Guide & FAQ
Home Building Cost Calculator
How the calculator works, what the inputs mean, and answers to the most common questions.
Using the calculator
The calculator is designed for larger screens — it works best on a desktop or laptop rather than a mobile phone. It takes around 10 minutes to complete all five steps and will produce an itemised budget estimate across low and high cost ranges.
What you can do with your costing:
- Get a ballpark estimate before you even have a plan
- Use it for Council costings
- Identify where to reduce cost before things are locked in
- Save your costing and retrieve it later with a unique code
- Email yourself a one-page summary
Why we built it
For every new home or renovation, there needs to be a way of estimating cost before designs are finalised. The old method — square metre rates — is so basic it's barely useful. It doesn't account for site conditions, quality of materials, fitout complexity, or any of the factors that actually drive cost.
My goal was to design a simple but significantly more accurate tool that anyone can use for free. It took around two years to design, test and build with the help of a programmer. We use it ourselves — for Council costings, for feasibility checks, and for helping clients understand their budget before we start drawing.
Note: Where pricing is critical, always engage a Quantity Surveyor or get builders' quotes. This calculator is designed for the budgeting, feasibility and concept stages — not as a substitute for professional cost planning.
FAQ
Frequently asked questions
What is the quality scale?
The quality scale is a way of adjusting the estimate based on the complexity, level of detailing and quality of materials for your project. For example, homes with high ceilings and more complex roof forms sit at a higher quality scale — and a higher cost. Rather than guessing the number yourself, turn on 'Auto' and answer the 12-question checklist. It will select the quality scale for you based on your specific project characteristics.
How accurate is the costing?
It's designed to give a quick, reasonably accurate estimate — not a definitive cost plan. It won't replace a Quantity Surveyor's Cost Summary Report, and it doesn't incorporate every variable a builder's tender would cover. That said, we know builders who use it as a starting point for cost conversations with potential clients, and find it accurate enough for that purpose.
What is the contingency allowance for?
Contingency is an allowance for costs that will occur on your project but are hard to estimate precisely — unforeseen items, minor overruns, changes during construction. The automatic calculation suggests a contingency based on the size, quality level and other factors you've entered. It's higher for renovations (where unexpected conditions are more common) and lower for new builds. You can override it with a custom amount or remove it entirely.
Why does excavation cost so much?
The calculator assumes that excavation usually involves more than just digging — retaining walls, drainage and associated site works typically follow. It uses a rate that reflects the total cost of these items together rather than excavation alone. Since this is a whole-project estimating tool rather than a trade-by-trade breakdown, that's a more realistic reflection of what you'll actually spend.
Does the build date affect the cost?
Yes, but only if the planned build date is more than a year from today. This allows the calculator to factor in expected future cost increases for construction materials and labour. If your build is within the next 12 months, current rates apply.
Can I retrieve my costing with just the code?
Yes — and that's the simplest way. You can ignore the name and costing title fields if you want. Your unique retrieval code is shown on the summary page. Keep it somewhere handy and you can pick up exactly where you left off.
Do builders use this tool?
Yes — we know builders who use it to start a cost conversation with potential clients before going to the effort of a full tender. They find it accurate enough for that early-stage discussion.
Why is it labelled Beta?
In software terms, Beta means the tool is complete and usable but may still contain known or unknown issues. We update and correct it regularly, but we can't control every use case and take no responsibility for errors. Use it 'at your own risk' — and if accuracy is critical, engage a Quantity Surveyor instead.
How long did this calculator take to build?
About two years, working with a programmer. It went through multiple rounds of design, testing and refinement before we were happy with its accuracy and usability. It's been updated regularly since — the current version is 7.0, updated May 2026.
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