What's really surprising about the RIAI's current exhibition, the New Housing 2001, is not so much the fact that a lot of mould-breaking higher density housing is in the pipeline, but how much has already been built - not just in Dublin, but in other cities, towns and even villages.

Though the Residential Density Guidelines are just three years old, it is evident that they are having an impact on how we think of "housing".

In the past, few could imagine living in anything other than a semi-detached house in the suburbs; now, the choices available are considerably wider.

"We no longer talking about theory", says John Graby, the institute's director. "What's happening now is extraordinary - suddenly, higher density housing is being built throughout the country. And the 65 schemes featured in the exhibition are the most typical of what's being built at the moment."

It does not set out to promote any particular solutions, still less styles of architecture. The schemes on view range from the neo-vernacular approach of McCrossan O'Rourke in Ballsbridge Wood, for example, to the more radical modernism of Howley Harrington in Balgaddy, between Lucan and Clondalkin.

Some of the Dublin schemes are very familiar - A+D Wejchert's Smithfield Village, Anthony Reddy and Associates' work in the west end of Temple Bar and O'Mahony Pike's sought-after Herbert Park Lane apartments beside the River Dodder, which achieved a density of 90 units per acre without compromising quality.

Two major projects by O'Muire Smyth at Beresford Street and Benburb Street, each occupying a city block, demonstrate that decent private and semi-private open space can be provided in large apartment schemes. Indeed, it is hard to imagine apartments being designed nowadays without such residential amenities.

The sheer volume of work is impressive, covering outer suburban areas as well as high-profile inner city locations. The exhibition also includes infill projects in Cork and Limerick and some of the larger towns, large-scale urban extensions and at least one good example of what can be done to rejuvenate villages.

Gerry Cahill Architects' project for Avonbeg in Tallaght gives some idea of how to revive a jaded low-density housing scheme. Originally laid out as a row of villas, Avonbeg is having its gaps infilled to provide up to 90 additional units, in a wide range of types, for tenants of South Dublin County Council.

Cahill, wearing his hat as a partner in Urban Projects, is also involved at the upper end of the market with Clarion Quay in the Docklands area.

The apartments here, with their large balconies and winter gardens, are among the best yet built in Dublin and include a 20 per cent allocation for social housing.

Conroy Crowe Kelly's plans for Grange Road, near Baldoyle, illustrate what's in store for the city's 'north fringe'.

Putting 484 houses, 1,139 duplex units and 311 apartments on a 110-acre site might be regarded by some as cramming - until you look closely and see the generous provision of green spaces throughout.

The exhibition includes at least two schemes that proved very controversial - one at St Anne's, in Milltown, and the other on lands in Blanchardstown that used to be part of the James Connolly Memorial Hospital.

In both cases, they were opposed by local residents who believed the densities were too high.

But nobody is talking about "huge monolithic slabs", as John Graby puts it, still less tower blocks - unless you count OMP's elegant tower at Charlotte Quay. Most of the schemes are low to medium-rise and some are clearly inspired by Dutch examples, such as the Almeer housing area outside Amsterdam.

Much of what is on show is clearly recognisable as 'houses', even if many of them are actually duplex units over garden flats.

"I think what we are seeing is a changing social pattern", Graby says. "We are moving away from the idea of 2.5 children per family living happily together for the next 40 years."

He believes Part V of the Planning and Development Act, 2000 - the bitterly-fought provision laying down a 20 per cent requirement for social housing - will mean more people living in apartments and other relatively unconventional forms of housing.

"What we're trying to show is that higher density works".

Designing a housing estate with a density of, say, 20 or 30 units per acre involves a lot of work - and this represents both a problem and a challenge.

It is obviously a problem if the designer doesn't understand what's required, but resolving all of the issues can be quite a challenge even to the most thoughtful architects.

Several of the schemes featured in the exhibition are mixed use developments, adding another layer of complexity.

A new shopping centre in Balbriggan, for example, will have housing overhead. Infill schemes in Cork and Limerick also attempt to integrate apartments with retail, restaurant and leisure uses.

At the edges of towns, there is scope for experimentation - such as Emer O'Siochr·'s plan for an "eco-village" in Athlone, McCrossan O'Rourke's model scheme for Mulhuddart or OMP's daring plans for Pelletstown, off the Navan Road in Dublin, with its close integration of housing and rail-based public transport.

Some of what's on view hardly rates as "high density" at all, such as the 10 units per acre in one scheme planned for Ratoath, in Co Meath.

Others are quite modest in terms of their ambitions while a few are misconceived, such as the overblown neo-classical layout planned for a housing scheme off Swords main street.

One of the most useful, in terms of its applicability elsewhere, is a very civilised development of nine town houses laid out around a village green on a half-acre backland site in Ballycastle, Co Mayo, by Doherty Coleman Architects. If the public could be persuaded to see the sense of this, we might not have the Bungalow Blitz.

"The big change is that we can now show people what the future is going to be like", says John Graby, using photographs of completed schemes or computer-generated images of those yet to be built.

That should make it easily accessible to elected representatives, local and national officials and community groups.

The exhibition, which will tour a number of venues in the coming months, is to be supplemented by two seminars on higher density housing - one in Dublin on September 25th and the second in Cork on October 3rd - and by the publication of a book comprehensively illustrating all of the featured schemes.

Apart from photographs and photo-montages, the book will include more detailed information on each scheme, including plans, sections and elevations. Graby forecasts that it will be "the best copying book that architects doing housing could buy, because they will be able to see what works without having to visit every site".

Now, having grappled with the assembly of higher density housing and all that goes with it, architects probably need to focus more attention on the design of living spaces to move away from traditional, highly-compartmentalised internal layouts towards something much more fluid and flexible for the 21st century.

The New Housing 2001 exhibition continues at the Architecture Centre, 8 Merrion Square, until September 7th. It will travel later to Athlone, Cork, Mullingar and other venues.