Ash dieback on development sites and what to do
Ash dieback changes retention categories, felling consents and the biodiversity baseline at once. Treating it as a straightforward safety matter is how developers lose units and gain enforcement risk.
Ash dieback on a development site is three problems wearing one coat: an arboricultural safety problem, a consents problem and a biodiversity net gain problem. The instinct is to clear the declining ash early and tidily. That instinct is what produces enforcement notices, refused felling exemptions and a baseline calculation that counts habitat you no longer have. The order of operations matters: survey and score first, establish the consent position second, and only then decide what comes down and when.
Ash dieback is caused by the fungus Hymenoscyphus fraxineus. It kills most infected ash over a number of years, though a small proportion of trees show tolerance and survive, which is why the Forestry Commission advises against blanket precautionary felling.
How dieback is recorded in a tree survey
A BS 5837:2012 survey records each tree's species, dimensions, age class, condition and estimated remaining contribution, and assigns a retention category. For ash, the surveyor should go further and record a canopy decline score, based on the proportion of the crown that has been lost, together with the presence of basal lesions.
Basal lesions are the part developers underestimate. A tree can hold half its canopy and still be at the point of structural failure, because the fungus has weakened the collar and the root plate, often with secondary honey fungus. Trees with basal lesions fail suddenly and without the warning signs surveyors usually rely on, and that is what shortens the safe inspection interval.
A useful survey will therefore state, for each ash: the decline score, whether basal lesions are present, the target zone below the tree, and a re-inspection date. Where a scheme is on a multi-year programme, expect the arboriculturist to say plainly that the ash schedule will need updating before implementation. A tree survey that omits the re-inspection point is not much use eighteen months later.
Categorising declining ash for retention
BS 5837 sorts trees into category U, where the remaining life expectancy is under ten years, and categories A, B and C for trees with at least forty, twenty and ten years respectively. Advanced dieback pushes ash towards U, and that has a practical consequence: category U trees should not be treated as constraints on layout.
Two cautions sit against that. First, a category U classification is a statement about the tree's safe useful life, not a licence to fell, and it does not override a Tree Preservation Order or conservation area control. Second, moderately affected ash in categories B and C should be assessed honestly rather than opportunistically downgraded, because officers and tree officers read a lot of these reports and a site where every ash happens to be category U attracts attention.
Where dieback is early, retention with monitoring is often the right answer. Where the tree sits away from any target, retaining it as standing deadwood, reduced to a monolith, keeps the habitat value and removes the safety issue.
Safety, target zones and phased removal
Risk is a product of the tree's condition and what sits beneath it. An advanced-decline ash over a public footpath, a road or a plot that will shortly become a construction compound is a different proposition from the same tree in the middle of a field.
That gives you a sequencing tool. Phase removals by target zone rather than felling the whole population at once:
- Trees over highways, footpaths, site accesses and neighbouring properties, dealt with first.
- Trees within the construction footprint, timed to the enabling works.
- Trees in low-target areas, retained under monitoring, reduced to monoliths where the ecology is worth keeping.
- Trees on boundaries in shared ownership, where the consent position needs settling before anything happens.
Phasing also spreads the arboricultural cost and avoids the stark visual change that a single clearance event creates. If any of the trees are shared with a neighbour, the ownership question is worth resolving in writing beforehand, as our note on boundary trees and shared ownership sets out.
Whether a protected ash can be felled without consent
This is where sites go wrong. Under a Tree Preservation Order, the exemption covers a tree that is dead, and work that is urgently necessary to remove an immediate risk of serious harm. A declining ash that is still alive is not covered simply because it will probably die. Since the Localism Act 2011 amended section 198 of the Town and Country Planning Act 1990, dying and dangerous trees no longer sit inside the automatic exemption in the way many people remember.
For a dead protected tree, five working days' written notice to the local planning authority is normally required before the work, except where the risk is immediate. There is also a duty to plant a replacement tree, which the authority can enforce. In a conservation area, works to trees over the relevant size need a section 211 notice with six weeks for the authority to respond, subject to similar exemptions.
Separately, felling outside gardens may need a felling licence from the Forestry Commission once the volume crosses the quarterly threshold, unless the felling is needed to implement a full planning permission. Two consent regimes, two sets of paperwork, and a criminal offence at the end of getting it wrong. The penalties for damaging a protected tree are worth reading before anyone books a contractor.
The effect on the biodiversity baseline
Biodiversity net gain is where premature felling costs real money. The statutory metric measures habitat area, distinctiveness and condition at the pre-development baseline. Clear a woodland or a line of ash before that baseline is taken and, on the face of it, the site looks less costly to develop.
The regime anticipates that. Under the Environment Act 2021 arrangements, where habitat has been degraded since 30 January 2020 without planning permission, the authority can require the baseline to be assessed as it was before the degradation. So the pre-emptive clearance does not reduce the obligation. It just removes your evidence of what was there and hands the authority a worse assumption. Our article on clearing a site before you apply covers the wider version of this trap.
Ash dieback also affects condition scoring rather than only area. A woodland parcel dominated by declining ash may score down on condition, which lowers the baseline units legitimately, as long as the assessment is done by a competent person and the reasoning is documented. That is a defensible reduction. Felling first and arguing afterwards is not.
Replacement planting and species diversity
Replacement planting after ash loss should not be ash. It should not be a monoculture of anything else either. Mixed native planting with a range of genera spreads the risk of the next pathogen, and authorities increasingly ask for that explicitly in landscape conditions.
Illustrative example. On a 3 hectare edge-of-settlement site with a boundary line of twenty mature ash, an approach that tends to work is: fell the fifteen trees with advanced decline and basal lesions in two phases, retain three as monoliths in the open space, keep two early-decline trees under annual inspection, and replant the boundary as a mixed hedgerow with standards of oak, field maple, hornbeam and small-leaved lime. The numbers here are illustrative, but the structure, mixed genera and staggered ages, is what a landscape officer looks for.
Set out the specification, the establishment period and the replacement-on-failure requirement in the Landscape Strategy, and expect the maintenance obligations to be secured for thirty years where the planting is doing biodiversity net gain work.
Managing the visual change on a large site
Losing a mature ash population changes how a site reads from every public viewpoint, and on a scheme in a sensitive landscape that becomes a planning issue rather than a maintenance one. Consultees notice when a scheme's landscape drawings show a green edge that will not exist after the arboricultural works.
Show the change honestly. A drawing set that includes the post-felling condition, the year-one planting and a year-fifteen projection gives the officer something to weigh, and it stops the difference between the visualisations and the reality becoming an objection at committee. Where the loss is large, advance planting in the first available season, ahead of the main works, buys establishment time that no amount of specification can.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. Tree surveys and biodiversity metric calculations should be prepared by a suitably qualified professional, and this is general guidance, not legal advice: check the position with your local planning authority before relying on it.
Frequently asked questions
Can I fell an ash with a Tree Preservation Order because it has dieback?
Not automatically. The exemption covers dead trees and work urgently necessary to remove an immediate risk of serious harm. A living tree in decline sits outside that, so you normally need consent through an application to the authority. For a dead protected tree, five working days notice is usually required and a replacement tree must be planted.
Does ash dieback reduce my biodiversity net gain obligation?
It can legitimately reduce the baseline through condition scoring, because a woodland dominated by declining ash may score lower on condition. It does not reduce the obligation if you fell first. Where habitat has been degraded since 30 January 2020 without permission, the authority can assess the baseline as it stood before the clearance took place.
Do I need a felling licence as well as tree consent?
Possibly. Felling outside gardens above the quarterly volume threshold needs a licence from the Forestry Commission, and that is separate from any Tree Preservation Order or conservation area control. Felling required to implement a full planning permission is generally exempt from the licence, but the tree consent position still has to be checked with the local planning authority.
Should every affected ash come down?
No. The Forestry Commission advises against blanket precautionary felling, partly because a small proportion of ash show tolerance and partly because low-target trees pose little risk. Phasing removals by target zone, retaining early-decline trees under monitoring and leaving some as standing monoliths keeps habitat value while dealing with the trees that actually threaten people or property.
How often should declining ash be re-inspected?
More often than healthy trees, and the interval should be set by the arboriculturist against the decline score and the targets below. Trees with basal lesions can fail without the usual warning signs, so where a tree overhangs a road, footpath or working area the interval is normally short. Update the survey before implementation on any multi-year programme.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.