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 department said.

It noted there was no need to seal the house off given it posed no obvious risk to the building’s overall structure or a retaining wall along the slope, but occupants were told to temporarily fence off their outdoor swimming pool over safety concerns.

Only residents at house No 72 had received an evacuation order, which came on Saturday night, 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 was also allegedly demolished to make room for basement windows, they said.

“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 also uncovered an illegally built swimming pool on the slope below the house, but the government said it would need more time to determine if the unapproved structure had contributed to damage to the retaining wall.

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The Buildings Department stressed it would work with the Geotechnical Engineering Office to investigate whether the illegal structures were linked to the landslide.

A spokesman for the office on Monday night said officers had checked part of the government slope that was not affected by the landslide and found no abnormalities. He added authorities would complete the inspection by Tuesday.

The Buildings Department 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 owners later agreed to let the officers enter the site on Tuesday.

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 with caution, especially when constructing pools, since the water posed additional problems.

He pointed to a landslide in 1994 that struck Kwun Lung Lau, a public housing estate in Kennedy Town, and left three people injured and five dead.

Authorities later found that long-term water damage to a nearby slope caused by leaking pipes had triggered the landslide. Officials subsequently issued guidelines for building water facilities on such terrain to prevent further incidents.

03:45

Hong Kong issues its longest black rainstorm alert ever as city pounded by sudden torrential rain

Hong Kong issues its longest black rainstorm alert ever as city pounded by sudden torrential rain

“It has been 30 years since then, and people in the industry should know these rules well,” he said.

“But if these really are unauthorised constructions, and someone who was not a professional conducted them, they may be unfamiliar with the government’s stringent guidelines. I would be a little worried in that case.”

While Chan said he could not determine whether the illegal structures had contributed to the landslide, he noted that aerial photographs showed at least four areas on the slope top had possible water seepage.

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“When water is continuously leaking, it affects the stability of the slope … and can lead to landslides.”

Veteran structural engineer Ngai Hok-yan on Monday said he believed the unauthorised structures could contribute to the instability of the slope.

“Whether it is inside or on the slope, if you put a basement there it will be an additional load on the slope,” he said. “Rainwater can also seep into the soil more easily. When the slope is saturated, it can become highly unstable.”

Remedial construction work at house No 72 would be very difficult, he explained.

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“You can’t simply tear down the illegal basement. This might further destabilise the slope or the building above,” he said, noting Civil Engineering and Development Department officers might be needed to make assessments on how to remove the illegal structure safely.

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 died during flash flooding.