Tree constraints plans at concept design stage

A tree constraints plan is a design tool, and its place in the sequence is before the layout, not after it. BS 5837:2012 puts the survey first, the constraints plan second, and the design third, with the impact assessment following once there is a design to assess. Most project teams reverse the middle two, producing a layout and then asking an arboriculturist what it costs in trees. That order is what generates the redesigns.

The plan translates a schedule of trees into an envelope the architect can work inside. Root protection areas, present and ultimate crown spreads, shade, and the categories that say which trees are worth designing around.

What a constraints plan shows that a survey does not

A tree survey is a schedule and a position plan. It records each tree's species, height, stem diameter, crown spread, condition, estimated remaining contribution and its retention category, with U, A, B and C indicating whether removal is appropriate and how high the quality is.

The constraints plan takes that information and draws the consequences on the site plan:

• The root protection area for each retained tree, drawn as an area rather than described in a table.
• Existing crown spread, drawn to the measured radii rather than as a symmetrical circle.
• Ultimate crown spread for the species at maturity, which is usually larger and often much larger.
• Shade cast by the retained trees at the times of year and day that matter for the proposed use.
• The categories colour coded, so the design team can see at a glance which constraints are negotiable.

That last point is the practical value. A category C tree of low quality is a constraint you can decide to lose. A category A tree is one the layout has to accommodate, and knowing the difference before the first sketch saves weeks.

Root protection areas as a design envelope

The root protection area under BS 5837 is calculated from stem diameter: an area equivalent to a circle with a radius of twelve times the stem diameter, subject to a maximum area for a single stem of 707m², with separate formulae for multi-stemmed trees.

Two things about that calculation matter at concept stage.

It is an area, not necessarily a circle. Where the shape has to be modified for site conditions, for example because of a wall, a slope or an existing structure, the area must be retained and the shape adjusted. That gives the designer flexibility a circle does not, and it needs the arboriculturist to draw it rather than the architect to assume it.

And it is a minimum rather than a target. On a tree in poor soil, on a slope, or with known one-sided rooting, the arboriculturist may need to modify the area, and a plan that simply plots twelve times the diameter in every case has not been thought about.

Treat the plotted areas as no-build zones in the first instance. Foundations, drainage runs, service trenches, level changes and construction access all have to sit outside them unless a specific engineered solution is proposed. Service runs are the element most often forgotten, and the approach needed where they cannot be avoided is set out in utility trenching close to trees on a development site.

Shade patterns and unit orientation

Shade is the constraint that produces post-occupancy complaints, and it is the one most often left off the plan.

Plot the shadow cast by each retained tree at the sun positions that matter: through the middle of the day and in the afternoon, at the times of year when the garden and the habitable rooms are in use. Trees to the south and south-west of a dwelling are the ones that matter most, and a large crown within twice its height of a south-facing elevation will put living room windows and a substantial part of a garden in shade for much of the day in summer.

Three design responses work, and they can be combined.

1. Orient the unit so habitable rooms face away from the shade, with kitchens, bathrooms, stairs and circulation taking the shaded elevation.
2. Position the garden where it clears the shadow, which sometimes means a side garden rather than a rear one.
3. Increase the offset, which costs plot depth and is the honest answer where the tree is large and the unit is small.

Shade also matters for roof-mounted technology, since an array that performs now can be overshadowed as a tree matures. That interaction is covered in tree shading of solar arrays and future growth.

Future growth and ultimate crown spread

Designing to the current crown is how a development arrives at a planning application five years later asking to fell a protected tree.

Plot the ultimate spread as well as the present one. Species vary enormously, and a 6m crown on a young specimen of a large species will be several times that at maturity. Where the ultimate spread reaches a window or overhangs a garden, the applicant is designing in a future conflict, and the authority's tree officer will often spot it.

The same logic applies to height and to proximity on shrinkable soils. Where the geology indicates a clay with high shrink-swell potential, foundation design has to account for the mature tree, and that is a structural engineering input rather than an arboricultural one. Getting the constraints plan to the engineer early changes the foundation strategy and the cost.

Honest uncertainty: ultimate spread figures for a species are indicative, and a tree in a tight urban situation will not reach the dimensions it would in open ground. Say so in the plan's notes rather than presenting the figure as a prediction.

Setting buildings back rather than losing trees

The usual trade is plot depth against tree retention, and it is worth making deliberately rather than by default.

Options available before the layout is fixed, in rough order of preference:

• Move the building line. Simplest, costs plot area, and the one most likely to be accepted by the authority.
• Reduce the footprint or change the house type on the affected plots.
• Use a no-dig construction specification for drives and parking within a root protection area, with a cellular confinement system over the existing grade.
• Use a pile and beam or cantilevered foundation where a structure has to sit close to the area, designed with the arboriculturist.
• Accept the loss of a lower category tree and replant, which is the right answer for a category C specimen with limited remaining contribution.

Construction access is the constraint people forget until the contractor arrives. Scaffold positions, crane standing, material storage and site compound all need plotting on the constraints plan at the same time as the buildings, and the implications are set out in scaffolding and site access within a root protection area.

Using the plan in pre-application discussion

Tree officers respond well to a constraints plan, because it shows a design team that has understood the site before proposing anything.

Take the plan to the pre-application meeting with two or three layout options tested against it, and ask specific questions. Which trees does the authority regard as making the greatest contribution. Would it accept the loss of the category C group along the northern boundary with replanting. Does it have a view on no-dig construction in the root protection area of the oak.

Those answers are worth more than a general discussion about principle, and they can be recorded in the pre-application response, which carries weight later. A tree survey prepared to the standard, with the constraints plan alongside it, is what makes that conversation possible. Councils normally expect this work from a suitably qualified arboriculturist, and professional review of any draft is strongly advised.

Updating the plan, and what it costs to do this late

The constraints plan is not a one-off deliverable. It should be reissued as the layout develops, because the retained tree set changes, the construction access changes and the levels change.

The cost of producing it after the layout is fixed is not the arboriculturist's fee. It is the redesign: plots lost, house types changed, drainage rerouted, and in the worst case an application withdrawn and resubmitted. On a site with mature boundary trees, the difference between commissioning the plan at week two and at week twelve is usually measured in units.

So the practical rule is simple. Commission the survey and the constraints plan before the first layout, and give the arboriculturist the topographical survey and the drainage concept so the plan can be drawn against real levels. Then let the Landscape Strategy develop from it, rather than treating planting as something added at the end.

Enter your postcode on PlanScope to see which arboricultural and landscape documents your site is likely to need.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional 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.