For years, adding space to add value has been a sure-fire developer's trick to make money. In this episode, the amateur developers decide the way to do it is by digging out their basements. It's one of the riskiest ways of creating space, and to add to their gamble these developers start out just as the property market begins to stumble back in 2007. Peter and Louise Mauder in Bethnal Green, East London, and Chris and Jules Weedon in Shere, Surrey, are soon in a race against time to beat the falling market. But that's not their only problem. Sarah thinks both couples are missing a real trick that could rescue their developments. But will they follow her advice, and will it be enough to secure their future incomes?