The clients didn't want the extension to eat up the whole of their south-facing garden so instead it was decided to knock down the existing extension and design a new one to house the kitchen and then plunge the living/band/gym space beneath the ground.
"They wanted to add space to their house but still have a garden," says Gerry Salley (who established Crean Salley, with Desmond Crean, in 1992) as we gaze across to where the evidence of the underground world is marked by a rectangle of glass balustrades in the middle of the garden.
Peer down and you are met with a large sunken (as in a depth of 3.5m) patio with a glass wall facing onto it behind which is the spill-out space with sofa, gym equipment, drums and guitars in evidence. Natural light enters this space from the patio/light well as well as the top-lit stairs descending from the overground extension.
The connection between the upstairs and downstairs is very important, says Salley: "You need to get a lot of light into the space because people don't want to open the door and step down into a cool, dark place. A key to underground extensions is easy access." Sound travelling up the stairs also helps to make the basement more welcoming, he says.
Light-oozing elements, such as large folding and sliding glass doors, pale paint and furniture, as well as perforated metal, open-tread stairs, help prevent gloom.
Darkness can come in other forms when you dig down: when architect Tom de Paor designed sunken houses in Dublin 8 a few years ago, they found a burial-ground from a 16th century cholera outbreak.
The part of the Crean Salley extension that is above ground has a Frank Lloyd Wright feel to it with its brick walls (to match the existing house), walnut stained cedar cladding, timber windows, clay tiled and low pitch roof whose eaves sweep out well beyond the walls. "There is something very nice about a strong horizontal with overhanging eaves. There is a lot of glare here and this roof gives shade without losing a sense of light," says Salley.
While many of Crean Salley's underground projects have been built beneath gardens, there are also cases of basement living spaces being dug out directly under houses. Elements such as a timber or concrete floor, and depth and type of foundations all dictate how difficult, and how pricey, it will be to create such lairs (with a timber floor you can dig straight down from the ground floor while with concrete floors you may have to excavate from the side). As a rule of thumb underground extensions cost about a third more than standard extensions per sq m, says Salley, with costs depending on size, access, ground conditions and so on.
What such structures need is a strong structural steel frame and a concrete 'tank' (ie, a solid waterproof box) to prevent leaking (or, God forbid, flooding). They use either waterproof, reinforced concrete or pre-cast concrete panels with an external waterproof lining. In this case the basement was made with reinforced concrete, formed by steel shuttering that was craned in over the house, and it has a pre-cast concrete lid. The ceilings and walls are heavily insulated.
In this house, there is a system that pumps water into a tank when water climbs to a certain level and it will also discharge excess water into the local-area drainage system. The sump system comes with an alarm that alerts the owners' mobile phones (or the maintenance company) should water levels rise alarmingly.
All of that is to be expected when digging a deep hole and truck loads of earth came out of this plot, says Salley.
It is those drainage issues that are of a major concern when people apply for planning permission for subterranean spaces, says Tom Mahon of the planning department at Dún-Laoghaire Rathdown Council. "Any application for such an extension, or a house with a basement, will be refered to the Environmental Services Department for a report."
He says that they do get application for new-builds with basements and new extensions complete with underground spaces: "While we don't get a huge number it is increasing, particularly for one-off houses."
But in some areas of London, where more and more people are seeking new life beneath the ground, there are neighbourhoods where planners have required people to stagger the digging to ensure that the foundations of whole streets don't all wobble at the same time. Yet, with basements being de rigeur in many houses on the European Continent and North America, extending down instead of out or up could become a common concept here.
The result in this D4 plot is a real den of a place that makes you feel as if you are escaping from the world, in a way that brings out your inner troglodyte.
