin returns to see if the houses, and the families that built them, have stood the test of time, and finds himself genuinely inspired.