Kevin McCloud meets Spanish architect Jaime and his wife Mimi as they embark on a mission to convert a Grade II listed folly into a family home in Buckinghamshire. Giving themselves a wildly optimistic six month deadline to finish before the birth of their second child, it's not long before the couple hit trouble. Jaime puts all his energies and creativity into the project, using innovative 3D mapping to squeeze living spaces into the tower, but it proves to be an exhausting struggle.

Kevin McCloud meets Harry and Briony Anscombe, who have decided to move from London to Cornwall with their three young children and begin building work on an ambitious house, inspired by American modernist properties. With a budget of just £400,000, Harry opts to project manage himself, paying local builders by the hour, but his lack of experience in construction proves to be problematic.

Gallery owner Elinor and fitness entrepreneur Born set themselves the task of creating Britain's first hypoallergenic house in a leafy back garden plot in south west London. Driven by the need to alleviate their two young sons' life-threatening allergies, their new part-submerged, shed-inspired home, will be constructed using low toxin materials and deploy cutting-edge filtration systems to clean the air they breathe. Will their innovative new house deliver the benefits Elinor and Born are hoping for?

Steph and her husband Alex build a contemporary, cantilevered farmhouse on farmlands once owned by her grandfather in Herefordshire. Steph and Alex are told their ambitious design will cost £500,000, twice as much as they can borrow, but the pair decide to take a leap of faith and move into a draughty old caravan on site as they begin working on their new home.

Identical twins Nik and Jon Daughtry build two near-identical houses next door to each other in Sheffield, paying tribute to the city's industrial past. Their plans are incredibly ambitious, not only do they want to build the two houses for the price of one, but the materials and finish have to be absolutely perfect. With a tight budget, it's an almighty stretch from the start. As delays, overages and imperfections hit, it's clear the twins' intuitive and incredibly close relationship will be tested like never before.

Professional deep-sea diver Adrian Corrigall and his wife Megan plan to build their new family home in rural East Sussex almost entirely out of concrete, with construction involving cutting-edge technologies conceived in Switzerland and never used to build a house before. However, the perils of being a pioneer soon become evident, and with both schedule and budget under strain, Adrian is forced to resume work as a diver, taking him away from the project for a month at a time.

Kevin McCloud returns to Devon to meet Kevin McCabe, a man who wanted to build one of the biggest houses the programme has ever seen. Finally, seven years after it began, the herculean task of building this giant cob castle is complete. Has it proven to be the great home he originally intended, and does it prove as he hoped that cob is a viable material for contemporary architecture?