But Poon maintained the pool, used for his aquatic exercise, was risk-free in every sense, posing no structural danger to his property.

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“This is not a case of deliberate intent but an honest mistake,” his counsel Kim McCoy said in mitigation.

A spokesman for Cheng said she had no comment on the verdict and sentence. Experts said the HK$20,000 fine was appropriate considering the timely action taken by Poon to demolish the pool.

Poon, a former president of the Hong Kong Institution of Engineers, was accused of knowingly commencing or carrying out building works without obtaining approval from authorities and consent in writing. He pleaded not guilty to the summons in January. The offence carries a maximum sentence of a HK$400,000 (US$51,000) fine and two years in prison under the Buildings Ordinance.

The West Kowloon Court heard the facility in question was called an Endless Pool, built for exercise, like an underwater treadmill. It comprised lightweight steel panels enclosing a plastic liner capable of holding 14.4 cubic metres, or 14 tonnes, of water.

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It was not disputed that Poon had failed to submit plans for government approval.

At issue was whether the pool was considered a form of building, in which its construction called for works that required planning permission.

Choy had argued the pool was a permanent addition to Poon’s garden and should have been subjected to the Building Authority’s scrutiny to ensure the safety of the overall structure.

His expert, government surveyor Michael Pang Yuk-lung, said the pool was a building that would pose significant structural implications for any slab it rested on.

On Tuesday, the magistrate rejected Pang’s opinion after finding the surveyor had assumed the role of advocate for the prosecution case in which he appeared to have failed to detach himself sufficiently from the proceedings.

Meanwhile, the magistrate partially accepted evidence from the defence expert, Dr James Lau Chi-wang, a structural and geotechnical engineer.

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Dr Lau had argued that the movable pool was neither a building nor a structure. “It’s just a bag of water placed in the garden,” Lau testified last month.

While the magistrate accepted Lau’s conclusion that the pool posed no structural implications for the property because it was supported by precise calculations based on building plans, he rejected the defence suggestion that the pool was not a structure.

A movable object with no threat to safety could still be considered a structure, he said.

The magistrate observed that the pool was enclosed by timber panels matching the surrounding floor finishes that did not extend to its bottom, which suggested its design was intended to achieve consistency in appearance and form an integral part of the garden.

The bare ground underneath also meant dismantling the pool would require resurfacing, incurring additional time and cost implications.

The design, together with the fact that the pool was meant for aquatic exercise, pointed to a degree of permanence.

Considering it was not a portable facility and that it was of a substantial size and weight when filled with water, So was satisfied beyond reasonable doubt the pool was a structure, which constitutes building works under the ordinance.

In mitigation, McCoy said his client was a leader and pioneer in his industry, who has also been heavily engaged in public service and philanthropy, as attested by his participation in various government advisory bodies and a HK$100 million donation to the University of Science and Technology.

Poon was fined HK$20,000, to be paid in 14 days, for his offence after the court heard the highest fine it had handed out was HK$50,000.

The pool was dismantled less than two weeks after the Buildings Department’s inspection on January 9, 2018.