What an arboricultural impact assessment adds to a survey

A tree survey records what is on the site. An arboricultural impact assessment says what your design does to it. They are two stages of the same British Standard, they are usually written by the same person, and a council asking for an impact assessment will not accept a survey in its place.

The distinction matters commercially because the survey can be done before you have a design and the impact assessment cannot. Commission them in the wrong order and you pay for the second one twice.

The survey as a factual record

BS 5837:2012 sets out what a survey records for every tree on the site and every tree outside it whose crown or roots could be affected. Reference number, species, height, stem diameter measured at 1.5 metres, crown spread at four points, crown clearance above ground, age class, physiological and structural condition, preliminary management recommendations and an estimated remaining contribution in years.

Each tree is then categorised. Category U for trees that should be removed for sound arboricultural reasons regardless of any development. Category A for trees of high quality, B for moderate quality and C for low quality, with subcategories recording whether the value is arboricultural, landscape or cultural.

From the stem diameter the surveyor calculates a root protection area, which is an area equivalent to a circle of radius twelve times the stem diameter, subject to the cap the standard applies. That figure is arithmetic, not opinion, and it is the number the rest of the exercise turns on.

The output is a schedule plus a tree constraints plan showing crown spreads, root protection areas and shade patterns. At this point nothing has been assessed. The plan is a constraint drawing for the designer to work with, which is precisely what a tree survey is for.

The impact assessment as a response to a design

The impact assessment takes the layout and reports the consequences, tree by tree.

It identifies which trees are to be removed and why, distinguishing removals required by the development from removals that were arboriculturally justified anyway. It quantifies every encroachment into a root protection area, in square metres and as a percentage of that area. It addresses above-ground conflicts, such as a proposed roof line within the crown spread of a retained tree. It deals with construction access, material storage, site compound location and crane oversail.

And it states the residual effect after mitigation, which is what the case officer actually needs to write a recommendation.

The standard contemplates that minor incursions into a root protection area can be acceptable where the rest of the rooting volume is undisturbed and equivalent area is provided contiguously. That is a judgement for the arboriculturist on the facts, not a percentage anyone can apply mechanically, and the ways of reshaping a root protection area without reducing it are usually a better answer than arguing about the size of an incursion.

Two further documents normally follow: a tree protection plan showing barrier positions and ground protection, and an arboricultural method statement describing how the works within or adjacent to protected areas will be carried out. The barrier detail is not a formality, and tree protection barrier specification under BS 5837 sets out what the specification has to say.

Shade, levels and service routes

Three technical strands sit inside the impact assessment and each one catches people out.

Shade. The constraints plan shows the shadow cast by retained trees, and the assessment considers whether proposed habitable rooms, gardens and solar panels will be usable. This matters both ways. A scheme that puts a living room window two metres from the north side of a mature beech creates a future pressure to fell it, and officers refuse layouts for exactly that reason. Plot the shade at the equinox and at midsummer, and place the private amenity space accordingly.

Levels. Cut and fill within a root protection area is as damaging as excavation for foundations. Raising levels suffocates roots as effectively as lowering them severs them. Where a level change is unavoidable near a retained tree, the answer is a no-dig construction using cellular confinement over the existing grade, and that solution has to appear on the drawings rather than in a paragraph.

Services. Trenching for drainage, water, power and telecoms causes more root damage on small sites than the buildings do, because service routes are drawn late and run along the boundary where the trees are. Route services outside root protection areas, and where a crossing is unavoidable use trenchless techniques. Show the service corridors on the tree protection plan as well as on the drainage drawing.

Quantifying canopy loss

Councils increasingly want a number rather than a narrative, and giving them one makes the case easier to recommend.

Two cautions. Projected canopy at maturity is a long-dated promise, so express it at a realistic horizon such as 25 years rather than at full maturity. And where biodiversity net gain applies, trees are measured through the metric's own modules, which use a different unit from canopy area, so keep the two calculations separate and consistent.

Where replacement planting is substantial or the site has a wider green structure, a Landscape Strategy is the better home for the planting proposals, with the impact assessment cross-referring to it.

Where one document does both jobs

On a small site the two stages are routinely combined into a single report, and that is entirely proper provided the content of both is present.

A combined report works where the site has a handful of trees, the design is settled, and the arboriculturist was appointed early enough to have influenced it. It works badly where the survey was carried out years earlier by someone else, because the categories and root protection areas then rest on measurements nobody can verify.

What must not happen is a survey presented as an impact assessment. A schedule of trees with a constraints plan and no analysis of the proposed layout answers a different question, and validation teams have become quick at spotting it.

Check the survey date as well. Trees grow, stem diameters increase and root protection areas expand with them, so a survey more than about two years old usually needs updating before it supports an assessment.

What validation lists actually require

The national validation requirements do not include tree reports. The requirement comes from the local validation list, which most authorities publish, and the wording varies.

Common formulations require an arboricultural survey and impact assessment in accordance with BS 5837:2012 where there are trees on the site or on adjoining land whose canopy or root protection area falls within the site, or where any tree is protected by an order or stands in a conservation area. Some lists set a trigger by proximity, such as trees within a stated distance of the proposed works.

Read the list for the specific authority before commissioning, and read it again for what else it asks for, because many lists require the tree protection plan and method statement at application stage rather than by condition. Producing them later, under a condition, means the design is already fixed and the arboriculturist has no room to help.

Commissioning them in the right order

The sequence that works is short and people still get it wrong.

1. Appoint the arboriculturist before the layout is drawn.
2. Survey, categorise and produce the tree constraints plan.
3. Design against the constraints plan, with the arboriculturist reviewing iterations.
4. Produce the impact assessment against the settled layout.
5. Produce the tree protection plan and arboricultural method statement.
6. Agree the monitoring regime, because an arboricultural site monitoring record is what demonstrates compliance during construction.

Doing step four before step three produces a document that catalogues problems instead of solving them, and that is the version councils refuse.

Enter your postcode on PlanScope to see which reports a site with trees is likely to need.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified arboriculturist before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.