Architect Derek Tynan started by sinking the floor when he extended a Victorian villa in south Dublin. The result is a winner, says Emma Cullinan.

Going down is the new going up, quips the owner of a south Dublin house which has just been extended by DTA Architects. When we think of extensions we do indeed think of another storey being put on top of a building, or a structure being added to the rear of a house.

Yet this extension wraps around the side and back of a Victorian villa like a curved hand, clutching the house and filling the void between it and the home next door.

The existing house - which the owners have lived in since 1967 - was a traditional double-fronted home, with four rooms on the upper floor and two below.

The brief was for more space, a better quality of space, and increased light. There were a few definites, such as an extra bathroom and a wish to preserve the existing period rooms which all have strong characters.

These include a drawingroom (literally: pen and ink pictures among others are created here) and a needlecraft room.

The brief was abstract for good reason: the client says that he knows a good thing when he sees it but admits to not being able to create it himself despite having trained as an architect many years ago.

He also trusted Derek Tynan and "was very excited about working with him".

Tynan's response to the brief was to lower the floor level by about 2ft, where the extension starts.

The differentiation between the old and new structures is clear, in that one part is clearly period and the other is contemporary, but the pivotal, linking room marries the two perfectly.

It sits below a rear upper room (to the right as you face the front door) mirroring its form and thus squaring the house in your mind.

But it's lower than the existing ground floor, it has modern lighting flush with its ceiling and the fireplace is of a contemporary design. So the room belongs to both the old and the new.

It can be shut off into the existing house through the use of a closable timber wall, or it can open up into the new kitchen and on out into the garden.

Circling around the kitchen units the route returns to the pivotal sittingroom or heads straight along a hallway out to the front of the house. The sinking of the floor has allowed storage space and a new bathroom under the house, reached from this corridor.

On the first floor, manipulated floor levels have enabled an exit from an upper bedroom onto a cedar-clad terrace.

A side door in the same bedroom leads to a space that overlooks the kitchen through a frameless glass barrier (rigorously tested by three young boys last week).

Turning back along the former alleyway between houses, there is a bathroom whose window to the front is protected by slatted windows formed by a break in the cedar cladding on the front wall: details, details.

"When I first saw the section drawings I though, 'jays, how did he do it? It was a magical solution'," says the client, who Derek thinks was "fantastic to work with".

This isn't zap, bang, look-at-me architecture, it's a resolution of internal space. There is very little evidence of a new structure from the outside. Indeed, if you saw it from the rear you would think it was just another timber-clad box.

But that cedar cladding is continual rather than continuous. The slats thread the building together, always appearing in some form throughout the extension.

It runs through windows, slides across the top of the glass wall to the garden, presents as a generous bench in the garden and gathers into a nest of cedar on the terrace where, after a rainfall on the day I visited, it emits a comforting wet-wood scent.

The linking of the interior - with its spaces looking down and up into each other, and circulation around interior structures - doesn't appear to be simple, and yet all of the spaces flow effortlessly into each other. There is a clarity of thought about a complex structure, in which there were few, if any, right angles to work with.

This extension has almost imperceptibly clutched onto and sunk in against a handsome structure, creating a greater whole. It has also nearly doubled the size of the accommodation from around 139sq m (1,500sq ft) (with a previous extension) to 214sq m (2,300sq ft).

Such micro-surgery was mastered when Tynan worked with architect Rick Mather in London in the late 1970s. "The practice at that time was doing a lot of careful domestic work in Camden.

"In London people put money into their houses at a time when, in Ireland, people would move somewhere new if the existing house wasn't working. But that has changed dramatically."

There are now lots of good examples of extensions in Ireland (and bad) and here Tynan has shown how a good architect, client, engineer and contractor (Mick Deevy comes in for high praise) combination can work so well.

This €350,000 structure - which is all the client wished for "and more" - shows care in design and detail.

Here the spaces flow into each other; the differing levels don't jar; various elements match each other when they meet (the granite slabs in the kitchen are exactly the same size as the concrete paviers in the garden and all of the gaps between them line up); the points where different elements meet are resolved; elements are used in a logical way, they are not just tacked on because they are fashionable (such as that flowing cedar).

Problems are resolved in a way that hides the fact that there was a problem (walls meet at right angles in key areas, giving a sense of order, even though in less obvious places they hit walls at odd angles) - and each space has a use and is nice to be in.

The play of light is a delight. This is an east/west facing structure that gets no southern sun, yet it now scoops in light from large expanses of glass both at ground level and through voids above the kitchen counter, as well as down from the terrace.

There is one complaint, that a cat-flap in the brief never materialised. Yet as the grey house-cat clambers up the ladder created by the timber slats, the client agrees that there are perhaps compensations.

Tynan says that he judges the success of a project by whether the client invites him back to dinner, and the meal invitation has been forthcoming here.

As he straightens a speaker on the wall he laughs: "A contractor once said to me; 'You've been cursed with a good eye'."

He hasn't turned his back on that internal spirit level but has used it here to raise his client's spirits.