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.