You can be sure that the physical cost of the slabs and footings will constitute a significant proportion of the cost of your build. The engineering design cost is a relatively small proportion of the physical cost. The more complex the design requirements, the more costly the engineering. 

By definition, the design and construction of slabs and footings are the very foundation of your house. If something in the design or construction goes badly wrong, then the situation can be beyond economic recovery. If the slabs and footings are over-designed or constructed, then you are wasting your money. 

The aim of the engineering design is to provide a cost-effective solution that will limit any building movement to an acceptable level. There are many factors that affect the design and cost including: 

• The slope of the land – it is more complex and expensive to build on a sloping block. There is more excavation and cut/fill slabs often require piers. 
• The soil – clay soils require stiffer slabs and footings that can add a lot to the cost.  
• Whether or not there are or were large trees on or adjacent to the building – these affect the clay movements and so the footing size. 
• The shape of the house – complex shapes result in more complex and expensive slab and footing arrangements. 
• Steps in the house floor level add costs. 

• The size of the house – as well as simply more material, longer houses need more slab reinforcement to control shrinkage cracking.  
• The house superstructure materials – the more brittle the superstructure the heavier the footings need to be. So, a timber-clad frame has a lighter and cheaper slab than a full-brick building. 
• The flooring materials. Brittle floor coverings (tiles) should have heavier reinforcement and polished concrete is heavier again. 

So, the building shape and material impact the construction price heavily. These are set by you and your architect. The nature of the soil is the other major factor. To reduce the risk of poor performance and to get the balance between cost and outcome at its optimum, good quality information of the soil profile is needed. 

The residential slabs and footing code now require a physical investigation of the soil profile. One option is for you to commission a geotechnical investigation. Another is that the design is conducted to the likely soil profile and is checked during construction then the design reviewed if required. Some subdivisions come with soil reports.  

Should you wish to discuss this information feel free to contact me. 

Paul Davis, Principal Structural Engineer – Project X Solutions Pty Ltd