Technical evidence suggests the illegal basement at the home of former chief secretary Henry Tang Ying-yen was not built at the same time as the house but was added after its completion in 2007, a court heard yesterday.
Structural engineer James Lau became the second expert defence witness to raise the possibility during the Kowloon City Court trial of the building's architect, engineer and contractor.
Lau said the basement must have been added after the occupation permit was issued because "there are significant changes to the dimensions of beams and columns from the approved plan".
Architect Henry Ho Chung-yi, engineer Wong Pak-lam and contractor Hien Lee Engineering are each charged with one count of building without planning approval between 2005 and 2007 and one count of knowingly misrepresenting information to the Building Authority.
All three deny working on or having knowledge of the 2,400 sq ft basement that derailed Tang's bid to become chief executive.
The court yesterday saw for the first time photographs showing construction joints on the basement's columns. These are marks formed when concrete is poured more than once.
A highly technical debate lasted throughout last week in court over whether such marks could be found where the basement touched the rest of the house.
The prosecution's expert witness last year told the court that the lower ground floor structure was done in a single pour.
The prosecution's expert based his view on the lack of construction joints.
The defence argued the basement was added by partially demolishing finished slabs after the building was completed in compliance with approved plans. A Buildings Department occupation permit marked the completion of the house in early 2007.
The defendants, as the professional representatives of a construction project and responsible for obtaining such a permit upon fulfilment of their roles, would not be held accountable for unauthorised building works beyond that point, it asserted.
Sammy Chan, another defence expert witness who reached the same conclusion as Lau, had told the court last week it was technically impossible for the basement to have been excavated between October 10 and 24, 2005, and then hidden from planning inspectors, as alleged by prosecutor Keith Oderberg.
The trial before Acting Chief Magistrate Clement Lee Hing-nin is due to end next Thursday.