Nobody should under-estimate the massive scale of what has been proposed for Spencer Dock, the most pivotal site in Dublin's Docklands. Yet very few people seem to have fully grasped the truly gargantuan nature of this scheme and the impacts it would have at ground-level and on the city's skyline.

For a start, the 51-acre site is the largest ever in Dublin to come up for comprehensive redevelopment. It is, for example, twice the area of Temple Bar - and just think of all the urban diversity which can be experienced on its network of narrow streets compared to, say, the sterility of the IFSC.

It is clear that Spencer Dock would be nothing like Temple Bar under the master plan drawn up by the Irish emigre architect, Kevin Roche. It will, if anything, be more like the IFSC - but on an incomparably larger scale "more in common with Detroit than Dublin", as Cllr Ciaran Cuffe (Greens) has put it.

Mr Cuffe is an architect and knows how to read plans; most people don't. He can also imagine what the scheme as proposed would look and feel like if all of it was built; most people can't. No matter how many images are shown, the vast majority seem incapable of visualising a building until it is staring them in the face.

So let us tease out the scale of the Spencer Dock scheme. Here, for the purposes of comparison, think about Canary Wharf, the huge office complex on the Isle of Dogs in London's Docklands. Everything that's been built there so far, including Europe's tallest building, is less than what's been proposed for Spencer Dock. The first phase of Canary Wharf consists of 5.6 million sq. ft of space while 6 million sq. ft of space is planned for Spencer Dock, of which corporate offices would account for 2.2 million sq. ft. Only when its second phase is completed, providing an additional 5.7 million sq. ft, would Canary Wharf leap ahead.

Height is a real problem, too. According to studies commissioned by the Dublin Docklands Development Authority, any building over 40 metres at Spencer Dock would appear above the parapet of Holles Street Hospital, at the end of Dublin's Georgian Mile; the buildings proposed would be up to 95 metres high.

Their configuration is also problematic, with the tallest building, perhaps, inevitably positioned in the middle of the site. However, no detailed compositional studies have been submitted to justify the configuration proposed.

It is an open secret that the DDDA's draft master plan for Spencer Dock, prepared by Scott Tallon Walker and Urban Projects, proposes that any development should be capped at 3.5 million sq. ft, or 325,000 sq. m - not much more than half of what the developers, led by Treasury Holdings, now want. The draft master plan is also believed to recommend that no building on the site, other than the National Conference Centre, should be higher than 40 metres (132 feet). This would restore to the NCC that "landmark" quality it was supposed to have, according to Bord Failte's most recent brief for the project.

It is no wonder, therefore, that the DDDA and the developers fell out. If the draft master plan reflects its thinking - and it does - there was never any possibility of reaching agreement on the parameters of a Section 25 planning scheme, under which the project might have been taken out of the normal planning process.

Had they decided to avail of such a scheme, the developers would have had to scale down their ambitions and perhaps even agree to give the DDDA a "slice of the action" - something which it keenly sought. Instead, with relations in tatters, they calculated that they would receive a fairer hearing from Dublin Corporation.

John Fitzgerald, the highly pro-active city manager, has described the NCC as a "must have" project for Dublin and there is no doubt that he would like to "deliver" it for the Government - and be seen to do so. But he also has to bear in mind the "proper planning and development" not just of Spencer Dock but of the city as a whole.

Even before the planning application was lodged on March 2nd, a high-level project team had been set up. Last December, this team - which includes the chief planning officer, Pat McDonnell, and the city architect, Jim Barrett - met Kevin Roche and began a process of outlining some "key fundamentals" in developing the site.

An external expert in urban design - Michael Lowe, of London architects Arup Associates - was called in to advise, using insights from his own experience of large-scale schemes, such as Broadgate in London. His view was that the officials should put the Roche plan to one side and decide what they would like to see on the site.

This led to the preparation of a set of "development principles", which were presented to the City Council's planning committee earlier this month. And while this two-page document says Spencer Dock "offers a major opportunity for urban regeneration", the parameters it lays down can scarcely be met by the current development proposal.

"The layout and design of the centre must reflect the predominant street and spatial character of the city as a model for redevelopment", the guidelines say. "In terms of bulk and height, the development must recognise the sensitive character of the skyline of the city, including vistas, in locating any high buildings on the site".

There must be "respect for the amenities of existing residential communities in close proximity, especially in relation to bulk, shadow, outlook and physical presence". Within the site itself, there must be pleasant living and working conditions and clearly-defined public open spaces, without the unpleasant effects of wind or excessive shadow.

"The proposed centre, being located in a unique waterside setting, must demonstrate that in terms of its spaces, streets and buildings it can achieve the highest architectural and civic design standards", the corporation's guidelines insist, saying that any development at Spencer Dock must generate "an overall sense of place".

It is the DDDA's stated view that the height and density of the proposed development would produce a "poor quality environment" resulting in "serious injury" to the amenities of local residents through overshadowing and overlooking. Building heights would also cast shadows over many of the open spaces within the site.

In its submission to the corporation, the DDDA suggests that the present scheme "fails" because it has "ignored" one of the critical factors in urban design - that the quality of spaces between buildings must match the quality of the buildings themselves. It also opposes "fundamentally" a two-storey podium across much of the site.

Both the DDDA and the corporation have question marks over the proposed 3,000 apartments to be provided. Since all of these would be in high-rise blocks, it is unlikely that they would be suitable for family living - a goal in Docklands - and inconceivable that they could be used to provide the required 15 per cent social housing. According to the developers, the density of the Spencer Dock scheme is "not excessive". Density is measured by plot ratio, a formula which involves dividing the gross floor area of the any development by its site area. In this case, the plot ratio is given as 2.58:1 (excluding the National Conference Centre), which seems tolerable.

However, it would appear that the plot ratio has been calculated across the entire site - including parts which could never be developed, such as a 600-metre stretch of the Royal Canal, the roads that traverse the site and the camps hires along the Liffey and on both sides of Spencer Dock.

Traffic is also a huge issue. The implications of providing more than 7,300 car-parking spaces on-site in an area with a "sparse and discontinuous" road network - to quote the developers' environmental impact statement - can barely be imagined. Without a new bridge at Macken Street and the Port Tunnel, it would mean gridlock.

To avoid such a scenario, the corporation's development principles say the phasing of any development at Spencer Dock must proceed in tandem with provision of the necessary transport infrastructure, particularly public transport. More leisure facilities are also needed to ensure that the site does not become dead after dark.

The corporation has made it clear "significant changes" will have to be made to the present proposal before planning permission could be granted and it is seeking further information from the developers. But would even an amended version of the scheme, with floors lopped off its 26 buildings, overcome its failings in urban design terms?