A detached pair sits as two separate buildings with a gap between them, and each can often sit on its own clean lot.

An attached duplex shares a common party wall on the boundary or internal line, using land more efficiently on a narrow block.

Attached dwellings must meet fire separation and sound insulation between the two homes, and the shared wall usually becomes common property.

Decide attached versus detached early, because it drives both the building detail and the title outcome.

The National Construction Code requires a fire-resisting separating wall between adjoining dwellings, continuous from the footing to the roof.

It also sets sound insulation, both airborne and for some elements impact, so the two homes do not hear each other.

Service penetrations, the roof junction and the floor junction are the common weak points and must be detailed and inspected carefully.

Your Relevant Building Surveyor checks these at building permit stage and at the prescribed inspections.

The Victorian Building Regulations require an owner to protect adjoining property that could be affected by the building work.

Where protection work is needed, you must serve the adjoining owner the prescribed notice and reach agreement, or follow the dispute process, before starting that work.

Keep dilapidation records, including dated photos of neighbouring property, before you start, especially for excavation near a boundary.

A builder experienced in attached dwellings will handle these notices and junction details as routine.