And now... it's the Tate hotel

There's a certain kind of pedestrian precinct that breaks hearts with its well-intentioned tedium, its concrete planters and jokey sculptures (The Shopper, Kids), well-chosen Scandinavian lighting fixtures and interesting chequerboard paving patterns.

It's more common in middle-sized towns than in central London. So imagine the distress on finding that Tate Modern, the museum that has done so well by being clever, has released an image of its future self which, apart from a garnish of glassy high-tech classiness, and a certain upping of the scale, looks scarily like just such a precinct.

The image is a view of the currently desultory area to the south of Tate Modern, after transformation by a putative public-private partnership, and it shows every tic and commonplace of every precinct, plaza and mall everywhere from Plymouth to Gateshead at any time from the 1940s to now. Kneejerk pedestrianisation. Public art. Trees with everything or rather small green blobs of non-specific species. A big glass roof joining the Tate to an office development. Interesting paving. People wandering limply. People drinking coffee. Shops everywhere. Improbably blue sky. Azure Thames. Canopies. Banners, for Christ's sake.

The Tate has done well out of being different. Five million people have gone there, not least because it is not your usual airport/shopping mall/stainless-steel-andcaffe-latte theatre of tedium. Now, as if London were suffering vertigo over a place where culture precedes retail, the place risks being well and truly Starbucked. This is the district that fertilised Shakespeare, but the view is as poetic as a Microsoft manual.

The only intriguing element is a rumoured hotel on Tate land, which would be a first for national museums. Imagine: a British Museum Hilton or a V&A Hyatt or, more plausibly given the Tate's connections with a well-known New York hotelier, a Schrager Tate.

Luckily, the precinct is far from a fixed proposal. It is an illustration in an "urban study" that the Tate has commissioned from Mike Davies of the Richard Rogers Partnership, which looks at ways in which the site's hinterland might be improved, with the help of the investment the gallery is bringing to the area. It's a novelty for museums to turn into town planners, but it's admirable that the Tate should be taking an interest in its neighbourhood.

Nor is there anything wrong with the study's other agenda, which is that Tate Modern wants to double in size and needs to find the money to do so, an ambition which emerges coyly from the urban study. It was known that the Tate wanted to take over and convert the still-functioning electrical facility that fills the southern section of the former Bankside power station. It was also known that the Tate fancied some vast underground concrete cylinders which were once oil tanks. What's new is a new block, as high and long as the existing building, that shimmers into the dreaded perspective view like a sleek slice of Pompidou Centre grafted on to the Tate's scumbled back.

Tate director Sir Nicholas Serota wants more space because there are living artists out there, especially in America, who are reaching a certain age and are "looking for places where their work can be seen": Elsworth Kelly, for example, or Robert Rauschenburg, or Jasper Johns. The hope is to seduce them with beautiful expanses of new gallery, so the Tate can have many versions of its room of paintings given to them by Mark Rothko. Serota also wants to create a mother ship where other, smaller craft, dedicated to architecture or photography or design, can berth.

The other reason for getting bigger is that museums always get bigger, especially museums of contemporary art, and especially now. The Guggenheim, the Wal-Mart of modern art, has three branches planned in Brazil, one in Tokyo, another opening soon in Las Vegas and a Frank Gehry extravaganza proposed for lower Manhattan. Even a doubled Tate Modern will still be smaller than the Museum of Modern Art in New York, once its current expansion is complete. The Tate doesn't want to be left out.

Serota's problem is paying for this city of art, now that the Lottery boom is over, and here he can't help noticing that surrounding landowners have done well out of the upping of land values brought about by the Tate's arrival. So the study calls for "co-ordinated development", which means developers of surrounding sites chipping in for improvements, to the mutual benefit of commerce and art. It also shows something called "additional Tate development", which means profitable construction on the Tate's land, which means the hotel.