Two identical houses. Same plan, same spec, same year. One in Corrimal on a flat block with sandy soil and a sewer main running conveniently down the low side. The other about four streets away, reactive clay, a metre and a half of old fill from when someone levelled the yard in the seventies, and a driveway you could barely reverse a ute up. Difference between the two before either slab was poured: somewhere around thirty thousand dollars.
Not one dollar of that was the house. All of it was the ground.
Site costs are the least understood line in a new build quote and the one most likely to move after you've signed. I want to walk through what actually sits in there, because once you can read the line you can price it properly, and once you can price it properly you can get it fixed rather than estimated.
This isn't the "what does a house cost" question — that's covered in how much it costs to build a house in NSW, which has the per-square-metre ranges and the soft costs. This is the narrower and nastier question of the allowance sitting underneath those numbers.
Why it's an allowance and not a price
Here's the honest mechanics of it. When a builder quotes you a new home before anyone has drilled a hole in your block, they cannot know what the footings need to be. So they put in a number based on what a typical block in the area costs to prepare, they call it a site cost allowance or a provisional sum, and the contract says that if the real cost comes in higher you pay the difference, and if it comes in lower you get it back.
That's not a scam. It's the only honest way to quote before the investigation is done. What is a bit of a scam is setting the allowance low to make the headline number look competitive, knowing full well that reactive clay across most of the Illawarra makes that allowance unachievable. You compare three quotes, the one with the skinny site allowance looks best, and eight months later it isn't. Comparing quotes properly is half of choosing a builder without getting burnt, and the site allowance is where I'd look first.
Ask one question of every quote: what soil classification is this allowance based on? If the answer is vague, the allowance is decoration.
Soil classification, which decides the slab
A geotechnical engineer drills a few bore holes, looks at what comes up, and gives your block a classification. The classes run roughly from stable sand and rock at one end, through increasingly reactive clay in the middle, to problem sites at the other end — sites with uncontrolled fill, mine subsidence, soft or landslip-prone ground, where the standard solutions don't apply and it has to be individually engineered.
What the classification does is tell the engineer how much the ground is going to move as it wets and dries through the seasons. Clay swells when it's wet and shrinks when it's dry, and a slab sitting on it has to be stiff enough to ride that movement without cracking the house apart. More movement means a deeper edge beam, more internal beams, more steel, more concrete, sometimes piers down to something that doesn't move.
Around the Illawarra, moderately to highly reactive clay is the common answer rather than the unlucky one. Plenty of the escarpment side sits on it. So if a quote has been built on the assumption of a benign site, treat the allowance with real suspicion. And a problem-site classification is a different order of magnitude again, because at that point the footing stops being a catalogue detail and becomes a design.
The design rules for all of this sit in the residential volume of the National Construction Code, which references the Australian Standard for residential slabs and footings. Volume Two of the NCC is the one that applies to houses. Worth knowing it exists, mostly so you can ask your builder which classification the engineering has been done to.
Rock, and fill, and the difference between them
Rock is expensive but at least it's honest. Once the excavator hits sandstone, you're into rock saws or a hammer, and it's priced by the hour and by the cubic metre carted away. It's slow and loud and the neighbours will hear about it, but you find out quickly and the number is what it is.
Fill is worse, and it's more common than rock in built-up suburbs. Someone at some point pushed dirt around to make a flat yard, or backfilled a gully, or buried the demolition rubble from the previous house. That fill isn't engineered, nobody compacted it, and you cannot found a slab on it. So it either gets dug out and replaced with properly compacted material, or you go through it with piers to competent ground below.
The reason fill hurts is that it's invisible until someone drills. It doesn't show on a survey, it doesn't show in the real estate listing, and a couple of bore holes can miss the extent of it even when they find it. On a rebuild block especially, where there's been a house and sixty years of yard landscaping, I'd budget mentally for the possibility rather than being blindsided by it.
Where the services are, and how far
Your block has power, water, sewer and stormwater somewhere. What matters is where.
Sewer is the one that costs. Waste has to run downhill to the connection point, so if your junction is on the high side of the block and the new house wants its bathrooms on the low side, you're either re-planning the house or paying to relocate the connection. Worse, if the main runs across your land, anything you build near it triggers a build-over assessment with the water authority, and possibly a change to the footing design to bridge it. Get the sewer diagram before the design is locked, not after.
Stormwater has the same logic. Water has to get to a legal discharge point under gravity, and on a block that falls away from the street that point isn't obvious. Some sites end up needing a charged line, or an on-site detention tank if council conditions one, and neither is trivial.
Power is usually a reconnection rather than a new connection on a rebuild block, which is one of the quiet advantages of rebuilding over buying raw land — the money side of that comparison is in what a knockdown rebuild costs. Even so, an old single-phase supply feeding a new house with ducted air conditioning, induction cooking and a car charger may want an upgrade, and if the pole is on the far side of the street or the supply has to go underground, that's a real cost with a real lead time.
The block itself
Slope is the obvious one and it deserves its own treatment, which it gets in building on a sloping block. Short version: cut costs money, fill costs money, retaining what you cut costs more money again, and anything close to a boundary needs engineering and often a neighbour's cooperation.
Access is the underrated one. A concrete truck needs to get its chute where the pour is or you're hiring a pump. A crane needing to lift roof trusses over the house next door instead of standing on the block is a different day rate. A street too narrow to park a truck on means the trades are barrowing, and barrowing shows up in every trade's price whether they itemise it or not. I've had two jobs on the same street price differently purely because one had a verge you could stand a truck on and the other had a bus stop.
Trees. If council has conditioned a tree to stay, its protection zone is a no-go area for machines and material, which can push the whole build sequence around. And a tree you are allowed to remove still has to be removed, ground out, and the root zone made good before you can found on it.
Then the small stuff that adds up: temporary fencing, a site toilet, temporary power and water, a shaker grid at the crossover, sediment control, and the survey setout. Individually minor. Collectively a number you'd notice.
Turning an allowance into a fixed price
This is the actual advice, and it costs money up front to save money later.
Before you sign a building contract, pay for the geotechnical report and the detail survey yourself. They're your documents, they follow you if you change builders, and they're the two things that let a builder replace a guess with a price. A builder holding a real soil classification, a real contour survey and a real sewer diagram can engineer the footing and quote the earthworks properly. A builder holding none of those is estimating, and the estimate will be either optimistic or padded.
Then read the site cost clause. What's in the allowance, what's specifically excluded, and what happens to the difference in both directions. Rock is very often excluded entirely and priced as an extra when found, which is defensible, but you want to know it now. Same with piering.
And ask for the allowance to be converted to a fixed sum once the geotech is in. A builder who has the information and still won't fix the number is telling you something.
Fair Trading's guidance on building or renovating a home is worth a read on what a residential building contract has to contain generally, including how variations have to be documented. Most of the site-cost arguments I've heard about were really contract-administration arguments in disguise.
What I'd actually do
If I were spending my own money on a block I hadn't built on before, I'd order the geotech and the detail survey before I fell in love with a floor plan. A few thousand dollars, a couple of weeks, and it tells you whether the house you want is the house the ground will let you have. On a couple of jobs it's changed the design — single storey to two storey to shrink the footprint on bad ground, or the whole building envelope shifted six metres to get off the fill.
That's the useful thing about site costs. They feel like a tax you pay at the end. They're actually information you can have at the start, and having it early is the difference between a fixed price and a series of phone calls.
We build new homes across Wollongong, the Illawarra and the Southern Highlands, and we'd rather have the awkward ground conversation before you sign than after — you can see how we run those jobs on our new homes page. If you've got a block and you're not sure what's under it, send the address and any reports you already have through the contact page or call 0434 037 851.
Want a hand with this at your place? Get a free quote or call 0434 037 851.