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 McCloud meets Michelle Parsons, an artist and teacher who decided to seize the day and build her own private hideaway in a woodland environment in Essex after she recovered from a serious illness. Together with her architect husband David, Michelle planned to construct a sleek, black-clad, three bedroom house complete with an art studio. However, torrential rain and a potentially disastrous gas leak stalled progress on the build, and as construction gets under way, Kevin wonders whether the couple's decision to eliminate windows on two sides of the property may leave them living in a gloomy bunker.

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 meets a master craftsman whose dream is to build a castle made entirely of mud. Kevin McCabe is the leading living exponent of the ancient art of cob building - wrestling houses out of mud using his bare hands. But Kevin doesn't just want to build another cob house, he wants to build a cob castle. Not only that, Kevin also wants their gigantic new cob house to meet the highest environmental performance targets ever set. It is truly an almighty challenge. The house will be formed of two vast curved cob roundhouses - the largest of which is inspired by the natural geometry of a snail shell - connected by glazing and topped with undulating wild-flower meadow roofs to mirror the surrounding Devon countryside. But building it involves mixing and laying a mind boggling 2000 tonnes of cob, all during warm sunny weather. Kevin is determined to have the main cob walls up in a few months. But soon the sheer scale of the task becomes clear.

Andy and Nicki Bruce try to build an experimental flood-proof floating house on an island in the River Thames. But building such a complex design on a site with limited access is not easy. Andy and Nicki Bruce's amphibious house, the first of its kind in the UK. Their architect has come up with an ambitious, experimental design that has never been tried on a river bank in Britain and no one knows whether it will work. The Bruces decide to build on a site on a small island in the river. But it's not long before the project becomes bogged down in the logistical challenges of building such a complex design on a small island that is only accessible by a narrow footbridge.

Kevin McCloud follows the progress of 37-year-old Angelo Mastropietro, as he transforms a damp, dark and abandoned cave in Worcestershire. Having recently been diagnosed with multiple sclerosis, Angelo wants to create a retreat to help him cope with the news and become a 21st century caveman. He has a budget of pounds 100,000 and plans to almost single-handedly carve, cut and drill into the hillside to make the cave comfortable for contemporary living. However, there is no manual for a project of this kind, and the unique challenge soon becomes an obsession.

Identical twins Nik and Jon build near identical houses in Sheffield. Raised on steel stilts, these two extraordinary properties pay tribute to Sheffield's industrial past.

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 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.

Kevin McCloud follows a project to renovate and extend a classic 1920s cinema in Thorne, South Yorkshire. With little knowledge of restoration, Gwyn and Kate boldly set about turning the dilapidated building into a family home using masses of concrete after taking advice from an architect friend. The couple are hoping to create a hydraulic glass wall that opens up one side of the house to form a UV-filtered canopy, and there are also plans for a white roof extension that resembles a diving board.

Jonathan Broom and his wife Deborah have put everything on hold while he pursues his dream of building a mini Hollywood Hills-style mansion right in the gritty heart of north London. For ten years Jonathan Broom has been obsessed with building his own home.They finally stumble across a scrap of land right in the gritty heart of North London, but it's fraught with problems, and the only way they can build their wildly ambitious £1million pound family home, complete with nanny flat and swimming pool, is by sinking half of it six metres below ground. It's a project that pushes the couple and their builder to the very limit. Desperate to make every penny of the budget stretch, Jonathan strikes every deal he can, and even sells his business to raise funds. Originally scheduled to take a year, this giant hole in the ground consumes their money and lives for much longer.

Tamayo Hussey's missed Japan ever since she moved to the UK with her husband Nigel. To stave off the homesickness they've decided to transform a forester's lodge into a Japanese house complete with roof bath, tatami room and sliding paper walls. Keen to keep costs under control, Nigel and Tamayo decide to engage only the design skills of an architect and with no previous building experience they bravely go it alone, working without any detailed drawings and fire-fighting problems every step of the way. The wood they're using for the timber frame - Japanese larch - hasn't been used for building houses before in the UK, so no one can be sure it will be strong enough, the new interior walls don't meet properly and the replacement window design can't cope with the rain. But Nigel and Tamayo are determined to overcome any obstacle to get the Japanese house they're so desperate for.

Confirmed city dwellers Michael and Phil have moved to the country to run a farm, start a micro brewery and create in a vast, dramatic 21st-century farmhouse to live in. Michael Butcher and Phil Palmer were confirmed Londoners, loft-livers in the heart of Soho. Until they fell in love with Christmas Farm, near Newbury, and took the life changing decision to quit their urban media jobs and move to the country. They faced two big problems however: first there was an agricultural tie on the land, so Phil and Michael would have to become farmers; the second problem was the uninspiring faux-alpine timber chalet serving as a farmhouse - it had to go. But Phil and Michael have decided they don't want to build a regular brick farmhouse as a replacement. Determined to bring a bit of urban glamour to their new home, they want a 21st-century farmhouse unlike anything the area has seen before. Phil and Michael's version cleverly combines a vast, white, dramatic open-plan party pad upstairs, with muddy functional farming spaces below. As the builders go to work on the extraordinary hand-crafted flint exterior, complete with the largest sliding trapezoid window in Britain, Phil and Michael must juggle construction with farming the land. And in order to make it all work financially they decide to set up a new micro-brewery in the barn.

Peter Berkin and his wife Chard have decided to build a new home at the bottom of their garden. Problem is, right from the start they can't agree on any part of the design - even the basic shape. Peter wants the new house to feature a workshop where he can build a plane. He wants the house to be round. Chard however wants it to be square and practical. Peter aims to spend pounds 400K, Chard wants to keep the budget around pounds 200K. With Peter building as much of the house as he can himself, helped by his mates, and Chard taking control of the money and schedule, it looks like a recipe for disaster.

A young farmer and architect in Northern Ireland hopes to make a home out of four large shipping containers welded together to form a giant cross. Patrick Bradley has come up with a pounds 100k house design to be built out of four 45-foot shipping containers, welded together to form a giant cross and cantilevered over the top of a stream at a beautiful secluded spot on the family farm. His mum is hoping his new home will get Patrick a girlfriend. But his tiny budget is soon under threat.

Kevin McCloud returns to a remote part of France and a restoration project of a dilapidated manor house started back in 2003 by Denise Daniel and Doug Ibbs. When Denise Daniel and Doug Ibbs gave up everything in the UK to start a new life in a remote part of central France, they couldn't have known what was ahead of them. Most people would have regretted falling in love with a massive, dilapidated manor house with just four crumbling walls and half a roof, but not Doug and Deni. This impressive, extraordinary and utterly determined couple rolled up their sleeves and slogged away day and night, doing everything themselves, year after year, until slowly but surely they created a magnificent edifice out of the ruins they bought off the internet.

Kevin McCloud meets Clinton Dall from Sussex, a self-made businessman and father of four aiming to build one of the largest homes ever featured on the show. Dall wants the end project to be perfect - from the finish of his mirrored cruciform steel columns, to how the floor tiles line up with the dining room table, and is willing to spend whatever it takes to achieve his vision. However, the construction represents a huge risk, as down one road lies abject failure and financial ruin, while down the other, is the creation of something sublime and almost heroic.

Boat enthusiast James Strangeways invites Kevin McCloud to survey progress on the construction of his new home, which his architect nephew Ben Hebblethwaite has designed to be as 'un-houselike' as possible. James has spent the past 35 years travelling the canals and waterways of Britain, and despite never liking houses, has decided now is the time to put down roots on dry land. Ben hoped a home standing on stilts above a marsh, and incorporating ship-like qualities such as a keel-shaped roof, will be enough to satisfy his boat-mad uncle. However, Ben's contractor goes into liquidation a few months into the project, taking pounds 87,000 of the pounds 300,000 budget, and when James decides to make a few tweaks to the design, alarm-bells begin to ring for the young architect.

Kevin McCloud follows the progress of teacher Michele Long and architect Michael Howe, who want to restore and transform a 100-year-old blacksmiths in County Antrim near the coast of Northern Ireland. With virtually no practical experience, Michael wants to do as much of the project as possible by himself. However, the weather on the coastline is fierce, and the freezing weather coupled with a tight budget of 150k means the build is stretched from one to three years.

Kevin McCloud follows the progress of private chef Ed Versluys and pilates instructor Vicky Anderson, who want to convert a concrete cowshed in the Somerset countryside into a three bedroom home. With the help of one young builder and the knowledge they can learn from the internet, the couple plan to project manage the conversion themselves. However, they have only eight months and a budget of just over pounds 200k to make a warm and comfortable home with straw bale walls and wide expanses of glass.

Kevin McCloud follows the progress of Stephen Yeoman and Anita Findlay, who want to build a cutting-edge, post-industrial house covered in rusty metal. However, their prominent riverside plot in the traditional and architecturally conservative area of South Downs means everyone will be watching. The project proves to be a real rollercoaster when the fitting of the rusty steel cladding goes wrong, cash flow problems threaten to bring work to a halt, and the couple announce that a baby is on the way.

Kevin McCloud meets Tom and Danielle Raffield, whose lifelong passion has been steam-bending wood. They've used the technique to create extraordinary furniture and lighting, but now plan to use it on a larger scale. In an attempt to escape the confines of their tiny gamekeeper's lodge, they have decided to build a wavy wooden house in South Cornwall, completed with curved cladding, twisted furniture and interior walls covered with weathered timber. With a £100,000 budget, the pair have decided to do a lot of the building themselves, and Kevin is on hand to discover whether they're up to the challenge.