The Buildings Department also said it had applied for a court warrant to inspect a third property, house No 74, at Redhill Peninsula in Tai Tam. Occupants had earlier denied government employees access during their investigation, it added.
Secretary for Development Bernadette Linn Hon-ho on Sunday confirmed authorities had found illegal structures at house No 72 that unlawfully occupied government land.
The property was one of three residences, alongside No 70 and 74, damaged by the landslide on Saturday night, following record rainfall over Thursday night and Friday.
The government on Monday said a further inspection by Buildings Department and Geotechnical Engineering Office employees suggested house No 70 also had illegal structures.
“A five-by-13-metre [16.4-by-42.6-foot] unauthorised structure was found on the lowest podium of house No 70. A five-by-six-metre basement was also found behind it,” the Buildings Department said.
The department noted there was no need to seal the house off given it posed no evident risk to the building’s overall structure and the retaining wall near the slope on government land, but occupants were told to temporarily fence off their outdoor swimming pool over safety concerns.
Only residents at house No 72 have received an evacuation order so far as the property was considered at risk due to unstable soil. No visible dangers were found at No 70 and 74.
Authorities also revealed further details on Monday about the illegal structures uncovered at house No 72, saying an unauthorised basement with a 17-by-19-metre floor space was found underneath the property.
A section of the retaining wall along the slope was also illegally demolished to make room for basement windows, they added.
“These unlawful constructions pose a high risk to the structural safety of the entire building, so house No 72 must be temporarily closed to ensure safety,” authorities said.
Inspectors had also uncovered an illegally built swimming pool on the slope opposite the house, but the government said it would need more time to determine if the unapproved structure had contributed to damages to the retaining wall.
The Buildings Department also said it would apply for a court warrant to enter house No 74 after occupants refused entry to government officers sent to inspect the property.
The department stressed it would work with the Geotechnical Engineering Office to investigate whether these illegal structures had anything to do with the landslide.
Raymond Chan Kin-sek, a former head of the Geotechnical Engineering Office, told a morning radio show that building works on slope tops should be done cautiously, especially when constructing pools, since water posed additional problems.
He pointed to a landslide that struck Kwun Lung Lau, a public housing estate in Kennedy Town, in 1994 and left three people injured and five dead.
Authorities later found that long-term water damage to a nearby slope caused by leaky pipes had triggered the landslide. Officials subsequently issued guidelines for building water facilities on such terrain to prevent further incidents.
“I cannot say whether the landslide this time is related to the unauthorised constructions … but if a person unfamiliar with the government’s stringent guidelines was in charge of such works, then I am worried,” he said.
Discussing the incident in the Tai Tam, Chan said aerial photographs showed at least four areas on the slope top that had possible water seepage.
“When water is continuously leaking, it affects the stability of the slope … and can lead to landslides,” he explained.
Last Thursday, a record black rainstorm alert issued at 11.05pm lasted for more than 16 hours, with the city reporting the most amount of rain ever collected in an hour. The Observatory headquarters logged 158.1mm (6.2 inches) of rain between 11pm and midnight that day, the highest since records began in 1884.
The downpour brought the city to a standstill the next morning as streets were turned into rivers, drivers were left stranded in cars, restaurants and railway stations flooded and landslides struck near residential estates.
More than 140 people were sent to hospital and one man had died during flash flooding.