A house collapses after its owner digs basement illegally on Deshengmen Inner Street, Xicheng District in Beijing, on Jan 24, 2015. [Photo/IC]

In January, workers illegally digging a basement under a courtyard on Deshengmen Inner Street, Xicheng District, caused a street surface to collapse. Traffic on the northern second Ring Road of downtown Beijing was interrupted and four houses nearby collapsed.

The cave-in resulted in a crater 15 meters long, 5 meters wide and 10 meters deep. The accident later shocked the country after it turned out that the owner, Li Baojun, was a legislator in Xuzhou, a city in East China's Jiangsu province.

An investigation by Beijing News reporters shows that building a basement under a courtyard is gaining momentum in the capital. The reason is practical and economical - it can not only enlarge a living space but also add value to a house.

A real estate agent roughly estimated that a 300-square-meter courtyard can sell for 45 million yuan. If a basement is dug out, it can add 10 million yuan to the property's total value. Construction costs are about 1.2 million yuan.

Digging a basement without official permission is dangerous because the construction may break gas pipes or water pipes buried underground, endangering public safety. Digging is difficult to detect unless neighbors report it to the police.

Authorities in Beijing strengthened their effort to crack down on illegal basements after the legislator's case.

A notice issued by authorities says that law enforcement officials can enter a house to investigate after receiving a tip-off and noticing suspicious activity. Previously, they didn't have the right to go inside.

When illegal underground construction is found, the property owner can be ordered to restore the site to its original state. If the owner refuses to comply within three days, related departments can enforce the order to fill in the basement and bill the owner.

In October, an illegal 318-square-meter basement in Qinglong Hutong, Dongcheng District, was filled in after it was reported by local residents and confirmed by local police.