Impact Assessment – what do arborists need to know?
Arborists involved in report writing come to the role from many different paths — from practical tree work and horticulture, to landscape architecture, ecology or other science fields. When preparing Arboricultural Impact Assessments (AIAs), it’s helpful to remember that we’re participating in a broader planning process where many other consultants – including ecologists, heritage consultants, traffic engineers, and stormwater planners – are also assessing impacts in their area of expertise. While our focus is trees, we’re part of a multidisciplinary framework that relies on clear, consistent, and professional communication. Framing our work within this wider context helps ensure it’s understood and valued alongside other expert advice — and gives us an opportunity to meet the standard expected of all professional consultants in the development space.
In any form of development – whether constructing infrastructure, building or renovating homes, altering land use, or undertaking civil works – the impact assessment process is critical to ensure potential effects on the environment, people, and assets are systematically identified, evaluated, and managed. In the Australian planning and development context, impact assessments provide decision-makers with evidence-based analysis to approve, modify or reject proposals in line with statutory obligations, environmental sustainability, and community expectations.
This article details the fundamental concept of impact assessment, outlining its essential elements and steps, before narrowing focus on how these principles are applied in Arboricultural Impact Assessments (AIAs) – which assess how trees may be affected by proposed works, and guide measures to retain and protect them where possible.
What is an Impact Assessment?
An impact assessment is a structured process used to predict and evaluate the likely consequences of a proposed action or development. Its purpose is to:
- Inform decision-making by identifying potential risks and opportunities;
- Protect environmental and social values;
- Avoid adverse impacts through design amendments;
- Mitigate adverse impacts through protective measures;
- Ensure compliance with relevant planning instruments and legislation.
Impact assessments may vary in scope and complexity depending on the nature of the proposal. These can range from Environmental Impact Statements (EIS) for major developments, to specialist studies (e.g. traffic, heritage, or arboricultural assessments) forming part of a Development Application (DA) or planning permit. Most references to impact assessments refer to the planning stage before any works even begin, however some ongoing impact assessment is involved in monitoring works as they proceed.
Essential Elements of an Impact Assessment
In any given area of expertise, the following core elements are generally included:
| Description of proposal | Scope, location, and nature of the works being proposed. |
| Baseline conditions | Current site conditions and features that may be impacted. |
| Assessment of impacts | Identification and analysis of likely direct, indirect, and cumulative effects. |
| Significance of impact | Evaluation of the scale, likelihood, and consequence of impacts. |
| Mitigation measures | Proposed strategies to avoid, minimise, or offset impacts. |
| Conclusion & recommendations | Summary of key findings and proposed conditions or actions. |
Process Steps in Conducting an Impact Assessment
A consistent methodology provides transparency and rigour. The typical steps are:
- Scoping – Define the extent of the assessment and key matters to address.
- Site Investigation – Collect data and document existing conditions.
- Impact Identification – Determine which features are likely to be affected and by what.
- Impact Evaluation – Use qualitative or quantitative methods to assess significance of the impacts.
- Mitigation Planning – Develop strategies to reduce or manage impacts.
- Reporting – Present findings in a structured, decision-ready format.
These steps are adapted as needed for the subject matter of the assessment.
Arboricultural Impact Assessment (AIA)
An Arboricultural Impact Assessment applies the above principles to assess how trees – particularly trees on and adjacent to a development site – may be affected by proposed works. This is a common requirement under Development Control Plans (DCPs) or local tree preservation policies, especially in urban or environmentally sensitive areas.
AIAs are often required to accompany Development Applications where:
- Trees are proposed to be removed, pruned or transplanted within a development;
- Trees are located near excavation, construction, or services; or
- Works are or may encroach within Tree Protection Zones (TPZs) as defined in AS 4970–2025 Protection of Trees on Development Sites.
Key Inclusions in an Arboricultural Impact Assessment
An AIA, as detailed in the 2025 Australian Standard AS4970, needs to follow a systematic approach, including:
| Component | Detail |
|---|---|
| Tree assessment survey and data | Inventory of existing trees, including species, size, health, condition, landscape significance, useful life expectancy, retention values. |
| Site and development plans | Plans showing layout, excavation, services, and proximity to trees. |
| Tree retention value | Assessment and categorisation of each tree’s arboricultural and landscape value combined with life expectancy. |
| Impact analysis | Identification of impacts (e.g. root zone disturbance, canopy pruning). |
| Application of AS 4970:2025 | TPZ and SRZ calculations, measured encroachments, and compliance with procedures and protection requirements. |
| Mitigation measures | Recommendations such as tree protection fencing, design changes, or monitoring. |
| Tree protection plan (or Tree impact plan) | Map and instructions for implementation of protection measures. |
| Conclusion and advice | Summary of tree impacts, retention feasibility, and conditions for approval. |
Recommended Design Modifications
Where feasible, Arboricultural Impact Assessments should include recommended design changes aimed at avoiding or reducing adverse impacts on trees. These suggestions are most valuable when made early in the planning and design process, enabling proponents and project designers to adjust their layouts or construction methodology before impacts are locked in.
Design changes are considered the highest-order mitigation measure in the impact hierarchy, as they can eliminate or significantly reduce the need for tree removal, complex work methods and protection measures, or offsetting.
Examples of typical design modifications include:
- Relocating built structures (e.g. dwellings, driveways, car parks) outside the Tree Protection Zones (TPZs) of high-value trees;
- Reducing building footprints or modifying basement excavation to preserve root zones;
- Shifting underground services away from trees, or boring under root zones where alignment cannot be changed;
- Using permeable or suspended paving systems to maintain soil permeability and root function;
- Adjusting site grading to minimise cut and/or fill near retained trees.
These recommendations do not have to be reflected in the final design, but their inclusion in the report demonstrates that arboricultural advice was considered early, and that opportunities to reduce impact were explored in line with best practice and planning expectations.
In best-practice impact assessments, it is both common and strongly encouraged to include a separate section or discussion of recommended design changes. These recommendations ideally occur:
- Early in the planning process – to influence design before it becomes fixed or costly to amend, and
- Within the impact assessment report – to formally document how the proposal could be improved to avoid or reduce impacts, even if not adopted in the final design.
Why it’s important:
- Encourages proactive mitigation rather than reactive justification;
- Demonstrates that the assessor considered alternatives and sought to minimise harm;
- Provides councils and consent authorities with evidence of good faith efforts to retain valuable trees or features;
- Supports compliance with avoid-minimise-mitigate hierarchy found in many environmental planning frameworks.
Arboriculturists are encouraged to use a structured format – such as the table below – to present design change recommendations clearly and concisely. This approach assists planners, designers, and assessing officers in understanding the arboricultural rationale and prioritising actions accordingly.
Table 1. Sample Design Modification Table for Arboricultural Impact Reduction
The below example table format can be used in preliminary impact assessment advice or within the AIA report to clearly present specific design recommendations aimed at avoiding or minimising tree impacts. Alternatively this information may be presented in the table of all tree assessment data.
| Tree No. | Tree Description | Identified Impact from Proposal | Current Encroach- ment % | Proposed Design Modification | Impact Reduction Outcome | Resulting Encroach- ment % | Priority |
|---|---|---|---|---|---|---|---|
| 23 | Angophora costata, 15m NRZ, Good health, High retention value | Proposed driveway encroaches 35% into TPZ | 35% | Shift driveway alignment 1.5m north, above grade paving | Reduces TPZ encroachment to <10% | <10% | High |
| 5 | Eucalyptus robusta, 13m NRZ, Medium ret. value | Stormwater line crosses through SRZ | 28% | Reroute service to the west or use tunnel boring | Avoids root severance in SRZ | 0% | High |
| 7 | Group of native shrubs, Medium ret. value | Site grading cuts into TPZ by >500mm depth | 20% for each tree | Maintain existing soil level in TPZ | Maintains soil stability and root health | 0% | Medium |
| 31 | Lophostemon confertus, 12m, Good condition | Building eaves encroach into canopy zone | 20% crown | Reduce roof overhang by 1m | Reduces future pruning and conflict | 5% crown pruning | Low |
| 12 | Jacaranda mimosifolia, 10m, Fair condition | Basement excavation within 2m of trunk | 40% | Reduce basement extent or maintain existing wall | Maintains structural roots & existing soil | 15% | Low |
Note: This table is illustrative and needs to reflect site-specific conditions, arboricultural judgement, and local planning controls.
Table 2. Sample Tree Impact Assessment Data Table
This table is a sample of how an arborist might present the encroachments and impacts in the AIA report to clearly show the trees that are expected to be impacted, details of the impacts and tree protection recommendations.
| Tree No. | Tree Species | Size & Retention Value | Development Impact of Proposal | Encroach- ment % | Proposed Design Modification | Tree Protection Recommendations | Outcome |
|---|---|---|---|---|---|---|---|
| 5 | Eucalyptus robusta | 13m NRZ, Medium | Stormwater line crosses through SRZ – major encroachment | 28% Major | Reroute service to the west or use tunnel boring | Ground & trunk protection & fencing if retained | Remove |
| 7 | Group of native shrubs 100mm DSH | Medium ret. value | Site grading cuts into TPZ by >500mm depth – moderate encroachments | 18% for each tree | Maintain existing soil level in TPZ | Exclude all works with fencing | Retain |
| 12 | Jacaranda mimosifolia | 10m, Fair condition | Basement excavation within 2m of trunk – major root encroachment | 40% Major | Reduce basement extent or maintain existing wall | – | Remove |
| 23 | Angophora costata | 15m NRZ High | Proposed driveway is major TPZ encroachment | 35% Major | Shift driveway alignment 1.5m north, above grade paving | Requires ground & trunk protection, careful excavation | Retain |
| 31 | Lophostemon confertus | 12m, Good condition | Building eaves encroach into canopy zone – major crown encroachment | 20% crown | Reduce roof overhang by 1m | Branch and trunk protection, Project Arborist inspection | Retain & prune |
Note: This table is illustrative and needs to reflect site-specific conditions, arboricultural judgement, and local planning controls.
Summary
As arborists contributing to development projects, our reports often play a key role in shaping design decisions and planning outcomes. That’s why Arboricultural Impact Assessments need to do more than outline which trees are proposed for removal or retention. A robust impact analysis should clearly explain the reasoning behind tree removal decisions, and assess how retained trees will be affected by construction activities, works, and site changes. By applying consistent methodology, referencing standards like AS 4970–2025, and communicating our findings with clarity, we can ensure that arboricultural advice is genuinely useful in the planning process. In doing so, we contribute to better, more informed outcomes — not just for projects and approval pathways, but for the long-term health of our urban landscapes.
