Report guide
Arboricultural Impact Assessment (AIA)
An Arboricultural Impact Assessment tests a specific design — buildings, driveways, services and excavation — against each retained tree's Notional Root Zone (NRZ) and Structural Root Zone (SRZ), quantifying encroachment and recommending mitigation.
What an AIA typically includes
- NRZ (from DSH) and SRZ (from DARB) calculations to AS 4970:2025 for every tree affected by the proposal.
- Overlay of the design footprint on each zone to identify and measure encroachment.
- Classification of encroachment as minor (up to 10% of the NRZ area), moderate (more than 10% and up to 20%) or major (more than 20%, or any SRZ intrusion), with compensating TPZ area where required.
- The proposed Tree Protection Zone — the area to be physically fenced off — for each retained tree.
- Recommendations such as root investigation, engineering solutions (e.g. pier and beam footings) or redesign.
When you need one
Most councils require an AIA as part of a development application whenever proposed works are within, or close to, a tree's NRZ. It's typically prepared once a concept design exists, following an earlier Preliminary Arborist Report.
Calculating encroachment
Encroachment area is the portion of a tree's NRZ circle that a design footprint covers. Use the Encroachment Zone Calculator to draw the footprint to scale and classify it under the AS 4970:2025 minor, moderate and major bands before committing to a design.
Related tools and guides
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