Budgeting for a garage: Stage 1 - Concrete foundations and soil classifications

Budgeting for a garage: Stage 1 - Concrete foundations and soil classifications
It’s important to know the building and budgeting process to avoid expensive mistakes. Part 1 - Concrete foundations and soil classifications

For a homeowner, having your own garage can be a great addition to your property. But it’s important to know the building and budgeting process, or else you might encounter major (and expensive) mistakes. 


A lack of understanding of the building process is the reason so many homeowners blow out their budgets. It can also cause costly delays because you’re constantly adjusting the timeline and making changes on site. Even just preparing a site can be an intensive process. 


So, to help you build your ideal garage, we will cover the entire process in a series of blogs, starting with the first stage: Concrete foundations and soil classifications.


This stage is the biggest source of budget blowouts because homeowners take them for granted and don’t plan for it enough. 


Understanding what this stage entails will help you prepare for the costs and requirements of building your garage. 

Considerations for the concrete foundations of your garage


It’s important to have a clear idea about your site and the structure you want, as this will affect the costs and work needed for your foundations.


Here are some of the factors you need to consider:


  • Accessibility - If your site isn’t easily accessible for concreters, the prices will get higher. You also need to consider if you’ll need a concrete pump if your site cannot be accessed by a concrete truck.


  • Size - The size of the concrete slab and foundations will depend on how big your structure is because it will show how much material and human resources you need to do the job.


  • Thickness - It’s critical to get the right thickness for your concrete slab. If it’s too thin, it will crack under the weight of the garage and everything else you put in it. Too thick, and it becomes unnecessarily expensive.


  • Type - There are different kinds of concrete slabs, and they each have different purposes and price points:

  • The ground slab is solid concrete poured directly into the excavated trenches. 

  • The raft slab is reinforced with beams and applied on sites that aren’t level.

  • The suspended slab is elevated from the ground and used for sites with un-compacted fills.


So while you may have an estimate of costs, if you don’t factor in allowances, your budget might not be enough.


For example, a calculator estimate would show you that it costs $11,520 to pour a 12 meter by 12 meters concrete slab. 


But in actuality, the total project can reach up to $30,000 after you consider the total amount of concrete, materials, and workforce needed.


The price also varies depending on your location. In Australia, the average cost of a concrete per square metre is:


  • Victoria and Queensland - $80 per square metre

  • Western Australia  - $70 per square metre

  • New South Wales - $95 per square metre


It’s best to consult with concrete experts once you start planning the foundations for your garage. At Shed Bonanza, we connect our customers with professional concreters so that they get a fair price.

What are soil classifications?


It’s important to identify the soil classification of your site because it will affect the foundations of your garage. 


First, a geological engineer will conduct a soil test to learn how reactive the soil is (i.e. how much the soil will shrink or swell with changes in moisture). 


This will let them identify which of the following soil classifications apply to your site:


  • A – Non‐reactive – sand and rock sites – little or no ground movement from moisture changes

  • S – Slightly reactive clay sites

  • M, M–D – Moderate reactive clay or silt – movement from moisture changes

  • H1, H1–D – Highly reactive clay sites

  • H2, H2–D – Very highly reactive

  • E, E–D – Extremely reactive

  • P – Soft soil – reactive sites subject to abnormal moisture conditions or sites which cannot be classified otherwise.


Identifying the right classification minimises cracks on the floor and foundation movement because you’ve matched the concrete slab to the right soil conditions.


Here are some tips to further minimise cracks and foundation movement:


  • Inform your builder whether you plan to grow, keep, or remove trees near your garage so they can take this into account when building.

  • Plant new trees away from the garage so that the roots can grow (and won’t interfere with moisture conditions of the foundation).

  • Avoid placing paving, paths, and garden beds near the garage so that water doesn’t flow towards its foundations.


It’s important to note that every site is different. Even if you have estimates, the actual costs for each site can vary depending on the requirements, the site, and the structure. 

If you have questions about concrete foundations and soil classifications for your garage, give us a call.