Every home is only as good as the ground it sits on. You can choose the best materials and hire the best builder, but if the footings don't suit the soil, cracks, sticking doors and costly repairs can follow within a few years.
The ground under a Northern Beaches block can be clean beach sand, reactive clay, sandstone a metre down, or a mix of all three across one site. The only way to know is to test it.
That's the job of a geotechnical report. At Greenwood Consulting Engineers, we design footings and slabs around these reports every week, and we investigate the damage when they're skipped. Here's what a report tells you, what it costs, and why it pays to get one before design starts.
1. Different soils need different foundations
Soil behaves very differently depending on what it's made of and how wet it gets. Some soils barely move. Others swell after rain and shrink in a dry spell, lifting and dropping your house with them.
| Ground type | How it behaves | Footings that often suit |
|---|---|---|
| Rock (e.g. sandstone) | Very stable once you reach sound rock | Pad footings or piers founded on rock |
| Sand and gravel | Doesn't swell or shrink, but loose sand can settle | Slab or strip footings on well-compacted ground |
| Silt | Can lose strength when wet and settle over time | Deeper footings or piers to firmer material |
| Clay | Swells when wet, shrinks when dry | Stiffened raft slab, or piers down to stable ground |
| Fill (placed soil) | Unpredictable; can compress for years | Piers or piles through the fill to natural ground |
These are general patterns only. The right footing depends on the test results, the depth of each layer and what you're building.
Our take: Two neighbouring blocks can need completely different footings. Never assume your soil matches the house next door.
2. Your site class under AS 2870
The headline result of a geotechnical report is your site classification. Australian Standard AS 2870 (Residential slabs and footings) sorts sites by how much the ground surface is expected to move as moisture changes. Your engineer uses that class to size the slab and footings.
| Site class | What it means | Expected surface movement |
|---|---|---|
| A | Stable, non-reactive (most sand and rock sites) | Little or none |
| S | Slightly reactive clay | Up to 20 mm |
| M | Moderately reactive clay or silt | 20–40 mm |
| H1 | Highly reactive clay | 40–60 mm |
| H2 | Highly reactive clay | 60–75 mm |
| E | Extremely reactive clay | More than 75 mm |
| P | Problem site: soft soil, uncontrolled fill, landslip risk or other abnormal conditions | Needs specific engineering design |
The more reactive the site, the deeper and stiffer the footings need to be. A Class H2 slab can need much deeper edge beams and more steel than a Class A slab of the same size.
Our take: A Class P result isn't a deal-breaker. It just means the footings must be designed by an engineer rather than taken from standard tables.
3. What a report costs, and what it saves
A geotechnical report for a typical residential block costs roughly $1,500 to $4,000, depending on site access, the number of boreholes and how deep the testing goes. On a project worth hundreds of thousands of dollars, that's a small line item.
Without one, your engineer has to design for the worst case or guess. Either way, you tend to pay for it:
• Over-design. Footings sized conservatively "just in case" mean extra concrete, steel and excavation you may never have needed.
• Surprises during construction. Hitting soft ground, fill or rock in the wrong place mid-dig triggers redesign, extra piers and builder variations, often while trades wait on site.
• Repairs later. Underpinning a cracked house costs many times more than getting the footings right the first time.
Our take: On a reactive clay site, an accurate report can easily pay for itself in concrete and steel alone.
4. What goes wrong when the ground is guessed
In our remedial work, we regularly see the results of footings that didn't suit the ground. The three most common problems are:
• Consolidation. Soft clay, silt or poorly compacted fill slowly squeezes down under the weight of the house. The building settles, sometimes for years after it's finished.
• Differential settlement. One part of the house moves more than another, for example where half the slab sits on rock and half on fill. This is what causes diagonal cracks above doors and windows, sloping floors and doors that jam.
• Foundation failure. In the worst cases the footings can no longer carry the building safely, and major underpinning or partial rebuilding is needed.
Our take: Cracks are a symptom, not the problem. Patching them without understanding the ground underneath usually means they come back.
5. Get it right from the start
The best time to test the ground is before the architect finalises the design, not after the excavator arrives. Early testing gives you:
• Accurate design. Footings sized for your actual ground, not a guess.
• Cost certainty. Fewer variations and surprises once building starts.
• Lower risk. Problems like fill or landslip risk are found while there's still time to plan around them.
Here's how it usually works:
1. Early chat. Talk to your structural engineer about the site and the house you have in mind.
2. Site investigation. A geotechnical engineer drills boreholes or test pits and tests the soil.
3. The report. You receive the site classification and recommendations for footings, excavation and drainage.
4. Footing design. Your structural engineer designs the slab and footings to suit, ready for approval and construction.
Our take: Ordering the report early also helps with council. Sloping or landslip-mapped sites often need geotechnical input with the development application anyway.
Ground conditions on the Northern Beaches
Our local ground is more varied than most of Sydney, often within a single street.
• Steep sandstone slopes. Around Bilgola, Clareville, Bayview and the escarpments, rock can be close to the surface on the high side and deep on the low side. Footings often need to step down the slope to reach it.
• Coastal sands. Flat beachside suburbs like Collaroy, Narrabeen and Mona Vale often sit on sand. It's non-reactive, but loose sand and a high water table need care.
• Clay and fill. Older lots, filled gullies and some low-lying areas can hide reactive clay or uncontrolled fill that only shows up in testing.
• Landslip risk. Parts of the Northern Beaches are mapped by council as landslip-prone. Building there usually needs a geotechnical assessment before approval.
Our take: On a sloping block, the geotechnical report and the structural design work hand in hand. Getting both teams talking early saves time at council and on site.
Frequently asked questions
Do I need a geotechnical report for a renovation? Often, yes, if you're adding a storey, extending the footprint or building a new deck, pool or retaining wall. Your engineer needs to know the ground can take the extra load.
How long does it take? Site testing usually takes a day or less. The report typically follows within a couple of weeks, depending on the geotechnical firm's workload.
Can I use my neighbour's report? It can give a useful hint, but ground can change over a few metres. Most engineers and certifiers will want testing on your own site.
My site is Class A. Do I still need an engineer? Yes. Class A is good news, but slabs and footings still need designing, and things like slope, wind and drainage still apply.
Jargon buster
• Geotechnical engineer: An engineer who investigates and tests soil and rock to work out how the ground will behave.
• Borehole: A narrow hole drilled into the ground to take soil samples and see what's below.
• Reactive soil: Soil, usually clay, that swells when wet and shrinks when dry.
• Site classification: The AS 2870 rating (A, S, M, H1, H2, E or P) that describes how much your ground is expected to move.
• Raft slab: A concrete slab stiffened with deep beams so the house moves as one unit on reactive ground.
• Pier: A concrete column cast into a drilled hole to carry loads down to firm ground or rock.
• Underpinning: Strengthening or deepening the footings of an existing building, usually to fix settlement.
Start with the ground, build with confidence
A geotechnical report is one of the smallest costs on any build, and one of the few that can save you many times its price. It turns guesswork into a footing design that suits your block, your budget and your home.
Planning a new home, extension or knockdown rebuild on the Northern Beaches? Send us your plans and site details. We'll tell you whether you need a geotechnical report, help coordinate it, and design footings to match.
Greenwood Consulting Engineers is registered as a professional engineer and design practitioner (structural) under the NSW Design and Building Practitioners Act 2020. [Insert registration numbers.]
Book a pre-design consultation · Phone 0424 784 706 · Email admin@greenwoodengineers.com.au · Suite 13, 90 Mona Vale Road, Mona Vale NSW 2103