Before any serious design work starts, BS5837 expects you to gather the right information on trees and turn it into a clear tree constraints plan. This early phase underpins everything that follows, from layout decisions to planning submissions.
Step 1: Topographical and Site Survey
The starting point is a good topographical survey. BS5837 says this should:
- Be to a known scale, with north point, scale bar and clear paper size.
- Plot all trees on or close to the site with stem diameter of 75 mm or more at 1.5 m height, plus off‑site trees that could influence the project.
- Record spot levels across the site, including at the base of trees, edges, embankments, ditches and retaining features.
- Show other relevant features: buildings, boundaries, streams, hedges, stumps and underground/overhead services.
This plan becomes the base drawing for the tree constraints plan.
Step 2: Tree Survey – What Data Is Collected?
An arboriculturist then undertakes a BS5837 tree survey, using the topo as their base. Each individual tree, group or woodland is recorded in a schedule and plotted on the plan.
For each tree (or group) the survey should capture:
- Reference number linked to the plan, and usually a physical tag on the stem.
- Species (common name, with scientific name key if needed).
- Height and stem diameter (measured to BS5837 conventions, including special cases like multi‑stem trees).
- Crown spread at the four cardinal points, plus height and direction of the first significant branch and canopy base.
- Life stage (young, semi‑mature, early mature, mature, over‑mature).
- Structural and physiological condition, with notes on defects, decay, pests, disease and any urgent work.
- Estimated remaining contribution in years (10, 10–20, 20–40, 40+).
- BS5837 category grade (U, A, B or C, with subcategories 1/2/3).
This converts individual trees into usable design data rather than just “green blobs” on a drawing.
Step 3: Tree Quality Grading – What Is a Constraint?
The BS5837 category tells you how important each tree is to retain. In Phase 1 you’re not yet deciding exactly what to keep or remove, but you are flagging where the main constraints lie.
- Category A (light green): high quality, long life, major design constraint.
- Category B (blue): moderate quality, still a significant constraint.
- Category C (grey): low quality/young, less of a constraint, often replaceable.
- Category U (dark red): unsuitable for retention beyond 10 years, normally removed for safety/management reasons.
On the constraints plan, these categories are both coloured and referenced so designers can see at a glance where the priority trees are.
Step 4: Root Protection Areas (RPAs)
The other key dataset for a constraints plan is the Root Protection Area. BS5837 defines the RPA as the minimum area around a tree that must be protected to keep it viable.
- For a single‑stem tree, the RPA radius is 12 × stem diameter, converted via a standard table and capped at 707 m².
- For multi‑stem trees, combined stem diameter is calculated using prescribed formulae and then converted to an equivalent circular area.
- RPAs are first plotted as circles but can be reshaped into polygons if roots are likely to be asymmetric (due to roads, buildings, slopes, drainage, etc.).
These RPAs become “no go” zones for normal construction and are a primary below‑ground constraint for layout.
Step 5: Building the Tree Constraints Plan
Once the data is collected, the tree constraints plan is simply all of that information overlaid on the topo. It is not yet a protection plan or method statement; it’s a design‑tool drawing that shows:
- All surveyed trees, numbered and colour‑graded by BS5837 category.
- The crown spreads, so above‑ground space and shading can be understood.
- The RPAs, as circles or adjusted polygons, to show below‑ground constraints.
- Other key contextual features (levels, boundaries, buildings, services) from the topo.
Design teams can then test early layout options against this constraints plan before committing to detailed design.
Why Doing Phase 1 Properly Saves Time
BS5837 is clear that tree survey and constraints work should happen before detailed design, not as an afterthought. Early, accurate constraints mapping:
- Reduces the risk of costly redesign when conflicts with important trees are found late.
- Gives planners confidence that trees are being treated as a genuine constraint from the outset.
- Helps identify realistic developable areas and where new planting could later provide mitigation.
