Site Costs article in New Home Buyer

New homes look pretty simple. There’s the roof, which sits on the bricks. The bricks rest on the slab. The slab rests on the ground. Except if the slab really just rested on the ground, your house would soon begin to resemble that glass vase you dropped and tried to glue back together 10 years ago.

The ground beneath your feet isn’t stable enough to support the weight of a house. Soil can expand when wet, contract as it dries and compact over time. The effects of this movement are easy to see in the cracks in the plaster and concrete of many older homes. Newer homes are subject to far fewer cracks than older homes, largely because most new homes feature piers to the underside of the slab.

Piers are columns of concrete poured into deep holes in the ground, measured in lineal metres. They rest on the more stable soil or rock that lies deep beneath the surface dirt. The slab lies on top of these piers, and is therefore not as affected by the movement of the surface soil. Some slabs will require more piers than others, whilst some sites will require deeper piers to connect the slab to more compressed, stable soil. This means more lineal metres of concrete, and therefore, more cost.

Most builders will present piering in their quotes as a provisional allowance – for instance, they may allow for 50 lineal metres of piering. No builder can fix the cost of piering because no one knows what’s under the ground until someone starts drilling. The day piers are poured, a structural engineer will be on site. Piering and drilling will continue until the engineer’s satisfied that the slab complies with the Australian Standards. Hopefully, the amount of piering required will match the provisional allowance provided by the builder, or even be a little less. If more lineal metres of piering are needed, however, you will be charged for them.

You can, however, ensure that the builder’s piering estimate is as accurate as possible by providing the builder with additional information. If soil testing (also known as a bore hole test or geotechnical report) has been done on the site, providing the builder with this information will help them assess how much piering will be needed to reach stable soil. This report will also help firm up another variable cost in the tender. Included in the base price of most homes is an “M” class slab. “M” slabs are suitable to be laid on moderately reactive soil, meaning the land expands and contracts to a small extent. Land may also be classified as S, meaning stable, or H, which is the label attached to highly reactive soil. If your land is classified as highly reactive, and your builder has only allowed for an M class slab, you will have to pay the difference for thicker cement, and more piering.

If the land you’ve purchased is part of a new development, it’s possible that extensive cut and fill has occurred on the site before you bought it. If so, it’s important to get a fill diagram from the land vendor. Because recently laid fill still hasn’t compressed, your land may be subject to a great deal of movement. Further, to reach the solid soil that the piers must rest on, piers may have to be drilled to a much deeper extent than usual. The fill diagram will tell the builder’s estimator where and how much fill is on your land, so their estimate of how much piering you need will be that much more accurate.