With the Luas trundling across the back garden, A2 Architects felt that this house needed to address the people in transit, writes Emma Cullinan

ONLY minutes pass between trams as they clackety-clack past this house and passengers who gaze out of the window at back gardens blurring by will notice this home because it has, in effect, turned to face them.

The brief given to A2 Architects, who were appointed by a couple - with two children - who had just bought this house, was for "breathing" space.

The designers' response was that, since the Luas was now a feature of their back garden, this extension should create a new "front" to the house.

The three-and-a-half metre addition, across the extraordinary width of 12 metres (most houses measure seven to eight across), is now known as "Brick Thickness" and is designed to act as a buffer between public and private spaces.

Yet it is not so much a new front as another front and its design borrows from the other side of the house, in its form, craftsmanship and material content.

The Crampton-built home has a bay and porch to its original front and this undulating elevation has been referenced out back. So too have the bricks: the client loved the original mellow variegated bricks "which I have never seen anywhere else" and wanted something similar and especially a brick that didn't look new. So here is a polychromatic wall whose elements vary from purple through to black and which is held together with lime mortar. Black pigment in this results in a wall that you read as a whole - aided by the lack of expansion gaps that the lime allows - despite the fact that the wall is subtly multi-coloured and sparkles softly beneath the sun.

The variegation and the glimpses of glitter relieve what could be a foreboding expanse and this animation is taken through to the structure's form which winds around and up from the projecting bay of the sitting area into the kitchen and up to a first floor bedroom. The opening to one side of the bay reflects that in the upper bedroom - as if they are speaking to each other.

This is a firm that has so far tended to steer clear of mono-rectilinear forms. The last extension A2 built was a Douglas fir and plywood "tent" with an asymmetrical pitched roof that adds an interesting contribution to the flat roof/pitched roof debate: there are many ways of angling a pitch. It was done for practical reasons; to bring light in at optimum angles but the client had also mentioned something about wanting a tent.

Next on site will be a row of three houses (perhaps later expanded to six) in adjoining Dublin gardens, which also employs square voids within rectangles to bring in light and create courtyards and terraces.

These are all the sorts of urban infills that the firm will be concentrating on when co-principal Peter Carroll presents a plan for Dublin - along with Peter Cody of Boyd Cody Architects - at the inaugural Lisbon Architecture Triennale which runs from May 31st to July 31st (called Line to Surface: Urban Void/Extended City).

The windows in the Ranelagh extension are also designed to enliven the restrained elevation by being flush in places and recessed in others: some glass is set into thin iroko surrounds and other sections are held by thick frames. "We wanted to create incidents in the thickness," says Carroll.

Yet where it turns a corner the glass is unframed - instead an interior metal column takes the load at this point. It was important to the architects that the glass flowed around the structure. "We were keen to manipulate materials so that they turn around surfaces and you can follow the joinery in a continuous line," says Peter Carroll, who set up A2 Architects with former UCD classmate Caomhan Murphy in 2005. In the meantime Murphy had worked with Burke Kennedy Doyle in Ireland and Turner and Associates in Australia, and Carroll was at Murray O Laoire, O'Donnell and Tuomey, and then with Rafael Moneo in Spain.

While at O'Donnell and Tuomey, Carroll worked on the exquisite Milk Bar in Montague Street that runs from Camden Street to Harcourt Street in Dublin 2 and the Multi-denominational School in Ranelagh and its hallmarks are here. The exterior is reminiscent of the school and the fact that the interior is altogether softer than its skin is another similarity with that building.

Light beams in from above, split by baffles which help to shade the light and throw it around. These were also designed to stitch the old and the new part of the house together, says Carroll, and they are very large stitches indeed.

The kitchen to one side flows off the existing hallway and the sittingroom to the other side meets a downstairs livingroom. "Everyone who comes here says it is amazing how well the old and new parts of the house join together," says the owner.

The light bounces off the slightly sparkling terrazzo floor whose inset stones were chosen by the client from a selection of colours. This terrazzo mimics that used in the Milk Bar and wraps around surfaces in the same way. Here it leaves the floor and rises up certain surfaces much as the former chimney breast between the kitchen and dining area. It also flows through to the adjoining room.

The oak ceiling comes in the opposite direction, dropping down to become furniture in the livingroom and creating a bookshelf beneath a window seat, while in the next room the same oak is used in the kitchen units. "We tried to use as few materials as possible to keep it calm," says Carroll," and to concentrate on working in 3D to pull things together."

The two rooms can be divided by a huge sliding wooden door - again a reference to a door in the Multi-demoninational School - and also reminiscent of Gerrit Rietveld's work in the Schroder house, where sliding doors create smaller rooms from an open plan. In fact, the house in The Netherlands was built just before this one and, while Crampton built in a different style, that descended from the Edwardian look, both have the same sense of craftsmanship and thought. The chimney on this house, for instance, has soldier courses around its corner and horizontal stack bonding along its edge, indicating thought and craftsmanship. In the render at the front the mottled effect looks as if it had been created by fingers being pushed into it: the sure hand of a craftsman, or 'trace de la main" that Irish engineer Peter Rice felt was so important in a building.

This is a robust extension - which is just what the client asked for and her son has displayed its toughness by roller blading on the terrazzo without anyone panicking too much.

It is also a building that people can interact with in other ways. The kitchen cupboard doors have inserts instead of handles so you have to curve your hand into a hole to open them. There are external sills wide enough and high enough to be a bum rest and inside there are two generous shelves along interior windows. "People always gravitate to windows," says Cowell. The client confirms that these "seats" are well used: it's always a treat to be able to sit on and lean against a building for casual chats.

While the client is happy with the result, the "frontage" to the Luas has been compromised. When neighbours told her that they were catching the eyes of passengers on passing trams, she decided that there was a limit to just how open you could be and a high fence has now gone up at the end of the garden. But then all architects have to bow out when they leave site and allow clients to take ownership, in whatever form that takes.