Officials say 70 out of 85 homes they have looked into following a recent landslide at Redhill Peninsula are suspected of having unauthorised structures, with almost half also believed to have illegally encroached on government land.
The revelation came as the government announced it has ordered the owners of four luxury houses in the area to remove their unauthorised works within 150 days.
The illegal structures came to light following a landslide on September 8 that was triggered by a massive rainstorm that wreaked havoc across Hong Kong for many hours.
Some homes at the cliffside estate looked to be in danger of collapsing and experts suggested unauthorised works could have made the land unstable.
Speaking after a joint inspection operation between the Buildings and Lands departments on Friday, officials said they had tried, unsuccessfully, to enter 10 houses in the area. In some cases nobody answered the door, in others, they were refused permission to go inside.
"But we have left behind notices, stating clearly that we will go for an inspection again on Monday," said Keith Ko, a chief building surveyor at the Buildings Department.
"Hopefully the owners will cooperate with us and allow our colleagues into the houses for inspection. Otherwise the department will seek a search warrant from the court in accordance with the law."
The authorities have found serious issues regarding unauthorised structures at houses 70, 72, 74 and 76, the government said in a statement.
The illegal works include missing retaining walls, a basement on a government slope, the addition of extra floors and outdoor structures.
The owners have a month to submit a demolition proposal for vetting, and 150 days to fix the problems.
A government spokesman said further details regarding the other homes believed to have unauthorised additions cannot be disclosed at this point, so as not to affect any potential prosecutions.