[T]he role of reality in the production of an unsolicited project… is arguably what separates unsolicited architecture from so-called speculative or paper architecture. While Archigram’s visions of a walking city may have addressed a social need – for free and undetermined public event space – without financing or marketing, it comes across as entertainment. Which is of course, what it was intended to be, to the extent that it was even presented in comic book form. Which is also not to say that entertainment cannot inspire a real project, but that the strength of the unsolicited rests in its very tangible potential to be pursued through to realisation with the right political, financial and public support in place.
Read the rest of the (excellent) post, which includes several examples of unsolicited architecture.