Before the Slab: Soil Tests, Contour Surveys and the Site Costs That Blow Out Queensland Budgets

Most of the site costs for building a house are set before anyone pours concrete. The soil test decides the slab design, the contour survey decides the cut, fill and retaining, and the builder's tender turns both into allowances that either hold or turn into variations. In Queensland, those three documents explain most of the budget blowouts that start below the slab.

This guide covers what to do before building a house on land you already own: how to read your soil test and site class, what the contour survey tells you, how to read the site costs in a tender, and what to check before any digging starts, including the house next door.

The soil test and your site class

A soil test, often called a site classification, takes samples from boreholes on the block and gives it a site class under AS 2870, the Australian Standard for residential slabs and footings. The class describes how much the ground is expected to move as it gets wetter and drier, and it's the main input to the slab design.

• Class A and S: sand, rock or slightly reactive clay, with little or slight ground movement.
• Class M, H1, H2 and E: clay that moves more with moisture, from moderate (M) to extreme (E). The higher the class, the deeper and more heavily reinforced the slab beams usually are.
• Class P: a problem site, such as fill, soft or loose ground, trees, slope or abnormal moisture. Class E and P sites need footings designed by an engineer.

Because the class drives the slab, it's usually the biggest single site cost in a tender. In Queensland, before a regulated contract that includes footings or a slab is signed, the builder generally has to obtain foundations data such as the soil test. It's usually at your cost, and the builder gives you a copy once it's paid for. Read the class on the report, then check the tender is priced on that class.

The contour survey: how far the block falls

A contour survey, often a contour and detail survey, measures the levels across the block, along with features such as trees, existing structures, services and the levels at the boundaries. It shows how far the block falls and in which direction, and that decides three things:

• Cut and fill: how much soil is dug out on the high side and built up on the low side to make a level platform for the slab.
• Retaining walls: where the cut or fill needs holding back. In Queensland, a retaining wall more than 1 m high, or within 1.5 m of a building or another retaining wall, generally needs building approval and an engineer's design.
• Stormwater and sewer: where roof water can drain to and where the sewer can connect. A block that falls away from the street may need an easement or a pump. If a sewer, water or stormwater main runs through or beside the block, footings near it are assessed under the Queensland Development Code (MP 1.4).

Get the survey before the house design is fixed. A design drawn for a flat block and then placed on a sloping one is where cut, fill and retaining allowances start to grow.

What a soil test and contour survey cost depends on the block: its size, access for the drill rig, the number of boreholes and how much detail the survey picks up. Some builders arrange both before the tender and charge for them up front, while others ask you to arrange them. Either way, ask who owns the reports and whether the tender is priced on them.

Reading the site costs in a builder's tender

Site costs are the work needed to get from the block as it is to a level, serviced platform for the slab. Tenders show them in two ways: as fixed prices, and as allowances that can move.

Under the QBCC Act, the builder warrants that provisional sums and prime cost items have been calculated with reasonable care and skill, taking into account the information reasonably available, including the nature and location of the site. That's why the soil test and contour survey matter at tender time: an allowance set without them is easier to question.

The site lines that most often turn into extra cost:

• Rock excavation: usually a provisional sum, because no one knows how much rock there is until digging starts.
• Cut, fill and compaction: volumes worked out from levels that don't match the survey.
• Piers and deeper edge beams: needed where fill is deep or the ground is soft.
• Retaining walls: left out of the tender, or allowed for at a lower height than the survey shows.
• Stormwater and sewer connections: longer runs, pump-outs or connections through an easement.
• Clearing and demolition: trees, old structures and disposal, especially on knock-down rebuilds.

Ask the builder to list every provisional sum with its allowance, and to show how an overrun is charged. If a site cost isn't in the tender at all, ask why.

Site works and the slab are the base stage of a construction loan, and our guide to construction loan stages covers what the lender checks before each drawdown. If you're building as an owner-builder, there's no tender to check these lines against: the allowances are yours to manage, and owner-builder vs licensed builder sets out who carries each risk on each route.

Before any digging: services, structures and next door

Three things to check before the first machine arrives:

• Underground services: lodge a free Before You Dig Australia enquiry. Asset owners send plans of their pipes and cables, and are asked to respond within two business days. The plans are indicative only, and receiving them isn't approval to dig.
• Existing structures and trees: mark what's being removed and what has to be protected, including old footings, tanks and tree roots on knock-down rebuilds.
• Next door: on a sloping block or a knock-down rebuild, excavation and retaining can sit close to the boundary. Ask the builder and engineer how close the excavation comes to the neighbouring house, and how its footings are protected.

Record next door before excavation

When excavation and retaining go in close to a neighbour's house, any crack that appears later becomes a question: was it there before? If a neighbour complains to the QBCC about damage from building work next door, the question becomes what the work caused and what was already there. A dated record of the adjoining homes, captured before the first machine arrives, gives both sides something to check against.

rchive captures the adjoining homes in 3D LiDAR before excavation, the agreed rooms and the exterior, with each owner's consent. Our guide to pre-construction condition surveys explains how the capture runs and what it records that photos miss.

Everything poured into the slab disappears for good, too. Before plasterboard, the pre-plaster audit records where the waste pipes, floor wastes and conduits come up through the slab, before flooring and cabinetry cover them.

Site costs before the slab: FAQs