Fears have been raised that digging large basements could cause subsidence and affect the natural flow of the water table, increasing the risk of flooding. Camden’s new planning rules state extensions should be no bigger than the footprint of the house, and no more than three metres deep. Westminster is also reviewing its policy.
A council spokeswoman added: “Applicants must submit material to demonstrate that the proposed basement will not harm the local environment, including structural stability and drainage.” The move will be studied closely by planners across the capital.
John Walker, head of development planning at Westminster said: “We are currently reviewing our policies on basement extensions and will be consulting on a range of options shortly in our City Management Plan. Until the new plan is adopted we will judge each application on its merits.
Prime Minister David Cameron added an extra floor beneath his North Kensington home. Since then everyone from hedge fund millionaire Chris Rokos to comedian Ricky Gervais - who has an underground gym beneath his home in Hampstead - have latched on to the trend.
The cost of a basement varies wildly. But experts believe that a 700 square ft extension - excluding fit out - would cost around £100,000. Larger projects can cost seven figures.