Underpinning FAQ — Central Coast

It depends on the scope. Resin injection under a few affected sections is a smaller job than a partial underpin of several footings, and both are well below a full-perimeter underpin. The gap comes down to depth, soil, access and how many piers are needed. Council rules can add depth or paperwork. In the former Gosford area, Central Coast Council's DCP 2022 requires piers along a drainage easement boundary to go at least as deep as the invert of the pipe or culvert, with a structural engineer's pier-and-beam details submitted before approval. A structural engineer's report should come before any quote, so the price is for the cause, not the symptom.

Hairline cracks under 1 mm that don't grow are usually cosmetic. Cracks wider than 5 mm, diagonal cracks running from the corners of doors and windows, doors or windows that have started to bind, and cracks that come back after patching are the ones for an engineer to look at. Photograph each crack with a ruler and the date in shot. Then check again after the January to April rain. In the Bureau of Meteorology's Gosford record (1916 to 2013), those months each average roughly 135 to 155 mm, against about 70 mm in August and September. Cracks that open and close with the wet and dry months point to moisture change under the footing.

Under Chapter 3.7 of Central Coast Council's DCP 2022, a lot rated Hazard Category 2 or above for landslip needs a geotechnical report with a development application, and Category 4 needs a comprehensive report covering adjoining lots too. The chapter says who can sign it: a geotechnical engineer or geotechnical geologist on the National Professional Engineers Register (Level 3) or a current member of the Australian Geomechanics Society, with at least five years advising on building works over Hawkesbury Sandstone and Narrabeen Group strata in the Sydney Basin, or considerable experience with similar geology, and at least $10 million in professional indemnity cover. Confirm this before paying for a report.

In the version in force in 2026, Part C of Chapter 3.2 (Coastal Hazard Management) in Central Coast Council's DCP 2022 covered the open coast from Wamberal Beach to Patonga Beach. It required footings, including strip footings and isolated piers, to bear safely below or beyond the calculated zone of reduced foundation capacity. Where coastal forces applied, the engineer had to allow for possible dune slumping, loss of support and a changing water table. Council would not permit redevelopment of an existing building in a Coastal Hazard Area unless its foundations were shown to withstand the designated coastal processes and certified by a coastal and structural engineer. That version flagged a revision, so check the chapter in force.

It is worth ruling out. Subsidence Advisory NSW lists the Wyong, Hue Hue and Swansea North Entrance Mine Subsidence Districts, and their maps take in parts of the Central Coast Council area. The maps are labelled with localities including Jilliby, Dooralong, Wyong Creek, Kiar, Bushells Ridge, Doyalson, Lake Munmorah, Chain Valley Bay, Mannering Park, Budgewoi, San Remo, Gorokan and Lake Haven. They are only a general overview, so check your own address on the NSW Planning Portal Spatial Viewer. Homes damaged by mine subsidence are eligible for compensation if they were built with the correct approvals, and building or extending a home in a district needs Subsidence Advisory NSW approval first.

Not on its own. Underpinning fixes the footing, but it may not hold unless the moisture problem around it is dealt with too. That means fixing drainage, keeping garden beds and irrigation off the foundation line, and managing tree roots nearby. On the Peninsula, Central Coast Council's DCP 2022 names drainage black spots around Cogra Road, Rothwell Street, Moana Street and Angler Street in Woy Woy, and Shephard, Glenn and Carpenter Streets in Umina, where it says the drainage problems cannot be readily overcome. On streets like these, drainage belongs in the diagnosis from the start. Ask the contractor for drainage advice as part of the same job.