Yes — you can install an Expandify home on a property with an existing septic system, but approval depends on:

Whether the current system has capacity for another dwelling

Zoning and council rules for secondary dwellings

Site layout and space for expansion, if needed

If your current septic system is not suitable, your council will likely require you to:

Upgrade the system

Install a separate septic system

Provide a new wastewater absorption area

A formal wastewater assessment will usually be required, which is an additional cost separate to the Site Assessment. The builder can organise this for Complete Packages. For DIY Kits, your plumber or certifier will need to handle this independently.