Digging deep to create extra space
If moving isn’t an option, digging below ground level to create additional space is sometimes an alternative – but is it a cunning plan or a potential mistake of Titanic proportions?
Belgrave Mews is an “iceberg house” of 219sq m (2,360sq ft), in London, with a 33sq m (360sq ft) roof terrace, off-street parking and a price tag of €8.354 million (£6.95 million) through agents Landmass Developments. The ceiling heights at basement level are 3.2m. Walk- on glass flooring has been installed at ground level to help draw natural light below ground. To further illuminate the space, a high-spec, flush-with-the-ceiling lighting system by Lutron was installed.
Grosvenor Crescent Mews features a 100sq m atrium, so that even at basement level it’s possible to see the sky. A nine-metre water feature gives some sense of the scale of the build. This property sold in 2010 for about €9.04 million (£ 7.5 million).
Building work begins at Grosvenor Crescent Mews, a property in London’s Belgravia sold by Landmass Developments in 2010 for about €9.04 million (£7.5 million).
Marianne Davys, a London-based Irish architect working in Muswell Hill, is about to build a basement in her home. She got her next-door neighbours onside by designing and digging out their basement first to create the space above. “It’s not often you can say to a prospective builder ‘Here’s one I did earlier,’” she laughs. In her mirror-image semi, the floor-to- ceiling height at basement level will be 2.3 metres. The project will add 18sq m to the ground floor and 28sq m at basement level. The house was originally 78sq m, to which she has already added a 30sq m attic space from where she runs her business. The plan is that in future years, when she retires she can, by closing a fire door to the basement, turn the space into a self-contained holiday let, offering panoramic views over London.
If you’ve already colonised the attic but still need more space, digging below ground is a building method being used in London to maximise every square centimetre of space.
In Knightsbridge and Chelsea, vast, James Bond-style lairs extending three and four floors below ground level in “iceberg houses” – so-called because what you see above ground is only a small part of the entire square footage of the house – make sense because space is at a premium.
Phones 4 U billionaire John Caudwell has just been granted permission to extend and connect his two Mayfair properties, townhouses that back onto each other. He plans to add 1,300sq m (14,000sq ft) in an underground extension that will include a swimming pool, sauna, salon, bar, games room and car park. When completed, the 4,645sq m (50,000sq ft) mansion will be worth an estimated €303 million (£250 million).
But does excavation work make sense for ordinary home owners? “When it’s done properly, that is properly designed with light wells and adequate ventilation, it can add 10 to 15 per cent to the value of houses, especially in parts of Dublin 6 and Dublin 6W, where demand is high,” says agent Felicity Fox.
Landmass Developments is a London-based company that prides itself on its ability to maximise basement space. In any of their projects, floor-to-ceiling height at basement level is a minimum of 3.2 metres, says managing director Alan Waxman, who explains that nts the space feeling subterranean. In Ireland the building regulation floor-to-ceiling height is 2.4 metres. Landmass, who employ in-house architects and interior designers, also install atriums, to make the sky visible from below ground. They also insist on high-spec illumination systems by Lutron and walk-on glass floors on the ground floor to further draw natural light down.
These add-ons are expensive. Landmass quotes a construction cost of about €6,500 per sq m for a high-end basement in London, which compares favourably with the costings of Allister Coyne of Ailtireacht, a Dublin architectural practice that has designed many basement projects including a villa in Rathmines, Dublin 6.
According to Coyne, the average build cost in Dublin in 2014 is from €1,500 to €3,000 per square metre, depending on the level of finish you want. Digging down is 100 per cent more expensive again, he says, bringing the price to between €3,000 and €6,000 per square metre.
Employing an engineer with expertise in this area is crucial. John Pigott, director of Casey O’Rourke Associates, a Merrion Square-based consulting structural and civil engineering practice, calls basements “the most complicated forms of construction in city sites”, explaining that for “every square metre you dig down you will be removing two tons of earth”. If the average Dublin basement entails boring down approximately three metres, this creates six tons of what he calls “spoil generated earth” that has to be disposed of correctly. If it is contaminated, it will need to be disposed of in specific landfill sites, which pushes the disposal cost up dramatically. Peroleum, the result of a leaking oil tank, is a common contaminant.
A site investigation will give some idea of what is in the ground below, Pigott explains. The ground samples are sent off for chemical analysis. What the ground is made up of – clay, sand, rock – informs how you will dig it out. Much of Dublin has brown and black boulder clay, which Pigott considers to be “good ground”. The results of the investigation will also give you an idea of how much space you can hope to gain.