Fantasy can, increasingly readily, become reality. Zaha Hadid, for instance, first came to prominence over 20 years ago with her competition-winning design for the Peak Club in Hong Kong. It was never built, but Hadid's series of gouache paintings of the scheme - one of which is in the show - introduced her jagged, splintery style and announced the arrival of a genuine talent. Could she build it? Yes she could: today, she is building all over the world. As is Will Alsop, whose buildings become ever more energetically outré. Why is his competition-winning "Fourth Grace" proposal for the Liverpool waterfront in the exhibition at all? It's a real-life ongoing building project, a bit early to classify it as dreamland. Maybe its inclusion reflects the fact that Alsop tends to start with a hopeful approximation of a building, which then gets progressively modified.

And this presents us, the lovers of the glorious unbuilt, with something of a problem. Today, it is possible to build almost anything, in almost any shape. A few decades ago, Frank Gehry's extraordinary billowing designs would have seemed bonkers. Were it not for the arrival of a particular computer program which allowed him to scan his junk models into buildable form, they would have remained the visual mutterings of an eccentric. But he was not the first to benefit from technology in this way. The "Fantasy Architecture" show has some soft-pencil sketches of the Sydney Opera House, dating from 1961. Architect Jorn Utzon's designs for those overlapping shells were absurdly over-ambitious for the time. These sketches are not by him, but by the great engineer Ove Arup, working out a way to make it stand up. As we see, he succeeded.

So today, what was recently fantasy architecture is built, routinely. This ought to spoil the game, since the everyday cannot, by definition, be fantastical. But it has always been thus. How impossible was the idea of a Gothic cathedral in the late 12th century? The fantasists will always trump the pragmatists because all they have to do is imagine something one or two steps beyond today's norm. And then sit back and wait for everyone else to catch up.

Northern Gallery for Contemporary Art, Sunderland: http://www.ngca.co.uk