Super-Basements - The art of digging down
Architect David O'Shea of Dublin-based ODOS describes himself as claustrophobic, which makes working on basements "interesting at times".
O'Shea, who was one of the architects behind the recent high-end apartment complex at 55 Percy Place, is currently out to tender with a super-basement proposal for a house in London's Belgrave Square which he describes as being "somewhere between a single and a double basement".
In the course of his research, O'Shea has found numerous examples of successful super-basement projects from London and all over the world.
"They make sense if you are land-locked and have limited space. If you can go up, because of air rights, then you do, but if that's not possible going down is inoffensive visually.
"Typically the space is used for a swimming pool, games room, den or cinema - leisure spaces in which there is no huge need for natural light. The spaces lend themselves to that.
"Getting light into cavernous spaces in order that they are pleasant places in which to spend time in is a challenge. It can't just be about digging holes, and making non-meaningful spaces; there has to be a legacy."
O'Shea says that ODOS has had a few enquiries about super-basements from people living in the more expensive Dublin postcodes.
"We've poked down for wine cellars - those are effectively a half-height basement - and we are waiting to hear about a competition that we pitched for to put a 25m basement pool in a house in Dublin 4.
"In London, of course, the budgets are bigger. There's an element of keeping up with the Joneses about it; they are all driving one another. In some places there is over-intensification, and property owners are trying to squeeze in too much. There's only so much a site can take.
"The impact on neighbouring properties is a major issue; you have to be sure that the work is not going to undermine their structural stability. You need to retain the foundations to neighbouring properties, so there are multiple preparatory surveys to be done. The whole process is fraught with danger and you need an excellent builder.
"Water is a big consideration too, as the work you are doing is changing the water attenuation. You are effectively placing a big void in the water table that causes water to be displaced, and there are implications in terms of pre-existing drains that may need to be re-routed. You need a cogent plan as to how to navigate that safely, and hope that you don't hit an old historic river!
"On one project in Enniskerry we hit rock formations that no one knew were there and we had to redesign the foundations. Unfortunately you don't always know what's there until you start digging, so you need the client to be on board with that and to understand the cost implications.
"The planners are generally okay with construction that's below ground, especially if the garden is going back on top. They approach it from a density point of view.
"In terms of cost, building underground is very expensive to do, so it only makes sense if things are really tight in terms of land mass. Cost-wise, a good rule of thumb is that anything underground costs almost double the equivalent above ground. Generally we say above-ground costs are in the region of €1,800-2,100 per sqm, and they would average €3,000-3,500 per sqm below ground."
Given that Dublin is nothing like as densely populated as other capital cities, and that there is not the same degree of pressure on space, it seems unlikely that the super-basement trend will take off in the suburbs to the extent that it has in London.