This is what i got back from my builder:
Coastal zoning is 1 km from breaking surf but there is variables with this to consider but this is a general rule.
In this zone there are things that change in construction for example stainless steel brick ties, coastal cement is required, the galvanising on any steel used may have to be upgraded, roof fixings have to be upgraded, any texture coating to the external May have to be impact coated, wind zoning for the roof may be higher therefore more expense
The costings on houses in this zone may vary slightly to quite substantial it really depends on the location.