Arboricultural Impact Assessment: What It Is and When You Need One

If you are planning to build, extend or clear a site that has trees on or beside it, sooner or later someone will mention an arboricultural impact assessment. It can sound like a lot of paperwork before you have even started, but it is really a straightforward piece of work that shows your local council you have thought about the trees and how to protect them.

At Canopy Consultancy we prepare these reports for homeowners, developers and architects across Surrey and the surrounding counties. This guide explains what the assessment is, when you need one, what goes into it, how long it takes and how to avoid the delays that catch many people out.

What is an arboricultural impact assessment?

Often shortened to AIA, it is a written report that looks at how your proposed development will affect the trees on and around the site. It is prepared by a qualified arboricultural consultant, and it sits alongside your planning application so the council can judge whether the trees will be protected properly.

The report answers three simple questions. Which trees are there, and how good are they? What will your scheme do to them, both during building work and afterwards? And what needs to be done to keep the important ones healthy, or to replace the ones that have to go?

It is not a way of stopping you from building. A good assessment is practical and balanced. It helps you design around the trees that matter, and it gives the planning officer the confidence to say yes.

When do you need one?

You will usually be asked for an assessment when there are trees on the site, or close enough to the boundary that their roots or branches could be affected. Common situations include:

• New houses, extensions, garages and outbuildings near existing trees.
• Changes to driveways, patios, drains and services that run close to roots.
• Sites with a Tree Preservation Order or in a conservation area.
• Larger schemes such as housing estates, commercial buildings and change of use applications.
• Applications where the council has already flagged trees as a concern in pre-application advice.

If you are not sure whether your project needs one, ask your planning officer early or speak to us first. Many councils publish a validation checklist that lists the documents they expect, and tree reports are usually on it whenever trees are within or next to the development area. If a TPO is involved, our guide to tree preservation orders and building work explains what that means in practice.

How the assessment fits with BS 5837

In the UK, tree reports for development follow a British Standard called BS 5837, which sets out how trees should be surveyed, graded and protected around construction. Councils expect your consultant to work to this standard, so it shapes almost everything in the report.

Under BS 5837, each tree is given a category. Category A trees are of high quality and value, B trees are of moderate quality, C trees are of low quality, and U trees are in such poor condition that they should be removed whatever happens. Those grades matter because they tell the designer which trees should drive the layout and which can reasonably give way.

The standard also defines a root protection area for each tree. This is the ground around the trunk that must be kept free of building work, digging and heavy machinery if the tree is to survive. Where a design needs to enter that area, the report has to explain why, and how the damage will be avoided.

What goes into an arboricultural impact assessment

Although every site is different, a typical report brings together several pieces of work.

A tree survey

The starting point is a survey of every tree that could be affected. The consultant records species, height, trunk diameter, spread, condition and life expectancy, then grades each tree. If you would like a better picture of this stage, read our guide to a tree survey for a planning application.

A constraints plan

This plan shows each tree, its category and its root protection area, so architects can see where building is sensible and where it is not. Ideally it is prepared before the design is fixed, because it is much cheaper to move a line on a drawing than to redesign a scheme later.

The assessment of impact

This is the heart of the report. The consultant compares your proposed layout with the survey and describes what will happen to each tree. Some will be untouched, some will be pruned, some may be affected by foundations, levels or services, and some will need to be removed. Each effect is explained in plain terms, with the reasoning behind it.

Mitigation and replacement planting

Where harm cannot be avoided, the report sets out how it will be reduced. That might mean a different foundation type, hand digging near roots, or moving a driveway a few metres. Where trees are lost, it often recommends replacement planting to keep the site green.

A method statement and protection measures

The report explains how the trees will be looked after on site. This covers fencing, ground protection, storage areas, access routes and the order of works, and it usually includes a tree protection plan showing exactly where each measure goes.

AIA or tree survey: what is the difference?

The two are often confused, and it is an understandable mix-up because the survey feeds into the assessment. A tree survey describes the trees as they are today. The assessment goes a step further and shows how your particular scheme will affect them.

If you are at the very beginning of a project, a survey may be all you need to guide the design. Once you have a layout, the council will normally expect the full assessment, together with a tree protection plan and method statement. Your consultant can tell you which stage you are at and what the council is likely to ask for.

The tree protection plan and method statement

This is a scaled drawing that shows the trees you are keeping, the protective fencing and the areas where work must not happen. The method statement describes how the work will be carried out around those trees, step by step.

Planning officers often attach a condition requiring both documents to be followed, and they may check on site. Having a clear plan also helps your builder, because everyone knows where the fence goes and what is allowed inside it.

What happens to the roots?

Most of the damage that building causes to trees happens underground, where it is easy to miss. Digging trenches, compacting soil, changing levels and laying new surfaces can all harm roots that you cannot see. Our guide to root location surveys explains how we investigate where roots actually are, which can sometimes free up space that the standard protection area would otherwise rule out.

How long does it take and what does it cost?

A straightforward domestic report can often be prepared within a week or two of the site visit. Larger developments take longer because there are more trees and more design changes to consider. It is worth building this time into your programme, since the council cannot validate an application that is missing a required report.

Cost depends on the number of trees, the size of the site and the complexity of the scheme. A single extension next to a couple of trees is a modest job. A multi-plot development with mature woodland on the boundary is a bigger piece of work. We are happy to give you a clear quote once we understand the project.

Common mistakes that cause delays

After years of seeing applications go through, the same problems come up again and again:

• Leaving the report until the last minute, so the design has already been fixed and cannot be changed around the trees.
• Forgetting about trees on neighbouring land that overhang or have roots under the site.
• Submitting a basic survey when the council has asked for a full assessment with a protection plan.
• Using a report that is out of date, or that does not follow BS 5837.
• Ignoring a Tree Preservation Order or conservation area status until the council raises it.

Most of these are easy to avoid if the consultant is involved early. A short conversation before the architect starts drawing can save weeks later on.

Why involve a consultant early?

The cheapest time to protect a tree is at the sketch stage. If the root protection areas are on the first drawing, the design naturally works around them. If they only appear after the plans are finished, you may be forced to change the layout, redo drawings and delay your application.

Early involvement also leads to better outcomes for the property itself. Mature trees add value, privacy and character, and well-planned protection means they stay healthy long after the building work is done. If you are weighing up a larger scheme, our article on pre-planning tree surveys shows how early advice can unlock more of a site.

What to look for in an arboricultural consultant

Not everyone who works with trees is qualified to produce an arboricultural impact assessment for a planning application. When you choose a consultant, check that they:

• Hold recognised arboricultural qualifications and relevant professional membership.
• Work to BS 5837 and understand how your local council applies it.
• Can explain their findings clearly without jargon.
• Are happy to talk to your architect and planning officer directly.
• Give you a clear timescale and a written quote.

It also helps to choose someone who is independent of the tree surgeon doing the physical work. That way the advice is based on what is right for the trees and the scheme, not on what work might follow.

What happens after the report is submitted?

Once your application is in, the council’s tree officer reads the report alongside the plans. They may ask a question or request a small change, such as moving fencing or adjusting a foundation detail, and a quick, clear reply from your consultant usually resolves it. If permission is granted, there will often be a condition requiring the protection measures to be installed before any work begins and kept in place until the end. Sticking to that condition protects the trees, your builder and your sign-off.

Frequently asked questions

Does every planning application need an arboricultural impact assessment?

No. If there are no trees on or near the site, you will not normally need one. If there are, the council will usually ask for a report, and the level of detail depends on how close the trees are and how much they matter.

Can the council refuse my application without one?

Yes. If the report is required and missing, the application may not be validated at all, or it may be refused because the council cannot judge the impact on the trees.

Will the report stop me removing a tree?

Not necessarily. It simply sets out the case. Trees in poor condition, or those that make the scheme impossible, can often be removed with the right justification and replacement planting. Protected trees need extra care and permission.

Can I use a report prepared for a previous application?

Only if the site and the proposal are still the same, and the report is recent enough. Trees grow and change, and layouts move, so an old report often needs updating.

Speak to Canopy Consultancy

If your project involves trees and you would like straight, practical advice, we would love to help. Get in touch with our team and tell us about the site, or take a look at our home page to see the services we offer. We will explain what the council is likely to need, give you a clear quote and help you get your application moving without unnecessary delay.