The third runway proposed for the airport at Chek Lap Kok could intrude into mainland waters, as planners try to avoid contaminated seabed mud pits and protect nearby residents from noise.
Central government approval would be needed for this option to proceed but a government source said the intrusion was minor and should not present a great problem.
At least three options for the runway were shown to Airport Authority board members after the authority commissioned consultants in July to study the feasibility of such a plan.
One would take in some of the 12 mud pits north of Chek Lap Kok, used since 1992 to dispose of contaminated mud dredged for large projects and maintenance of navigation channels, one of which is still in use.
The option that appears most feasible traverses sea borders but stays clear of the mud pits and residential developments, a board member and government source said.
'A runway built on contaminated mud might not be feasible,' the source said, adding that flight paths in the other two options could trigger more complaints from residents affected by noise pollution.
Complaints had been lodged by residents in Tuen Mun, Ma Wan, Sha Tin and the south of Hong Kong Island, the source said.