What could possibly be wrong with the Scottish National Gallery's proposals to create a "world class" gallery in which the nation's artistic treasures can be properly and permanently displayed?

Who wouldn't want to have Raeburn and Wilkie or Paolozzi and Peploe given centre stage rather than languish in the shadows, or make only occasional appearances, as if they were less worthy of attention than their peers?

The idea for the gallery on the Mound to give greater emphasis to Scottish art is excellent. Less wonderful is that in order to accomplish this, it is deemed necessary to encroach on to the east side of Princes Street Gardens, thereby eating up a substantial strip of what is known as "common good" land. It will take an Act of Parliament to transfer this land to the eager SNG, but by all accounts everyone involved is already purring and thus it seems destined to go ahead.

Since the extension will be done under the watchful eye of the gallery's panjandrums, the project will no doubt be executed with style. That, however, is not the point. The prospect of another bit of Edinburgh's public free space disappearing is profoundly dispiriting. Such a move is as much an act of aesthetic vandalism as if someone were to tip a pot of Dulux over a landscape by Craigie Aitchison or slash a John Bellany portrait. The gardens are every bit as beautiful and valuable as any work of art. Once lost, the metres swallowed up will never be seen again. A Scottish collection, by comparison, could be housed in a variety of other locations: in Glasgow, for instance, or in the heart of the country, in Perth.

You could argue, of course, that the battle for the soul of Princes Street Gardens was lost years ago. This end of the park, opposite Waverley Station, can be more like a big top than a place to linger and relax. For several weeks it hosts a tawdry Christmas market and a fairground and ice-rink so extensive and noisy their presence is heard as far away as Leith. When eventually the circus disappears, so has the grass, and for weeks Edinburgers are obliged to thole a cheerless, muddy site more like the Royal Highland Show after a cattle stampede than the urban oasis it was originally intended to be. What Sir Walter Scott must be thinking as he stares out across this annual desolation is probably unprintable.

It is not fair, of course, to pick on a single act of well-intentioned land grab. Should the gallery be given permission to proceed, it will be only the latest in a long list of depredations that mar the capital, and make one wonder if it is in safe hands. Those charged with the city's protection clearly do not have an artistic bone between them. Although it is only 20 years since Edinburgh was designated a Unesco World Heritage Centre, already some are questioning whether it deserves to keep that accolade. Increasingly I am in sympathy with that view.

Writing of Edinburgh's New Town, Unesco states: "the layout of streets and squares maintain their intactness." Has anyone from the committee strolled along George Street of late? The main artery of the New Town has at each end an austere, airy square. After the Edinburgh International Book Festival has packed up its tents, Charlotte Square is a wasteland until it recovers, but far worse is St Andrew Square. Formerly a haven of trees and wildflowers, padlocked to keep out the public, it is now a sleekly landscaped zone. That would be fine were it not for the Yule-tide stalls it hosts, whose hoardings give it the appearance of a refugee camp - and again, the grass takes months to relay. Then there are the regular art installations it displays, making it more like a kindergarden or modern art gallery than a park.

In treating these squares with so little care, the architectural and aesthetic integrity of one of the most elegant streets in one of the loveliest cities in the world is ruined. Not so long ago, Edinburgh was renowned for its air of superiority. Some of that attitude was pure snobbery, intended to keep people in their place. Some, however, was well-earned. The Athens of the North knew its worth, and fought to protect it. As a result, the capital was a byword for class. Sadly, that's no longer true.