Billionaire Sit Kwong Lam has lost a court case to keep floor deckings and an air-conditioning unit affixed to common property adjoining his Ardmore Park penthouse in Tanglin.

But the High Court did highlight that the case raised the "interesting" question of what the law means by common property after Dr Sit's lawyer argued that the works were installed on areas not visible or accessible to anyone.

Dr Sit, chairman of Hong Kong-listed Brightoil Petroleum Holdings and whose net worth is listed at US$2.1 billion (S$2.9 billion) by Forbes, went to court after the Strata Titles Board (STB) last year rejected his application to keep the installations. At issue were the timber deckings he had installed over the entire roof outside his unit, including the floor trap and drainage system, as well as two ledge areas that ran along a segment of his condo unit's external facade.

He had also installed an air-con vent on an adjoining wall, arguing that the penthouse ventilation system was inadequate.

The STB found he had breached by-laws made by the condo's management council under the Building Maintenance and Strata Management Act and the works were done on common property.

Dr Sit took his case to the High Court, where Judicial Commissioner Kannan Ramesh, in judgment grounds released on Tuesday, noted how "both parties spilt much ink and spent much energy" on the issue of what constitutes common property.

Dr Sit's lawyer, WongPartnership's Senior Counsel Alvin Yeo, had argued, among other things, that the ledges could not be accessed by any unit owner and the council had not shown evidence of common usage or common purpose for the other two areas affected. The areas did not, therefore, constitute common property.

The council, defended by lawyers Subramaniam Pillai and Perera Randall Mingyang of Colin Ng & Partners, countered the areas were common property and findings of fact by the board substantiated by evidence were not open to challenge.

The judicial commissioner found that the works were situated in common property which was areas not for the exclusive use of the occupiers of Dr Sit's unit.

The judge added the council's refusal to consent to the timber deckings was not unreasonable, noting that the evidence did not show a compelling case for installation.

The judge noted that the general body had also refused support for Dr Sit's use of the property at the April 2015 annual general meeting.

Referring to the ledges, he added that "the appellant took matters into his own hands and acted inappropriately by doing something he was barred from doing, whatever his motivations might have been".

"To hold that the (council) withheld consent unreasonably in such circumstances would countenance such conduct," he noted.

Dr Sit is studying the decision with his lawyers before deciding whether to proceed with the appeal.