Property buyers could be paying more than HK$100,000 for exterior walls they do not need, as developers build thicker outer walls to inflate their saleable area, a green group says. The trend was spotted by Green Sense, which said the thickness of prefabricated external walls had nearly doubled from between 12.5cm and 18cm from 2003 to 2006, to an average of 30cm in the years after 2006. The walls are an approved green feature that the government has exempted from the plot ratio. A 697 square foot flat at Sino Group's One Madison development in Cheung Sha Wan, for example, has an actual and saleable area of 511 sq ft - which includes balconies and 30cm external walls. 'The government gave 44 square feet away for developers as a reward for these prefabricated external walls, but they build them as thick as they can and we foot the bill,' said Roy Tam Hoi-bong, of Green Sense. Apart from green features, developers also include clubhouses, common areas such as corridors, columns and lift lobbies in properties' saleable areas. Lohas Park, developed by Cheung Kong (Holdings), for example, was found to have included a 29 sq ft column in the saleable areas of its flats.The cost of a One Madison apartment is about HK$5,452 per sq ft. That would mean the buyer pays HK$119,942 for the 30cm external walls, Green Sense says. Tam said there was no logical reason for external wall thickness to have doubled in five years, especially when Sun Hung Kai Properties still builds flats with 15cm exterior walls. He called for the government to stop developers from installing extra-thick walls and to require firms to list wall thicknesses in sales brochures. A Cheung Kong spokesman said all its sales were in line with legislation but declined to comment on individual cases.