What Is an Arboricultural Impact Assessment?
An Arboricultural Impact Assessment, often shortened to AIA, is a report that considers how proposed development may affect trees on or close to a site.
It is commonly used as part of the BS5837 process for planning applications where trees are relevant. A tree survey records the existing trees. A Tree Constraints Plan shows the main tree-related constraints. An Arboricultural Impact Assessment then considers how the proposed layout, access, demolition, construction and future use of the site interact with those constraints.
The need for an AIA depends on the site, the proposal, the trees present and the requirements of the local planning authority. Where trees may be affected by development, or where trees could influence the design, layout or construction of a scheme, an Arboricultural Impact Assessment is usually an essential part of the BS5837 process and helps demonstrate how tree-related issues have been considered.
What does an Arboricultural Impact Assessment do?
An Arboricultural Impact Assessment assesses the impact of a proposed development on trees.
It considers:
the direct and indirect effects of the proposed development on existing trees;
which trees are proposed for retention and which are proposed for removal;
the impact of the proposal on Root Protection Areas and canopy spreads;
the effects of buildings, access routes, services, drainage, level changes and hard surfacing on retained trees;
the relationship between retained trees and the completed development, including shading, dominance and future growth;
whether the proposal is likely to result in future pressure for pruning, management or removal of retained trees;
opportunities for tree retention, protection and mitigation;
whether specialist construction techniques, tree protection measures or further arboricultural information may be required.
In simple terms, an AIA helps explain whether a proposed development can be delivered while properly considering the trees on and around the site.
How does an AIA relate to a BS5837 tree survey?
An Arboricultural Impact Assessment is closely linked to a BS5837 tree survey, but it is not the same thing.
A useful way to understand the difference is:
the tree survey records information about the existing trees;
the Tree Constraints Plan shows the constraints those trees present;
the Arboricultural Impact Assessment considers how the proposed development affects, and is affected by, those trees;
the Tree Protection Plan and Arboricultural Method Statement set out how retained trees will be protected during demolition and construction.
The first stage will usually be a tree survey and Tree Constraints Plan to inform the design. Once a layout has been prepared, an AIA may then be used to assess the impact of that layout.
When might an Arboricultural Impact Assessment be needed?
An Arboricultural Impact Assessment may be needed where proposed development could affect trees, or where trees could influence the design, layout or construction of the scheme.
This may include:
new dwellings and residential developments;
replacement dwellings;
householder developments, including extensions and outbuildings;
garden plots and infill development;
mixed-use or commercial redevelopment sites;
access roads, driveways and parking areas;
hard surfacing, landscaping and public realm works near trees;
demolition works close to retained trees;
development affecting trees on neighbouring land;
proposals involving excavation, level changes, drainage or service installation near trees;
works within or close to Root Protection Areas;
sites containing protected trees, including Tree Preservation Orders;
sites within Conservation Areas where trees may be affected;
applications where the local planning authority has requested arboricultural information;
developments where retained trees are likely to influence the design, layout, construction or future use of the site.
An AIA may be requested where the local planning authority needs to understand how trees have been considered as part of the proposal.
What does an AIA usually consider?
The exact content of an Arboricultural Impact Assessment will depend on the site and proposal. However, the following issues are commonly considered.
1. Tree removals and retention
An AIA will identify which trees are proposed for retention and which, if any, are proposed for removal.
Tree removal may be proposed for different reasons. It may relate to the development layout, tree condition, site constraints or the practical requirements of construction. The assessment should explain the arboricultural implications of any proposed tree removals and how they relate to the wider proposal.
Where tree removals are proposed, the assessment may also identify whether replacement planting is proposed as part of the scheme. BS5837 does not create a blanket requirement for mitigation planting in every case, but replacement planting may be considered where appropriate, particularly where tree losses are relevant to the planning balance or where the local planning authority has specific requirements.
Where trees are to be retained, the AIA should consider whether they can be successfully integrated into the proposed development and whether any protection or mitigation measures may be needed to support their long-term retention.
2. Root Protection Areas
Retained trees have Root Protection Areas, often shortened to RPAs.
A Root Protection Area is the minimum area around a tree deemed to contain sufficient roots and rooting volume to maintain the tree’s viability. The AIA considers whether the proposed development conflicts with these areas.
This may include assessing:
building footprints;
foundations;
excavation;
hard surfacing;
changes in levels;
drainage and other services;
construction access;
temporary working areas;
landscaping;
opportunities to improve the rooting environment, where relevant.
Works within Root Protection Areas need careful assessment. In some cases, they may require design changes, specialist construction methods, ground protection or arboricultural supervision.
An RPA does not automatically mean development cannot happen. It does mean the relationship between the proposal and the retained tree needs to be properly considered.
3. Canopy spread and above-ground constraints
Trees do not only affect development through their roots.
An AIA may also consider above-ground constraints, including:
canopy spread;
branch clearance;
shade;
future growth;
proximity to buildings;
relationship with windows, gardens and usable outdoor space;
potential pruning requirements.
These issues matter because retained trees need to work alongside the completed development, not just survive the construction phase.
For example, a layout may avoid the main Root Protection Area but still place a building very close to a large retained crown. That could create future pressure for heavy pruning or removal, particularly where the tree causes shading, overhang or a perceived nuisance.
4. Future pressure on trees
An Arboricultural Impact Assessment will also consider whether retained trees could create future concerns for occupants or site users.
This might include issues such as:
excessive shading;
close proximity to buildings;
dominance over small gardens;
seasonal debris;
pressure for pruning or removal after the development is occupied.
This does not mean every retained tree close to a building is unsuitable. The assessment should be balanced and site-specific. The aim is to consider whether the proposed relationship between trees and the completed development is reasonable and sustainable.
5. Mitigation
Where potential impacts are identified, the AIA may recommend mitigation measures.
This could include:
layout changes;
retaining higher-category trees;
avoiding unnecessary tree removal;
adjusting building positions, access routes or hard surfacing layouts;
using specialist or sympathetic engineering and construction methods;
providing replacement planting.
Depending on the proposal, further documents may be required, including a Tree Protection Plan and Arboricultural Method Statement.
What information is needed to prepare an AIA?
An Arboricultural Impact Assessment needs both BS5837 tree survey information and proposal information.
Useful information includes:
existing site plan;
red line boundary;
proposed layout;
topographical survey where available;
proposed levels;
drainage or service routes where relevant;
demolition plans;
access and construction information;
planning authority comments where relevant;
Tree Constraints Plan;
details of trees proposed for retention or removal;
proposed landscaping plans.
If the project is still at an early stage, the first step will be a tree survey and Tree Constraints Plan. These can help inform the design before a full AIA is prepared.
Is an Arboricultural Impact Assessment always required?
Not every project will need the same level of arboricultural information.
At the earliest stage, a BS5837 tree survey and Tree Constraints Plan are usually prepared first to identify tree-related constraints and help inform the design. Once a layout or development proposal has been prepared, an Arboricultural Impact Assessment is usually the stage used to evaluate how the proposal affects, and is affected by, the trees on or near the site.
Where a planning application is being submitted and trees could be affected by development, an AIA is usually required to explain how the proposal has responded to tree constraints and what impacts are anticipated.
The exact scope of the assessment will depend on the site, the trees, the proposal and the planning context, but where trees are a material consideration, an Arboricultural Impact Assessment is commonly expected as part of the supporting BS5837 documentation.
Common misunderstandings about Arboricultural Impact Assessments
“An AIA is the same as a tree survey.”
It is not quite the same.
A tree survey records information about the trees. An Arboricultural Impact Assessment considers how the proposed development affects, and is affected by, those trees.
They are closely connected, but they have different roles within the BS5837 process.
“You cannot build within a Root Protection Area.”
Not necessarily.
A Root Protection Area is an important consideration in the BS5837 process, but its presence does not automatically mean development cannot take place nearby or within part of the RPA. An Arboricultural Impact Assessment may consider whether appropriate mitigation measures, design changes or specialist construction techniques could reduce potential impacts on retained trees. Depending on the circumstances, this might include alternative foundation designs, no-dig surfacing, careful service routing or other arboricultural measures. Each situation should be assessed on its own merits. The purpose of the assessment is to identify potential impacts and determine whether the proposed development can be achieved while appropriately considering the needs of retained trees.
“Trees outside the site boundary can be ignored.”
Neighbouring trees can still be relevant.
Their roots or crowns may extend into the site, and construction activity close to the boundary could still affect them. This is why BS5837 tree surveys include trees on and adjacent to the site where they may influence the development.
“Root Protection Areas are the only issue.”
Root Protection Areas are important, but they are not the only consideration.
Canopy spread, shade and future growth can also influence whether a proposal is compatible with retained trees.
“An AIA guarantees planning permission.”
An Arboricultural Impact Assessment does not guarantee planning permission.
It provides tree-related information to support the planning process. Planning decisions can also involve design, highways, ecology, drainage, heritage, amenity, policy and other considerations.
How Flow Tree Consultancy can help
If you are preparing a planning application and trees may affect the design, layout or construction of the proposal, Flow Tree Consultancy can provide BS5837 tree surveys, Arboricultural Impact Assessments and related planning documents.
This may include Tree Surveys, Tree Constraints Plans, Arboricultural Impact Assessments, Tree Protection Plans, Arboricultural Method Statements and Arboricultural Clerk of Works input.
For more information, visit the Planning and Development service page, or send an enquiry through the form.
Where available, it is useful to provide the site address, red line boundary, proposed drawings, topographical survey, planning reference and any comments already received from the local planning authority.
About Flow Tree Consultancy
Flow Tree Consultancy is an independent arboricultural consultancy providing clear, professional advice on tree-related matters, including planning and development, tree safety and property risk.
The consultancy provides BS5837 surveys, Arboricultural Impact Assessments and arboricultural reports for planning applications, helping homeowners, architects, developers and planning consultants understand tree constraints and provide clear arboricultural information to support the planning process.
Last reviewed: May 2026
