The department’s senior building surveyor Robin Leung Chi-tim said the initial findings showed that there were 10 unauthorised structures.

“Under the arrangement of the authorised personnel, we conducted an investigation into house three and four [at Villa De Mer]. The two houses were structurally safe but we found there were some unauthorised building works that were not under the permitted building plans,” Leung said outside the villa.

Teresa Cheng must clear the air over illegal structures at her home

The illegal structures in each house included a rooftop structure, a basement, a horizontal extension on the ground floor, and glass canopies outside their car parks.

The two basements were about 538 sq ft each. Cheng’s was 2.5 metres in height while Poon’s stood 3.5 metres high.

Poon’s house had an illegal garden pool, measuring about 5 by 2.5 metres.

In space-starved Hong Kong, illegal structures and basements have been a contentious issue – especially as a number of high-ranking officials were revealed to have violated building regulations by expanding their private premises in the world’s least affordable city to own a home.

In the run-up to the chief executive election in 2012, the revelation of a 2,400 sq ft illegal basement at former minister Henry Tang Ying-yen’s private residence was cited as a main reason for his eventual defeat at the polls.

Former chief executive Leung Chun-ying, who beat Tang, also had his credibility questioned after he was found to have unauthorised structures at his home at The Peak.

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Reports on Saturday revealed that structures identified at the homes of both Cheng and a property next door belonging to her husband in Tuen Mun were suspected to be illegal.

Leung said the department had never received any applications from the owners or authorised personnel for any alterations or additions to the two houses.

They had also not received any reports about unauthorised building works for the two houses in the past decade before receiving media enquiries about suspected illegal structures as early as December 27. Officials did not know of the owners’ identity until the media enquiries last Friday.

Leung would not divulge if Cheng, 59, or Poon, 78, were present during the three-hour inspection, and also said a further analysis would be needed to determine when the structures were built.

“The investigation is still ongoing … we will take appropriate actions according to the procedures of the Buildings Ordinance and policies related to unauthorised building works,” Leung said, when asked if there was a specific time frame for the landlords to remove the structures.

Leung added that they would remain in close dialogue with the authorised personnel, who would have to submit a remedial action plan for the properties.

Construction workers, who came in a truck piled with bamboo scaffolding, also entered Poon’s house on Tuesday afternoon. It is unclear if they were preparing to remove any structures.

Poon said he regretted overlooking whether there were any illegal structures at the time when he bought the house in October 2012.

“When I bought the house, I did not consult professionals as to whether there were any unauthorised building works. The pace of my work was quite hectic at the time. In retrospect, I did not make the proper arrangements,” Poon said in a written statement.

“As a professional engineer and someone who has always been committed to contributing to the community, I understand that my performance has fallen short of the expectations of various sectors of the community. For this, I would like to express my regret and am sorry. I will learn from this lesson and strive to do my best in the future,” he said.

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Poon said he would fully cooperate with Buildings Department officials and make restorations and amendments to his property as soon as possible. There was no mention of his wife’s property in the statement.

Land Registry records show that Cheng bought her house in 2008 for HK$26 million (US$3.3 million) and Poon bought his in 2012 for HK$27 million.

Midland Realty’s residential division chief executive Sammy Po Siu-ming estimated that Cheng’s villa would be worth HK$35 million.

An advertisement on a real estate search platform gohome.com.hk stated that rent was HK$57,000 a month for one of the houses in the development.

Property owners in the city are not allowed to make alterations or additions to the buildings, unless they are minor interior renovations, without prior approval of the Building Authority as they may pose structural or fire safety risks. They are required to first submit building plan proposals to the Authority and are only allowed to make changes if the building plans are approved.