The excavation will descend over 50 feet below the surface say reports, deeper than the height of nearby houses, and will feature a 45-foot swimming pool, hot tub, sauna, massage room, ballroom, covered courtyards, staff accommodation, parking, a turning circle and car lift.
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The property developer painted the mews house in March last year after she was denied permission for a basement below the property.
Profits at the estate agent plummeted 42% in the first half of the year.
Ionut Gheorghe, 27, was detained in Los Molinos near Madrid on Friday by Spanish police under a European Arrest Warrant.