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?

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.

Condition survey
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.

You will also need to get planning permission, and get the neighbours onside, Pigott counsels. A detached house is the easiest type of building to dig down because you don’t have to worry about your development causing movement in any adjoining properties. Before building works commence in semi-detached or terraced properties, make sure your contractor carries out a “condition survey” of the neighbouring properties, photographing the condition of the houses inside and out before the works start and again when the works are completed, Pigott advises.

If you live in a semi-detached or terraced house it also makes financial sense to talk to your adjoining neighbours about your plans. You might even convince them to give their home the same below-ground treatment. Pigott points out that you pay a large premium to get the equipment to site. “If that can be spread between two or three neighbours the economies of scale of the job become more attractive.”

Before you start, you need to do a cost valuation exercise, counsels Marianne Davys, a London-based Irish architect practising in Muswell Hill, who has completed numerous small residential basements including the excavation of her next-door neighbour’s house. “Basement projects are expensive and technically much more difficult to design, build and waterproof than above ground extensions. Arranging insurance can also be complicated and expensive. In most cases water has to be pumped from the basement level for the rest of the life of the property.”

While it is an idea that makes sense in central London, where property values are very high, elsewhere “it is wise to consider carefully whether the value of the property when the project is complete will be proportionate to the amount of money you spend on the project,” she cautions.

Before you do any digging, Felicity Fox suggests you consult an agent in your area and take their advice on whether it is a good idea for you and the type of home you own.

"If you live in a two-up, two-down terraced house where the market for your property type is mainly first-time buyers and you want another room, you may be better off moving to a three-bedroom house elsewhere rather than extending down as your enlarged house may not suit the buyers looking in your area," she says.

Is your house suitable? What it costs and where it works
Putting in a basement costs from €3,000 to €6,000 per square metre in Dublin. In theory, it suits all house types. Detached houses fare best because they have no adjoining neighbours whose properties may be affected by such structural works. Before building works begin in semi-detached or terraced properties, make sure your contractor carries out a "condition survey" of the adjoining properties, photographing the condition of the houses inside and out before the works start and again when they are completed.