Designed by Terry Farrell and built by John Laing, the developer Regalian Properties PLC approached the Government in 1987 to see if they had any interest in the proposed building. At the same time MI5 was seeking alternative accommodation and co-location of the two services was studied. In the end this proposal was abandoned due to the lack of buildings of adequate size (existing or proposed) and the security considerations of providing a single target for attacks. In July 1988 Prime Minister Margaret Thatcher approved the purchase of the new building for the SIS. At this stage the government proposed to pay for the building outright in order to maintain secrecy over the intended use of the site. It is important to note that at this time the existence of MI6 was not officially acknowledged.

The building design was reviewed to incorporate the necessary protection for Britain's foreign intelligence gathering agency. This includes overall increased security, extensive computer suites, technical areas, bomb blast protection, emergency back-up systems and protection against electronic eavesdropping. While the details and cost of construction have been released, about ten years after the original National Audit Office (NAO) report was written, some of the service's special requirements remain classified. The NAO report Thames House and Vauxhall Cross has certain details omitted, describing in detail the cost and problems of certain modifications but not what these are. Rob Humphrey's London: The Rough Guide suggests one of these omitted modifications is a tunnel beneath the Thames to Whitehall. The NAO put the final cost at PS135.05m for site purchase and the basic building, or PS152.6m including the service's special requirements.