In the years that I have worked with builders, I have seen a lot of people get financially burnt because certain site conditions blow out the cost of their build. They say " a project should be 90% planning and 10% doing" and that is 100% right. It is imperative to obtain all the necessary site reports before starting any design, or even purchasing a block of land - particularly sloping sites. Reports can include soil test reports, percolation reports, bushfire reports, slope stability reports, noise overlays, heritage overlays, building envelopes etc. Any one of these can have a significant impact on the design and cost to build your home. For example, designing a home for Bal 19 when the block actually turns out to be a Bal 29 can result in thousands of dollars in additional materials and upgraded cladding and windows. A soil test may uncover very unstable soil, or even mines or covered springs and screw piers may be required costing upwards of $10,000.
So which reports do you need? This is where you should engage a building certifier. Certifiers can produce a Search Pack and tell you what reports are needed and do all the necessary searches including providing a registered plan. They cost appox $300-500 but are definitely worth it and can (and almost always do) saves thousands. We have even found that council requirements have been updated and some blocks can no longer be built on.