Building on a new-estate block in Canberra: fill, rock and slopes

What do site reports for new-estate blocks show?

That one block can hold controlled fill, highly reactive clay and shallow rock at once. The clearest local examples are three site classification reports the Ginninderry estate published for blocks in Macnamara and Strathnairn.

Site classification rates how much the soil on your block moves as it wets and dries. AS 2870 sorts sites into classes A, S, M, H1, H2, E and P; our guide to Canberra soils and footings explains them. Here is what the three reports found.

Ginninderry site classification reports, published by the developer. Each describes one block only.

These are reports for individual blocks. They don't give the class of a suburb, an estate or the block next door, and no official Canberra map of site classes exists. They also say the footing system must be confirmed by a structural engineer.

Why does fill matter on a new block?

Because the class depends on whether the fill was controlled. The sample reports show the difference: where fill was placed and checked under AS 3798, the report rates the block with it, as "H1* (highly reactive/filled)". Where fill isn't controlled, the reports warn that "Class P conditions" apply.

The second point is timing. A site report describes the ground on the day it was tested. If your builder cuts a pad, or more fill goes in for a driveway or yard, the reports say the class must be reassessed. So check the date of your report against what has happened on the block since.

What if there's rock close to the surface?

It changes both the digging and the footings. Geoscience Australia's notes on Canberra's geology record that "where hard rock occurs near or at the surface the necessary blasting adds to excavation costs". The same notes say shears and joints in the rock control how stable a rock cutting is; one slide on the Tuggeranong Parkway was later rock-bolted.

Canberra's rock is often deeply and unevenly weathered. Rock can be near the surface on one part of a block and metres down on another, which is exactly what the Strathnairn report found: H1* on its worst-case profile, and Class S conditions over shallow rock on part of the block. A footing design has to deal with both under the one house.

The soil mapping agrees. The Williamsdale soil landscape, which most Canberra suburbs sit on, notes that "a considerable range in profile form occurs within a small area".

How do slopes, cut and fill shape a new-estate design?

A sloping block means cut, fill and usually retaining walls, and the ACT limits all three near boundaries. Many of Canberra's newest estates are on hilly ground, so a level block is the exception there rather than the rule.

The rules that shape the design:

• Cut and fill near boundaries. For DA-exempt single dwellings, the total cut or fill within 1.5 m of a side or rear boundary can't exceed 1.5 m.
• Retaining walls. No building approval is needed for a wall up to 1.2 m high, measured to the lowest ground next to it. For planning, the limit drops to 0.4 m in the front zone and within 1.5 m of a side or rear boundary. Our guide to retaining wall rules in the ACT has the full tables, and retaining wall engineering starts at $890 + GST ($979 inc GST).
• Pools on a slope. A pool on a sloping block can need retaining and a check against the house footings. See pool engineering, from $990 + GST ($1,089 inc GST).

On a slope the footing design also has to step with the ground, which is where the site classification and the cut and fill plan meet. Footing and slab design with site classification starts at $1,290 + GST ($1,419 inc GST), with any soil test at cost.

What should you ask for when you buy a new-estate block?

The paperwork that tells an engineer what's in the ground. Ask the developer or your conveyancer for these before you sign the building contract:

• The site classification report for your block, with its date.
• Any records of fill placed on the block during the estate works.
• A contour or level plan showing the slope and any planned cut.
• Service plans showing sewer and stormwater mains near the block.
• Any bushfire (BAL) requirement for the block, which a bushfire consultant or the controls set. We don't do BAL assessments.

What protections come with a new home in the ACT?

A statutory warranty, residential building insurance and, for bigger projects, a property developer licence. The warranty covers structural elements for 6 years and other work for 2 years, from completion, and passes to later owners. Structural elements include the weatherproofing of external walls and the roof.

Residential building insurance has a minimum cover of $200,000 for 5 years after the certificate of occupancy, but policies may limit claims to the builder's insolvency, death or disappearance. Neither the warranty nor the insurance applies to residential work under $12,000 or to owner-builder work. Since 1 October 2026, projects of 3 or more dwellings also need a property developer licence, and the Registrar can order serious defects fixed up to 10 years after completion.

If something goes wrong in a new home, a building defect report, from $1,290 + GST ($1,419 inc GST), documents it and can support a dispute with your builder.

Is Googong different?

Yes: Googong is in New South Wales. NSW rules apply there instead of the ACT limits on this page, and we work with QPRC and your certifier. Googong's first residents arrived in February 2014, and completion is expected around 2034. See structural engineering in Googong.

When should you call a structural engineer?

As soon as you have the site report and a house design, and before the earthworks are fixed. That is when a footing, a wall or a step in the slab is cheapest to change.

• Footings and slab for the new house: footings, slabs and site classification.
• Walls for the cut and fill: retaining wall engineering.
• Problems in a new home: a building defect report.

We cover the new estates across Canberra at the same prices, including Whitlam and Denman Prospect in the Molonglo Valley and Strathnairn and Macnamara in west Belconnen. We're engineers only: we don't build, sell house-and-land packages or recommend builders, so our design isn't tied to anyone's contract. Send the site report and plans, add up to 5 files, and we reply with a fixed quote in under 20 minutes on average, during business hours.