Prestigious projects like this are normally put out to what's called a "design competition" rather than a tender as such. The competion briefing would include any restrictions imposed by the outline planning permission (i.e. height, footprint, overall accommodation etc.) - full planning permission would only be granted upon submission of a detailed design but this is normally a formality if the outline conditions have been adhered to. It's possible that due to the nature of the project and the fact that the site will become US sovereign territory the normal planning process would not apply. Whilst not being particularly attracted to the proposed design, I feel its impact on the locale will be less than the overbearing and bombastic structure currently occupying the west end of Grosvenor Sq.