We've tried expanding our cities outwards and upwards - but could another solution be to build underground? Emma Cullinan looks at a lecture hall built beneath a garden in D4

The debate about where to build more housing and offices has concentrated on whether we should keep creeping outwards, by adding more buildings onto city outskirts, or upwards, increasing density by building tower blocks.

Yet a new building in Dublin 4 points in a new direction: the lecture hall behind the Institution of Engineers of Ireland building at 22 Clyde Road is underground.

"This could help us look at the way we think about the inner city," says Niall McCullough, of McCullough Mulvin Architects, who worked on the project with his colleague Ruth O'Herlihy.

He's captivated by this idea of life going on beneath the ground: "It's like looking into water and seeing fish swimming beneath the surface." Here it seems as if nature has been drawn from the earth to create the structure. The lecture hall is covered in raw timber. "The technical term is lumps of spruce," grins O'Herlihy. "We wanted something strong and gutsy rather than sharp-edged cedar cladding."

This will eventually be covered in the evergreen jasmine which has already been planted and whose young tendrils are reaching out, like monkey's arms, trying to clasp onto the network of yachting wire that has been laid for them. This robust, untreated timber seems far removed from what we've come to expect from McCullough Mulvin, designers of serene edifices such as the Ussher Library in Trinity College, Dublin, in concrete and granite, and the pared cedar façade in Donegal Civic Offices.

The Clyde Road site, next to a protected structure, is tiny: there was just a garden - between the period building and a two-storey mews building - and car-park on which to build. The lecture hall is an independent shell beneath the former garden, which has now been replanted on the sloped planks that slide up the rear of the mews office building.

Beneath this lumber thatch - which incorporates "leaky-pipe" irrigation - is a layering of materials by green and flat-roof specialists Erisco Bauder. The building is artificially vented but it's buried nature reduces the need for heating, due to heavy insulation. Working for an organisation that represents 21,000 engineers in Ireland could have proved difficult if everyone had been given a say-so on the project. In this case two firms interfaced with the architects: Lee McCullough & Partners, on the structural side, and Homan O'Brien & Associates on services.

As well as being space savers, underground buildings are also created for ecological, defensive and symbolic reasons. Architect Daniel Libeskind gets quite emotional when he talks of descending into the foundations of the Twin Towers, where thick walls hold back the Hudson River: this is where the Ground Zero Memorial in New York is being built.

Skidmore, Owings and Merrill, one of the firms of architects involved in the Ground Zero project, is also designing an underground museum of the United States Army, in Virginia.

American architect Malcolm Wells is zealous about underground buildings, because they leave landscapes blot-free. Having begun his architectural career by building towering offices he was persuaded to descend into the earth by a visit to Frank Lloyd Wright's Taliesan West complex, where he was soothed by its underground theatre which was cooling in the desert climate. Like many ecological architects, the buildings, or roofs in this case, wear their green with pride - and anyone with a celebratory bent can enjoy Underground America Day with him on May 14th.

These verdant veneers are in marked contrast to the work of Japanese architect Tadao Ando whose underground buildings are given composed, tranquil concrete forms above ground from where you submerge: his Koshino House is half underground, as is his Forest of Tomba Museum.

He was one of a number of architects to design buildings at the Vitra furniture headquarters in Germany and responded to the site by putting his conference pavilion partly below ground. This was both out of respect for the surrounding cherry orchard and as a counterpoint to a nearby building by the expressive Frank Gehry.

"When visiting the site I was struck by the quality of movement that the Frank Gehry Design Museum projected so powerfully," said Tando. "Opposite Gehry's architecture of movement I introduced the element of stillness."

And that's the overwhelming quality of underground buildings: their serenity. As you sink into the lecture hall beneath McCullough Mulvin's planked roof, all is quiet. No need for double or triple glazing beneath the city: the thick "walls" quell traffic noise. Such spaces suit McCullough Mulvin well as many of their interiors have an ecclesiastical quality, such as the cathedral-like atrium in the Ussher Library and the Sienna Monastery in Drogheda, whose limestone-floored chapel has an opening in its timber ceiling to let light in above the altar. Such use of light creates a spirituality that has been used in churches worldwide; the penetration of sunlight into Newgrange creates a universal thrill that illustrates why bringing natural light into an underground structure lifts it into so much more than an artificially lit basement office.

In the engineers' lecture hall, light penetrates through two skylights, but the artificial lighting has been well thought out too. "It's not necessary to always listen to lectures beneath bright, fluorescent lights," says McCullough. The fluted birch wall panels are absolutely beautiful and add to the spiritual feel in an Alvar Aalto-esque way. I am assured by O'Herlihy that the fluting was for acoustic effect rather than aesthetic reasons but the person who designed these acoustic panels - used for decades in many buildings - must have had a secret mission to make walls beautiful.

In this tight site, the architects have also added another building to the rear, adding an extension to an archive store and lecture room they designed here in 1990. The new block steps lightly away from its parent, leaving a gap through which to enter the building. Cantilevering out is the first floor with an overwhelming amount of brick relieved by a huge corner window. You could imagine this extension as an upside-down version of the buried lecture hall. Inside, the new offices link well with the Georgian structure to the front, plastered throughout as if the interior of a period home.