Reference Page
This page explains the terms used in our tree survey schedules and sets out the limitations that apply to them. It is referenced from our drawings, schedules and reports where those terms appear without being explained in full in the accompanying text.
It covers two distinct types of survey. Please make sure you are reading the section that matches the document you hold, because the two methods answer different questions and are not interchangeable.
Start here
The heading of your schedule, or the title block of your drawing, will tell you which of the following applies. If your document refers to BS5837, root protection areas or tree categories, use the first section. If it refers to QTRA, risk of harm or an advisory risk threshold, use the second.
Undertaken to inform design, planning and construction. It records the quality of the trees present and the constraints they pose, so that they can be properly considered in a development proposal. It is not a risk assessment.
Undertaken to quantify the risk of harm arising from tree failure and to recommend proportionate action. It assesses safety, not suitability for retention within a development.
A note on both. All of our inspections are visual and made from ground level. No survey of any kind can identify defects that are not visible, and no assessment can make a tree safe. What both methods do is provide a reasoned, recorded and defensible basis for decisions about trees on the date those decisions were made.
Part One
Trees in relation to design, demolition and construction: Recommendations.
A BS5837 survey records the trees on and adjacent to a site, assesses their quality and remaining contribution, and identifies the space each needs below and above ground. Its purpose is to allow trees to be considered properly at the design stage, so that those worth keeping are kept and given the conditions they need to survive construction, and so that the constraints they impose are understood before a layout is fixed.
The survey produces a schedule of trees (explained in the key below) and, where commissioned, a tree constraints plan. Later stages such as an arboricultural impact assessment, method statement or tree protection plan build on this data, but are separate pieces of work.
A BS5837 survey is not a tree risk assessment and must not be used or relied upon as one. Defects are considered only insofar as they bear on a tree's health, longevity and the appropriateness of retaining it through development. A tree may be recorded in good condition for BS5837 purposes and still present a risk that this survey was not scoped to quantify. Where trees are close to occupied buildings, roads, footpaths or other areas people use, a separate risk assessment should be commissioned (see Part Two).
All trees over 75mm diameter, measured at 1.5m from adjacent ground level, within the site were surveyed, together with those located outside the site but within twelve times their stem diameter of the site boundary, as per section 4.4.2.1 of BS5837:2012. Trees beyond that distance were not recorded, and their omission should not be read as a statement about their condition.
Tree positions were located by GPS, by triangulation to base mapping (for example Ordnance Survey MasterMap®) or by reference to a supplied topographical survey. Positions taken from a topographical survey are subject to the limitations and accuracy of that survey and of the surveyor who produced it. Our own GPS and triangulation measurements aim to be accurate to within 0.5m, in accordance with the expectations of BS5837:2012.
Unique and sequential identification numbers have been allocated that reflect the nature of the arboricultural feature recorded (individual trees, groups, hedges and woodlands). Unless noted in the survey schedule, none of the trees have been tagged. Where a site is revisited, or surveyed by others, references may not correspond between surveys.
All tree survey data was collected from ground level, without detailed individual or physical examination. No aerial inspection, decay detection or root excavation was undertaken, so the disclosure of hidden or internal defects cannot be expected. Where a tree warrants further investigation, this is stated in the schedule.
Inspection is commonly restricted where trees are ivy-clad, where basal growth or other vegetation obscures lower stems and root collars, where trees stand wholly or partly on neighbouring land, or where ditches, fencing, watercourses or dense undergrowth prevent close approach. Any such restriction affecting a particular tree is recorded against it in the schedule, and should be read as qualifying the observations made.
Hazards are considered insofar as they affect the longevity of the trees and the appropriateness of retention. The survey does not constitute a risk assessment and should not be used as such.
Height, crown spread, first significant branch height and average canopy height were estimated unless otherwise stated. Stem diameters were measured directly where the stem was accessible; where it was not, the diameter shown is calculated, and the schedule column heading identifies the value as calculated or actual. Root protection areas derived from calculated diameters carry the same uncertainty.
Trees are dynamic living organisms whose condition can change rapidly, and which can be subject to damage by extreme weather conditions. Tree inspection details and recommendations can only be assumed to be accurate for one year from the date of inspection. They should be reviewed after that date, or sooner following any severe weather event or any change of circumstance on or adjacent to the site.
The survey is necessarily invalid if development, construction or tree works other than those recommended within it are undertaken upon or in proximity to the subject trees. If any of these have occurred since the date of inspection, the data should not be relied upon without reassessment.
Where a soil assessment (including soil plasticity indices) is absent from the received documentation, no specific assessment of the influence of trees in relation to soil volume change can be undertaken, whether as per NHBC Standards chapter 4.2 or section 4.3 of BS5837:2012. Any foundation design implications of tree proximity remain a matter for the project's structural engineer.
Where the assessment relies on information supplied by others (topographical surveys, base mapping, constraints data, ecological or soils reports) that information is taken in good faith. Sorensen Tree Consultancy Ltd is not responsible for the related accuracy of any third party information, or for conclusions which depend upon it.
All conclusions and recommendations of the assessment are necessarily the product of the author's experience and qualified opinion, formed on the basis of what was visible and reasonably ascertainable on the date of inspection.
Part One · Schedule key
Our BS5837 schedules are issued as a spreadsheet with the column layout shown below. The four crown spread measurements sit beneath a single heading; every other heading spans both header rows.
| Tree ID | Common Name | Botanical Name | Height (m) | Calc. / Actual Stem Dia. (mm) |
Spread (m) | Height (m) & Orientation of First Significant Branch |
Avg. Canopy Height (m) |
Life Stage | General Observations | Preliminary Recommendations |
Estimated Remaining Contribution (Years) |
BS5837: 2012 Category |
RPA Radius (m) |
RPA m² | |||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| N | E | S | W | ||||||||||||||
| T1 | Oak | Quercus robur | 11 | 420 | 5 | 4.5 | 4.5 | 5 | 2.5 - N | 5 | Early Mature | Base, main stem and branches obscured by Ivy. No significant visible defects. | No work at this time. | 40 + Years | B2 | 5.0 | 80 |
| T2 | Field Maple | Acer campestre | 9 | 370 | 4 | 4.5 | 4 | 4 | 0.5 - N | 5 | Early Mature | Base and lower main stems obscured by vegetation. No significant visible defects. | No work at this time. | 40 + Years | C2 | 4.4 | 62 |
| T3 | Ash | Fraxinus excelsior | 16 | 920 | 9 | 11 | 8 | 8 | 3 - W | 5 | Mature | Base inaccessible so stem diameter estimated. Historical crown lifting over field. Localised decay associated with branch stubs on eastern limb indicated by adjacent Inonotus hispidus bracket. | No work at this time. | 40 + Years | B2;1 | 11.0 | 383 |
Illustrative rows only. The data shown is taken from a real schedule but does not relate to your site.
| Column | Meaning |
|---|---|
| Tree ID | Alphanumeric reference assigned to allow cross referencing between all plans and within the main text. The prefix reflects the type of feature recorded: T individual tree, G group, H hedge, W woodland. |
| Common Name | The everyday name of the species. |
| Botanical Name | The scientific name. Where a tree could only be identified to genus, the species is given as sp.; where several species are present within a group, as spp. |
| Height (m) | Overall height above adjacent ground level, estimated unless otherwise stated. |
| Calc. / Actual Stem Dia. (mm) | Stem diameter is measured at 1.5m from ground level for trees with a single stem at that height, subject to the requirements of Annex C of BS5837:2012. For trees with two to five stems, the square root of the sum of the squared diameters of the stems is used, while the calculated diameter for trees with more than five stems uses the square root of the square of the mean diameter multiplied by the number of stems. All calculations are in accordance with section 4.6.1 of BS5837:2012 and are available on request. |
| Spread (m), N / E / S / W | Lateral canopy spread as measured at the four cardinal points (north, south, east and west). Four figures are given rather than one because crowns are rarely symmetrical, and asymmetry is what determines where a crown actually conflicts with a proposal. |
| Height (m) & Orientation of First Significant Branch | The orientation (N, S, E or W) and height above adjacent ground level of the first significant branch, specifically branches over 25mm measured at the point where they arise from the stem. |
| Avg. Canopy Height (m) | Average canopy height based on an estimated assessment of the height of the most significant part, giving the usable clearance below the tree. |
| Life Stage | Defines the relative stage of the tree's development within the environment. See the life stage table below. This is a developmental judgement, not an age in years. |
| General Observations | Comments including details of any identified structural defects, notes on form and location, and any restriction on the inspection itself. Where the entry notes that the tree was ivy-clad or otherwise obscured, the observations that follow are qualified accordingly. |
| Preliminary Recommendations | Subsequent recommendations include any necessary remedial work specifications and reference to any replacement planting, subject to details within the main conclusions. They are described as preliminary because they precede design: further work may be required once a layout is settled, and it will then be set out in an impact assessment or method statement. |
| Estimated Remaining Contribution (Years) | The estimated remaining contribution of the tree (as per section 4.4.2.5 of BS5837:2012) in the existing environment, precluding any development. This information typically informs the categorisation of the trees in the adjacent column. It is expressed in the bands under 10, 10+, 20+ and 40+ years, and is a judgement about useful life expectancy in the current context rather than a prediction of when the tree will die. |
| BS5837: 2012 Category | Tree category in accordance with section 4.5 and Table 1 of BS5837:2012. Retention categories include A, B and C, with those rated U typically identified for removal. Subcategories (numbers) reflect particular values to aid assessment. Text is coloured to correspond with the plans. See the tables below. |
| RPA Radius (m) | The same root protection area shown as an idealised radial model, expressed as a radius in metres. The shape of the area on the constraints plans will reflect site conditions and tree habit, so the plan and this column will not always agree. |
| RPA m² | The Root Protection Area (RPA) of the tree, calculated from the stem diameter as per section 4.6.1 of BS5837:2012. Where the calculated area exceeds 707m² it is capped at that figure. |
Every tree is placed in one of four categories reflecting its quality and the value of retaining it. Categories A, B and C are then given one or more subcategory numbers describing why the tree has that value. The colours below are those used on our constraints plans and in the category column of the schedule.
| Cat. | Quality and value |
|---|---|
| U | Unsuitable for retention. Trees in such a condition that they cannot realistically be retained as living trees in the context of the current land use for longer than ten years. Category U trees may still have ecological value, and their removal is not automatic, but they should not constrain a design. |
| A | High quality. Trees of good condition with an estimated remaining contribution of at least forty years. Their retention should be a significant driver of the design. |
| B | Moderate quality. Trees of at least twenty years' remaining contribution, which may have defects or be of lesser quality than category A but which merit retention. |
| C | Low quality. Trees of at least ten years' remaining contribution, or young trees with a stem diameter below 150mm. Their retention should not materially constrain development. |
| Subcat. | Reason for the value attributed |
|---|---|
| 1 | Arboricultural qualities. Condition, form, structure, size, rarity or exemplary specimen quality. |
| 2 | Landscape qualities. Visual and screening value, contribution to local character, and the tree's role in the wider setting. |
| 3 | Cultural values, including conservation. Historic, commemorative, veteran or ecological significance. |
Reading a compound category. Where a tree qualifies under more than one subcategory the numbers are separated by a semicolon, with the primary value given first. B2;1 therefore means a moderate quality tree valued principally for its landscape qualities and secondarily for its arboricultural qualities. It does not mean a category B2 tree and a separate category 1 tree. A single number, such as C2, means the tree's value rests on that one ground alone.
Life stage is expressed against the life expectancy (LE) of the species in its situation, not in years.
| Term | Definition |
|---|---|
| Newly Planted (NP) | Recently established and still reliant on aftercare. Readily replaced. |
| Young | Under 33% of life expectancy. |
| Early Mature | 33 to 66% of life expectancy. |
| Mature | 66 to 100% of life expectancy. |
| Late Mature | More than 100% of life expectancy. Typically in natural decline, with crown retrenchment and reducing vigour, which is not in itself a defect. |
| Veteran | As per Late Mature but with exceptional biodiversity, cultural and heritage value. |
| Dead | No living tissue evident. Retention as standing deadwood may still be appropriate where no unacceptable risk arises. |
Part Two
Quantified Tree Risk Assessment: risk of harm expressed as a number, not an adjective.
A QTRA assessment quantifies the probability that significant harm will arise from tree failure within a twelve-month period, and recommends proportionate action where that probability exceeds an agreed threshold. It exists so that decisions about tree safety can be made on evidence and recorded in a way that is auditable and defensible, rather than on impression.
Rather than labelling a tree "high" or "low" risk, QTRA combines three measurable components into a single figure, a one-in-n annual probability of significant harm, which can then be compared against thresholds derived from wider HSE guidance on tolerable risk.
QTRA does not eliminate risk, and no assessment can render a tree entirely safe. It also does not assess trees for their suitability for retention within a development proposal, does not categorise trees under BS5837, and does not constitute a condition survey of every tree present. Its scope is the risk of harm to the targets identified, on the date of inspection.
All investigations were undertaken visually from ground level without invasive physical examination. The disclosure of hidden defects cannot therefore be expected. Inspection is likely to be restricted where trees are ivy-clad or located wholly or partially on neighbouring land, where access is impeded by ditches or fencing, or where basal growth or other vegetation obscures lower stems and root collars. Where more detailed assessments are required, recommendations are set out in the schedule. Height, spread and other dimensions were estimated unless otherwise stated.
At the discretion of the surveyor, only trees that are identified as presenting unacceptable risks, and those requiring work to reduce that risk, may be recorded during inspection, to a level of detail sufficient to enable distinction and position. Similarly, at the discretion of the surveyor, trees that present obvious significant visible defects may be recorded to demonstrate that works are not required. The schedule is therefore not a list of every tree on site, and the absence of a tree from it is not an omission.
Target values reflect the usage of the area as observed, and as described to us, at the time of assessment. If the way land is used changes (a footpath is created, a car park or building is brought into use, an event is held, occupancy increases seasonally) the assessed risk changes with it, and the assessment should be reviewed.
Trees are living organisms whose condition changes over time and whose response to weather and other events cannot be predicted with certainty. This assessment reflects the condition of the trees as observed on the date of inspection and should not be relied upon indefinitely. Re-inspection at the interval recommended, or following any severe weather event, is advised.
Risk is quantified using the Quantified Tree Risk Assessment (QTRA) method. QTRA expresses the probability of significant harm arising within a twelve-month period. It does not eliminate risk, and no assessment can render a tree entirely safe.
Where a tree is identified as falling within, or close to, a statutory designation, this is flagged as a prompt to verify the position. Designation boundaries shown are indicative, derived from published mapping, and are not a substitute for confirmation with the Local Planning Authority. Responsibility for obtaining any necessary consent or serving any required notice before works are carried out rests with the tree owner or their appointed contractor.
Recommended works should be carried out by a competent arboricultural contractor in accordance with BS3998:2010. The assessed risk of harm assumes the recommendations are implemented within the priority period stated; where they are not, the risk should be taken to be that recorded before mitigation.
Recommended works are subject to wildlife legislation, including in relation to nesting birds, bats and other protected species. Nothing in this assessment removes the obligation on the owner or contractor to check for protected species before work begins.
This assessment presents the findings of the survey to which it relates and is subject to the terms of engagement agreed for that survey. It is prepared for the sole use of the client and no responsibility is accepted to any third party.
Part Two · Schedule key
QTRA schedules are issued as a spreadsheet with the layout below. Columns are arranged in four banded groups, shown in the upper header row: what the tree is, what was found, what the risk amounts to, and what we recommend is done about it.
| Identification | Tree Details | Risk Assessment | Actions | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Tree Ref. | UK Grid Easting | UK Grid Northing | Common Name | Height (m) | Stem Dia. (mm) | Life Stage | Comments | Most Likely Failure | Target Range | Size of Part | Probability of Failure | Risk of Harm | Advisory Risk Threshold | Recommendations | Action Priority |
| T1 | 598468.16 | 293414.40 | Beech | 18 | 1200 | Mature | Base, main stem and branches obscured by ivy. Large Ganoderma bracket to base… | Complete stem fracture (see comments) | 2 | 1 | 4 | 1 in 40,000 | Tolerable | Undertake reduction by the removal of the uppermost declining… | Medium (within 12 months) |
| T8 | 598633.77 | 293641.39 | Wild Cherry | 11 | 450 | Dead | Dead tree with visible horizontal crack across historic target… | Major deadwood (>25mm diameter or >1m length) | 5 | 3 | 4 | < 1 in 1,000,000 | Broadly Acceptable | N/A | No work required, risk is ALARP |
Illustrative rows only. The data shown is taken from a real schedule but does not relate to your site.
| Column | Meaning |
|---|---|
| Tree Ref. | Alphanumeric reference assigned to allow cross referencing between the plans, the main text and, where provided, the work priority dashboard. |
| UK Grid Easting / Northing | The tree's position as British National Grid coordinates, recorded so that it can be relocated on the ground or plotted in GIS. Where trees are untagged, these coordinates are the primary means of identification on a later visit. |
| Common Name | The everyday name of the species. |
| Height (m) | Overall height, estimated unless otherwise stated. Relevant to risk because it determines the fall distance and therefore what can be reached. |
| Stem Dia. (mm) | Stem diameter at 1.5m above ground level. |
| Life Stage | The relative stage of the tree's development, expressed against its life expectancy. The terms used are the same as those in the BS5837 key above. |
| Comments | The factual observations on which the assessment rests: defects seen, decay indicators, previous failures or management, and any restriction on the inspection. Where a failure entry reads "see comments", this is the column it refers to. |
| Most Likely Failure | The specific failure assessed, for example major deadwood, limb pullout, stem fracture or rootplate failure. QTRA assesses a defined failure rather than the tree in general, and the remaining risk columns all relate to this entry alone. |
| Target Range | A numbered band describing how heavily the area beneath and around the tree is used, and therefore how likely it is that something would be struck. Lower numbers mean higher usage, with range 1 the most heavily used. See the scale table below. |
| Size of Part | A numbered band for the size of the part most likely to fail, standing for the severity of the consequences. Lower numbers mean larger parts and more severe potential impact. |
| Probability of Failure | A numbered band for how likely that failure is within the next twelve months. Lower numbers mean failure is more likely. |
| Risk of Harm | The calculated output: the annual probability of significant harm, expressed as one in n. A figure of 1 in 40,000 means that, on the assessment made, significant harm from this failure would be expected once in forty thousand years of the conditions recorded. |
| Advisory Risk Threshold | Where that figure falls against the advisory thresholds: Broadly Acceptable, Tolerable or Unacceptable. See the table below. |
| Recommendations | The work specified, written as the minimum intervention needed to bring the risk within an acceptable range while retaining as much of the tree as possible. N/A means no work is specified. |
| Action Priority | The timeframe within which the work should be completed, agreed with the client in advance. See the priority table below. |
Each of the three inputs is a band on a numbered scale. The direction of these scales is the single thing most often misread: in every case a lower number is the more serious condition.
| Input | Scale | Direction |
|---|---|---|
| Target Range | Bands from most to least frequently occupied, based on how long people or property are present in the area over an average year. | 1 is the highest usage (for example a busy road or a well-used path), rising to the least frequently occupied band. |
| Size of Part | Bands for the size of the part assessed as most likely to fail. | 1 is the largest part and the most severe potential impact; the highest band is the smallest part. |
| Probability of Failure | Bands for the likelihood of the specified failure occurring within twelve months. | 1 is the most likely to fail, rising to the least likely. |
| Threshold | Risk of harm | What it means for you |
|---|---|---|
| Broadly Acceptable | Better than 1 in 1,000,000 | The risk is comparable to the background risks of everyday life. No action is required on risk grounds. |
| Tolerable | Between 1 in 1,000,000 and 1 in 10,000 | The risk may be accepted where it is justified by the benefits the tree provides and is reduced as low as reasonably practicable. Work may be recommended, and the reasoning is given in the recommendation. |
| Unacceptable | Worse than 1 in 10,000 | The risk should not be accepted. Action is recommended to reduce it, and the priority assigned reflects that. |
Priority bands are agreed with each client to suit their resources and budget cycle, so the exact wording may vary between commissions. The bands used in most of our schedules are:
| Priority | Meaning |
|---|---|
| Urgent | Immediate action required. Where used, this will be raised with you directly and not left to the schedule alone. |
| High | Within 6 months. |
| Medium | Within 12 months. |
| Low | As general management, or when funds allow. |
| No work required | Risk is ALARP (as low as reasonably practicable). The tree has been assessed and no action is recommended on risk grounds. |
Recording work as complete. Where your survey is issued alongside a work priority dashboard, marking an item complete records the action as reported to us. It is not confirmation that the works have been inspected or approved unless separately signed off.
Applies to both
This assessment is confidential to the client for the purposes of the brief, and no liability is accepted to any other parties. Beyond the remit of the brief (which, for a BS5837 survey, is expected to include planning submission) and notwithstanding section 47 of the Copyright, Designs and Patents Act 1988, it is not to be disclosed to other parties without the written consent of Sorensen Tree Consultancy Ltd.
Schedules and drawings carry a revision reference. Where a document has been reissued, only the current revision should be relied upon; earlier revisions are superseded in full and should not be read alongside it.
This page is provided as a permanent explanatory reference for documents we have issued. Because those documents may be read some years after issue, the version in force at the date of your document is the version that applies to it. Previous versions are listed below and can be provided on request.
Document control
Where a document we issued predates the current version of this page, the version in force at the date of that document applies to it. Superseded versions are retained and are available on request.
Current version: 2.0 · issued August 2026 · next review August 2027
Written, reviewed and maintained by Tony Sorensen BSc (Hons) Arb, MICFor, MArborA.
| Version | Issued | Summary of changes |
|---|---|---|
| 2.0 | Aug 2026 | Full review and rewrite. QTRA method, limitations and schedule key added. BS5837 limitations expanded and restructured. Schedule keys rebuilt as interactive tables matching the current spreadsheet formats. Life stage terminology updated, with early mature replacing semi-mature. Version control introduced. |
| 1.0 | Not recorded | Original BS5837 survey and assessment limitations, with key to survey terms supplied as an image. |
