The Foreign and Commonwealth Office believes it will require a bigger embassy to cement its influence in the region once the conflict with Al-Qaeda and the Taliban is over.
At present, the UK government operates from two relatively small buildings on a 8000 m2 site in Kabul, although it has also operated out of the Pakistani capital of Islamabad during the war.
An FCO source said: "Inevitably there will be a need for a new building after the war. The current buildings are part of the old British embassy site and they are not as large as we would like them to be."
The site is mostly unused at present. According to the source: "There is quite a lot of land because of our old colonial buildings. When our needs there declined [following the withdrawal of the British from the Indian subcontinent] our estate contracted."
The larger role of the embassy in international attempts to stabilise the region mean that much of its estate needs to be rebuilt. However, work is not expected to be complete for at least five years and invitations to tender are unlikely to be issued until the war is over.
Inevitably there will be a need for a new building after the war
An FCO spokesperson said a new scheme would not necessarily use the existing site. He said: "We have a long-term commitment to Afghanistan. We expect in the medium to longer term to review our accommodation there."
The project is one of several flagship embassies to be launched in the next 18 months.
Officials have discussed plans with architects to build an embassy in Beijing. A British official in China said a new building there would have to accommodate about 400 employees and would not be ready for at least three years. The project is complicated as the embassy is located in three buildings on different sites.
Another project likely to be advertised in the European Union's Official Journal in coming months is a revamp of the British embassy in Warsaw. An FCO spokesperson said: "We are looking into the possibility of upgrading the embassy and the [ambassador's] residence."