The Importance of a Geotechnical Report for Accurate Building Quotes

Most people think a build quote is based on their floor plan. In reality, it’s based on what the house is sitting on.

And that’s where most budgets blow up.

A geotechnical (soil) report tells your engineer and builder what’s actually under the ground -not what it looks like from the surface. Without it, foundation pricing is guesswork. With it, it’s engineering.

What a geotechnical report actually does

A geotechnical report involves drilling boreholes into your site and analysing the soil profile. It identifies:

• Soil type (clay, sand, rock, fill, etc)

• Depth to stable bearing layers

• Presence of rock, groundwater or uncontrolled fill

• How reactive the soil is (shrink–swell behaviour)

From this, engineers determine whether your house can sit on a standard slab, or whether it needs piers, screw piles, deeper footings, ground replacement, or rock excavation.

That one document drives almost every dollar in your foundation cost.

Why builders can’t price foundations without it

If a builder doesn’t have your soil report, they have two choices:

1. Guess conservatively and risk losing the job

2. Guess cheaply and win the job

Guess which one usually happens.

That’s how you end up with quotes that look great upfront… and then explode once excavation starts.

When the excavator hits soft clay, fill or rock, the builder suddenly “discovers” that:

• The slab needs piering

• The piers need to go deeper

• Rock needs breaking

• Soil needs to be removed and replaced

All of that becomes a variation. None of it was in the original price.

What actually drives foundation costs

Your soil conditions determine:

• Slab design – thicker slabs, waffle rafts, stiffened rafts

• Piering or piles – how many, how deep, and what type

• Rock excavation – hammering, saw cutting or blasting

• Fill replacement – removing unsuitable soil and importing engineered fill

  
  

A block that looks flat and perfect can easily need 4–6m deep piers if there’s soft clay underneath. Another block might look easy but sit on shallow rock that costs tens of thousands to excavate.

You don’t see any of this from the surface. The soil test does.

How costs blow out without soil data

When soil conditions aren’t known upfront, everything becomes reactive.

Piering gets added after excavation. Rock gets charged once it’s hit. Fill gets added once things start moving.

These are some of the most expensive items in residential construction - and they are usually charged at day-rate pricing, not contract pricing.

That’s how a “$25k site cost allowance” quietly turns into $80k.

What having a soil test upfront changes

When you get a geotechnical report before asking for quotes:

• Builders can price the actual foundation, not an allowance

• Quotes become comparable instead of guesswork

• Variations drop dramatically

• Disputes disappear

• Your budget becomes real

It turns your build from a gamble into a scoped project.

The bottom line

A soil test is not optional if you want an honest build price.

It’s the difference between:

“This is our estimate” and “This is what it will actually cost to support your house.”

If you’re building in NSW or anywhere in Australia, getting a geotechnical report before requesting quotes is the single easiest way to avoid foundation blowouts, disputes and budget shocks.

And it’s one of the first things we check when mapping approvals and site requirements properly - because everything else sits on it.