A new extension to this Dublin home plays some clever tricks with light, writes Emma Cullinan

This period-style house ticked all of the right boxes for two sisters who bought it as a new-build 12 years ago: bedrooms upstairs and, downstairs, a front lounge, diningroom and kitchen to the rear, and a garage to one side.

But gradually the shortcomings of the cellular rooms and arrangement of spaces began to be felt. The sisters entertain a lot and the thin kitchen and diningroom, side by side, left little room for a table and guests tended to squeeze into the kitchen with the cook. "We began by thinking we should get a new kitchen carpet," laughs one sister, "then talked about a conservatory and ended up with this extension."

An engineer she knew, Gabriel Corcoran, recommended architect Noel Brady of NJBA A+U. "We'd spoken with another architect who asked us what we'd like, but we didn't really know. Noel was full of ideas and he made models and computer images so that we knew what we were getting whereas with the other architect everything appeared to be in the ether."

The initial impression of the new extensions is of a simple space - essentially a room across the rear of the house with three pairs of high glass doors - but there are clever interventions that quietly heighten the richness of the space. Adding an extra room onto a house deepens the plan making it harder to attract natural light into its centre. Yet here daylight beams in through a long aperture in the roof that articulates the gap between the existing and the new structure.

Artificial lighting sits in above a beam here giving the same feeling of ambient lighting from a skyward direction after the natural light fades at night. This long opening brings in enough light to negate the need for light fittings in the main ceiling of the extension. While the resulting smooth surface may delight the occupants on aesthetic grounds it also provides a wonderful reflective surface for yet another architectural light trick.

"Light shelves" are more common in commercial buildings, but Brady has used one on the exterior of this extension to bounce light into the interior.

Internally you see the large glass doors with a band of windows above. On the outer face there are two projecting planes - one between the doors and clerestory windows and another above the clerestory.

They have striking aluminium bands on their front surfaces and this metal continues across the entire upper surface of the lower plane. Light falling onto this bounces up at an angle (through the clerestory windows) onto the smooth, angled ceiling and down into the interior.

Light also streaks down across the upwardly slanting top plane in through the illuminating gap in the roof. While sucking low winter sun into the building, the protruding planes also act as shades when the sun is high in the summer.

Brady had considered a pool of water instead of the aluminium - for its shimmering reflective qualities - but decided that the Irish climate would probably render it green and murky in a short space of time. I remark on Sri Lankan architect Geoffrey Bawa's creative use of reflective ponds and Brady cites Steven Holl's aquatic applications too. Brady, who teaches architecture at DIT as well as running his own practice, combines academic and practical knowledge to make informed decisions. Elements are used for a reason and are well designed. But he's not dogmatic and is prepared to discuss how to achieve a detail with a builder.

"You often see builders being given detail drawings that are beyond the scope of some of the best engineers in the world. Often the reality is that you have one man and a couple of labourers and the question is how you can achieve a lot with such resources," says Brady. "I do a lot of work on site with builders. I explain what I want to achieve and discuss, with the builder, how it can be achieved."

In this Dublin project both architect and clients agree they had a dream team, comprising engineer Gabriel Corcoran and builder Henry Ruane. "He missed the Galway races for first time in 20 years because he wanted to finish the job," says one of the owners. The clients took an interest in the work on site and were soon baffling friends and family with words such as "baffle" and "glulam beam". "I think my friend thought it was a dodgy beam, a sort of gombeen beam!"

The clients recognise that they got something beyond a standard extension from Noel and he, in turn, praises the clients' patience, not only during the build itself but in the run-up. "It's valuable if people allow time for the project to go through the proper legislative process and wait for the right builder," says Brady. "Many people think that if they phone an architect today they will have an extension in six months but this took two years."

The sisters have relished the slow approach and, even now, are quite prepared to wait for months if that's what it will take to find the right sofa for the space. This will give them time to get used to the new layout, which has thrown up surprises about how small spatial shifts can add volumes.

The kitchen - with new units from Ikea - has expanded sideways into a space behind the garage. This used to be a grass patch that had no useful relationship with the house or garden. There were worries about extending into the already small garden but, because the occupants can now see its complete width, as opposed to a vignette through the old kitchen window, it actually seems bigger. A covered terrace to one side of the extension creates a lovely transition from inside to out. Friends worried that this would waste space, and should be internal, but the sisters agree that they have all the interior space they need and this seating area has proved a bonus.

The thin diningroom beside the now expanded kitchen has been retained but its doors - at the extension end and into the kitchen - have been taken off creating a free-flowing circular route between the two rooms. Brady's young son unwittingly displays his dad's keen sense of space by running around from diningroom to kitchen giggling.

The sisters' goal of having a better space in which to entertain was put to the test over Christmas. "When we'd arrange to meet up with friends they all said, 'we'll come to your place' because they wanted to see the new extension. And the response has consistently been, 'wow'."