The Mayor of London has asked the US make alterations to its application to move its embassy to Nine Elms - and could ask it to contribute money towards extending the Northern line into Battersea.
Boris Johnson said proposals for the £275m development currently breached planning regulations and raised the possibility that money earmarked for the Crossrail project could be diverted.
The US announced it was planning to move to Battersea from Grosvenor Square site last October, citing security concerns and a lack of space as the reason behind the move.
Plans for the new site include a 30-metre security “blast zone” with a four-metre perimeter wall.
But in a letter sent to developers City Hall said current proposals breach the London Plan, the Mayor's planning guidelines, and face rejection.
It said the Mayor was concerned the wall meant the 97 metre building would be concealed from the river and that the embassy should be “visually permeable to preserve a sense of openness and interaction between the embassy and the surrounding area”.
It also asked for more eco-friendly changes.
The Government’s design watchdog, the Commission for Architecture and the Built Environment, also criticised the plans.
The Mayor also raised the prospect of channelling American cash to fund the extension of the Northern line, instead of imposing a suggested Crossrail charge on the Embassy - which he is seeking from major developers in central London.
He and other developers hope to add a branch from Kennington Tube station to Battersea Power Station via the embassy site at Nine Elms and the US could contribute up to £2.5m.
A spokesman for the Mayor said: “Current indicators show that the extension of the Northern line could be a central element in supporting the redevelopment of the area. The Mayor, Transport for London, and the borough councils have written to the government to outline the benefits of accelerating development.”
The US hopes to move to the two hectare site by 2016, with work beginning in 2013.
Draft plans for the building, which would be used by 800 staff and attract 1,100 visitors a day, have been submitted to Wandsworth Council.
The Americans will select an architect early next year from a shortlist of four, including the 92-year-old designer of the Louvre glass pyramid, Ieoh Ming Pei.
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