Castles in the sky

Which three British buildings that never were would you like to conjure into life with Millennium money? This idea came into my head the other day in Charing Cross Road (a London street bereft of good architecture) when I was mooching through Kerry Downes' veteran book on Nicholas Hawksmoor (1661-1736) and kept turning back to a design for a Baptistery this brilliant Baroque architect wanted to site at the west front of St Paul's Cathedral. The drawing dates from 1710, the year Wren's great temple was completed (the architect was 78 and still very much alive), and is a part of a fascinating plan to rein St Paul's around with a wedge-shaped piazza. The Baptistery would have been the pivot at the west and pointed end of Hawksmoor's wedge.

The plan is an excellent one, formal and grand, yet, because Hawksmoor envisaged an irregular wedge, particularly English in its romantic asymmetry and compact scale. The piazza, if built, was to have been flanked on all sides by simple and elegant five-storey arcaded buildings dressed, no doubt, in Portland stone. The line of old City streets and alleys would have been kept and traffic from these would have given on to the piazza through Hawksmoor's arcades.

The Baptistery itself is a handsome circular building, topped with a dome which Hawksmoor was to use again in his proposed design for the Radcliffe Library, Oxford (1733-34), which James Gibbs adapted for the building we see today. The Baptistery design suggests a typically solid Hawksmoor building, its base apparently sunk deep into the London clay, an illusion encouraging the onlooker to imagine that the structure had grown naturally from its setting, rather than having been plonked down as if from outer space. This effect is part of the reason that, if built, the baptistery and the piazza it heralded, would have been so very much a part of the architectural landscape of the City of London. It would have looked and it would have felt right.

The Baptistery, in case anyone is thinking ahead of me, would not have obscured the west front of St Paul's. Its dome would have been as high as the nave walls of the cathedral and it would thus have set up a glorious Baroque dialogue, with the smaller dome acting as standard bearer to the great dome behind, and the exquisite circular building turning pedestrians into the piazza, so as to enjoy a panoramic vista of St Paul's.

Had Hawksmoor's Baptistery and piazza been built, we would, Luftwaffe allowing, have a near-perfect setting for St Paul's, avoided the sad fate of the cathedral's east end (it faces nothing in particular, lines up on no axis), and, of course, we would not be faced with the unholy debate plaguing us today over the future of Paternoster Square.

Here is one instance when I would turn my back on present and future and, digging in to the Lottery coffers (a far greater bounty than the Coal Tax that paid for St Paul's and would have been needed to fund the piazza), would build Hawksmoor's piazza from solid stone, and save the cathedral so many of us love from the ignominy it faces today.

The Hawksmoor scheme would be expensive, but nothing like the cost of building Edwin Lutyens' Liverpool Cathedral, a design dating back to 1929 that was started (the present cathedral, "Paddy's Wigwam", stands at one end of where its Brobdingnagian crypt would be), but never finished. The problem for Lutyens (1869-1944) was also his opportunity for designing one of the greatest non-buildings of all time. Liverpool was the landing stage for Irish Catholics seeking food and work in the years following the terrible potato famine that killed one in eight people and sent a million more around the globe in search of food and work.

By the Twenties, Liverpool was a very Irish and a very Catholic city. Whilst the towering Gothic cathedral designed for the Anglican community by Sir Giles Gilbert Scott (1880-1960) soared above the industrial Mersey, Catholics were pinched into inadequate churches. But, while the idea of a great cathedral at Liverpool was a noble one, the city was never able to sustain the immense cost of this titanic project. Whether you consider a British cathedral bigger than St Paul's and rivalling only St Peter's in scale, pointless or not, there is no doubt that Lutyens' heroic domed temple would have been an architectural triumph.

Of course, anyone can argue that the very idea of building a classical temple on such a vast scale, and not scheduled for completion until well into the 21st century, was a nonsense to begin with. But this is to miss the point that here was no piece of passing architectural sophistry, no embarrassing historic pastiche, but an elemental building that we would have counted with the pyramids, with the world's greatest temples and monuments. It would, like Hawksmoor's buildings, have stood as far outside time as a building ever can. But where the design of St Paul's was an inspired compromise, the Cathedral of Christ the King, Liverpool, was fated to be scuppered.

What would the cathedral have been like? Vast, entered through a magnificent, yet stripped-to-basics triumphal arch, a tripartite entrance tunnel leading into an eye-boggling intersection of herculean stone vaults and, at the core, a mighty dome to rival the very best.

It would, unlike St Paul's, have been dark and mysterious within, more Byzantine than Baroque; and yet, by far the greater mystery was the manner in which Lutyens resolved the supremely complex geometry of his ambitious design. As the late Sir John Summerson wrote in a penetrating essay published in the catalogue of the Lutyens' exhibition held in the Hayward Gallery in 1981, "dimensional inter-dependence of this complexity has rarely, if ever, been achieved in a cathedral design and certainly in no cathedral ever built."

Lutyens was ever a creature of the greatest complexity, hiding, for example, a mind that could design one of the greatest cathedrals of all time behind a detached and bemused facade of terrible puns when anyone tried to probe his ideas too closely. When asked his opinion of dinner on one formal occasion by King George V, Lutyens pointed at the indifferent food on his plate and answered "this is the piece of cod that passeth all understanding". This is how he spoke to kings and clients, to journalists and family and is, why, of course, he was greatly loved.

His design for Liverpool Cathedral, however, is far from loveable: it is a thing of utter sublimity. It is also very far in form from St Paul's, even though Lutyens was Wren's most devoted fan. Unlike Wren, Lutyens was given a free hand at Liverpool. Whilst St Paul's is essentially a domed Gothic cathedral given Baroque form (its exterior apperance is far from being a true expression of its interior), Lutyens's Liverpool cathedral reveals his handling of interior spaces at every level and degree as would have been seen from the surrounds of Mount Pleasant, where Sir Frederick Gibberd's space-capsule cathedral stands, or along the axis that is Hope Street.

Construction of the cathedral began in 1933, the year Hitler came to power, and was halted by the world war Hitler provoked. If you ask nicely, and otherwise on certain days and at certain times, you can step down into the crypt and see just what a fine job Lutyens would have made of the whole.

The only British design to rival Lutyen's Liverpool cathedral was also never built, and this is my third Lottery-fuelled architectural fantasy. It is, of course, Wren's original "Greek Cross" design for St Paul's, a building of the greatest integrity and of a grace and gentleness (and gentlemanliness) that Luytens did not attempt in his more muscular cathedral. You will be able to see the great wooden model Wren had made of his design (it is big enough to walk into) when its restoration is complete and Martin Stancliffe, Surveyor of the Fabrick of St Paul's, has completed the creation of a museum in the, to date, forbidden heights of the cathedral's upper galleries.

Sadly, as only the maddest dogs among us would want to demolish the existing St Paul's (Hermann Goering had a go at the height of the Blitz), there would be no natural home for the earlier design. No, I am afraid that my proposals for building these superb domed cathedrals by two of our greatest architects must remain temples in the sky. And yet I have a feeling that the Hawksmoor scheme for St Paul's is perfectly feasible.

These are my suggestions. Now, tell me yours. They can reach back to the era of Stonehenge or up to the present day. They may well involve demolishing some banal horror and replacing it with a thing of beauty. All unreasonable requests considered.