What Are Site Works — and Why Did My Build Cost More Than the Quote?

Of all the questions I get from people who have already signed a building contract, the most frustrated ones are usually about site works.

"The builder quoted me $400,000 and now they want another $25,000 before they will start. What is going on?"

It is a fair question — and an important one. Site works costs are one of the most common sources of genuine financial shock in a new home build. They are also one of the most misunderstood line items in a building contract.

This guide explains what site works are, why they vary so much, and what you can do to protect yourself before you sign.

Site works is the collective term for everything that needs to happen to your block of land before the slab can be poured and the frame can go up.

Depending on your block, site works can include:

• Earthworks and levelling — cutting or filling the land to create a flat, stable building platform
• Retaining walls — structural walls to hold back soil on sloping blocks
• Soil treatment — chemical treatment to protect the slab from termites
• Engineered slab upgrades — a more complex slab design required by reactive or unstable soil
• Rock removal — breaking up and removing rock or hard material below the surface
• Stump removal — clearing old tree stumps or roots
• Drainage — managing stormwater runoff from the site
• Service connections — connecting sewer, water, power, and NBN to the boundary

On a flat, stable block with good soil, site works can be minimal — sometimes as low as a couple thousand dollars. On a sloping, rocky, or reactive-soil block, the same category of work can cost $30,000, $50,000, or more.

This is the part that catches people out.

Most volume builders advertise a base price for their home designs. That price covers the house itself — the frame, roof, walls, fit-out, and finishes. It does not cover the site-specific work required to prepare your particular block.

The reason is straightforward: the builder does not know what your block needs until they assess it. Every block is different. A flat, sandy block in a new estate costs almost nothing to prepare. A sloping block with clay soil and a high water table is a completely different proposition.

So builders separate the two. The house price is fixed. The site works are assessed separately, usually after you have signed the contract and the builder has conducted a soil test and site survey.

This is not inherently dishonest — it reflects genuine uncertainty. But it does mean that the number you see in the advertisement or the display home is not the number you will pay.

Here is where things go wrong.

Many building contracts include a site works allowance — a provisional sum that estimates what the site preparation will cost. Common allowances are $20,000, $25,000, or $30,000.

If the actual site works come in above that allowance, you pay the difference. If they come in below, you get a credit.

The problem is that allowances are often set low — sometimes unrealistically low — to make the contract price look more attractive. When the soil test comes back and the real site works cost is $25,000 against a $10,000 allowance, you are suddenly facing a $15,000 shortfall that was not in your budget.

This is the moment many people feel they have been misled. In some cases, they have been. In others, the builder genuinely could not have known without the soil test. The difficulty is that it can be hard to tell the difference.

Understanding what drives site works costs helps you assess the risk before you sign.

Australian soil is classified from Class A (stable, minimal movement) through to Class E and P (highly reactive or problem sites). The classification determines what type of slab is required.

A Class A or S site can use a standard slab. A Class H1 or H2 site needs a more heavily engineered slab with additional steel and concrete. A Class E or P site may require a waffle pod slab, pier and beam construction, or other specialist solutions.

Perth's soils vary significantly. Sandy coastal soils are generally stable. Clay-heavy soils in some outer suburbs can be highly reactive. Fill material — land that has been built up with imported soil — can be unpredictable.

A soil test before you sign a contract gives you the classification upfront, so you are not surprised later.

The steeper the block, the more earthworks are required to create a level building platform. This means excavation, fill, compaction, and often retaining walls.

Retaining walls in particular can be expensive. A single run of retaining wall can cost $500 to $1,500 per linear metre depending on height, material, and engineering requirements. A sloping block that needs 20 metres of retaining wall at 1.5 metres high can add $15,000 to $30,000 to your site costs before anything else is considered.

Rock below the surface is one of the most unpredictable site costs. It cannot always be detected before excavation begins. When rock is encountered, it needs to be broken up and removed — a slow, expensive process.

Some areas of Perth are known for limestone or rock close to the surface. If you are buying in one of those areas, ask the builder about the risk and whether the contract includes any provision for rock removal.

Blocks that have been built up with fill — common in some newer estates where land has been levelled across a development — can have variable compaction. Poorly compacted fill can require additional engineering to create a stable building platform.

Ask the developer or your builder whether the block contains fill material and, if so, what compaction testing has been done.

In most established Perth estates, sewer, water, power, and NBN connections are available at or near the boundary. But the cost of connecting from the boundary to the house varies depending on distance and site conditions.

In some outer estates or rural-residential areas, connections can involve significant additional cost. Always confirm what is included in the contract and what is a provisional sum.

You cannot eliminate site works costs — they are a real part of building on real land. But you can manage the risk.

Many builders will conduct a soil test as part of the pre-contract process. Some charge for this; others include it. Either way, it is worth doing before you commit.

A soil test gives you the classification upfront, which allows the builder to price the slab accurately rather than using a provisional allowance.

Some builders will offer a fixed-price site works quote after the soil test and site survey. This removes the provisional sum risk — you know exactly what you are paying before you sign.

Not all builders offer this, and it may cost more upfront than a provisional sum. But for many people, the certainty is worth it.

If the contract includes provisional sums for site works, ask the builder to justify the allowance. What does it cover? What is excluded? What happens if the actual cost exceeds the allowance?

Compare the allowance to what you know about the block. If the block is sloping and the site works allowance is $6,000, that is a red flag worth questioning.

A building consultant or quantity surveyor can review the contract and the site works allowances before you sign. This costs money — typically $500 to $1,500 — but can save you significantly more if it identifies an unrealistic allowance.

Read the contract carefully to understand what circumstances allow the builder to issue a variation — a formal request for additional payment. Site works variations are common and legitimate, but you should understand the process and your rights before you sign.

In Western Australia, building contracts are regulated under the Home Building Contracts Act 1991. Builders must provide a written contract for any residential building work over $7,500. Variations must be in writing and signed by both parties before the additional work begins.

If a builder asks you to pay more without a written variation, that is not compliant with the Act.

If you have already signed a contract and received a site works variation that you were not expecting, here is what to do.

Do not panic, but do not simply pay without understanding what you are paying for. Ask the builder for a detailed breakdown of the variation — what work is required, why it was not included in the original allowance, and what the cost is based on.

Compare the variation to the contract. Check what the original site works allowance covered and whether the additional work genuinely falls outside it.

Seek independent advice if the amount is significant. A building consultant, quantity surveyor, or solicitor experienced in building contracts can review the variation and advise whether it is reasonable and compliant.

Know your rights. Under the Home Building Contracts Act, you have the right to a written variation before additional work proceeds. If work has already been done without a signed variation, seek legal advice.

Consider mediation. Building disputes in Western Australia can be referred to the State Administrative Tribunal (SAT) or the Building Commission. These are formal processes, but they exist precisely for situations like this.

Not all builders handle site works the same way. Some are transparent and thorough in their pre-contract assessment. Others use low provisional sums as a sales tool, knowing that the real costs will emerge later.

One of the most valuable things I do for clients is help them understand what is in a building contract before they sign it — including the site works allowances and what they are actually based on. A contract that looks cheaper on paper can end up costing significantly more once the site works are finalised.

If you are comparing quotes from multiple builders, make sure you are comparing like for like. A quote that includes a realistic site works assessment is worth more than a lower quote with a provisional sum that does not reflect the actual block.

This guide is general information only and is current as at October 2026. Building contracts, site works costs, soil conditions, and regulatory requirements can change. Always read your contract carefully and seek independent legal or building advice before signing. If you have a dispute about site works costs, contact the Building Commission of Western Australia or seek legal advice from a solicitor experienced in building contracts.