We outline some often overlooked points around BNG that can help you plan for a more seamless project.
To make sure as little as possible comes as a surprise for those working towards achieving BNG, our experts have shared some points that are helpful to keep in mind.
1. The 10 metre watercourse rule
Under Biodiversity Net Gain (BNG) legislation in the UK, if a watercourse lies within 10 metres of your site boundary, it must be included in the metric.
This requires a separate type of survey called a ‘River Condition Assessment (RCA)’ which can only be done by a qualified MoRPh surveyor, and cannot be done during a normal BNG baseline survey.
Your BNG baseline assessment must cover the watercourse, and the 10% BNG requirement applies to the watercourse as well as to the terrestrial habitats on your site – even where the main channel lies just outside the site boundary.
Specifically, where the site boundary crosses into or includes the riparian zone of a watercourse (10 m from bank top for rivers/streams/canals, and 5 m for ditches), that length of the watercourse must be included in the metric.
Watercourses cannot be easily augmented, and without expert hydrological input any alterations can result in unintentional consequences. As such, the options for improving their condition can be somewhat limited, unlike with examples such as grassland or woodland. This makes it more complex to achieve and demonstrate a 10% gain.
Where on-site improvements cannot achieve the required uplift, the alternative is to secure off-site units through credits. For smaller sites, this can become a significant cost depending on the baseline habitat.
2. Culverts count as watercourses
With BNG legislation, the definition of a watercourse includes culverts. In BNG terms, a culvert is defined as “a covered channel or pipe designed to prevent the obstruction of a watercourse or drainage path by artificial construction.” Where water is culverted under a road this counts as a watercourse and as such, it should be part of your BNG calculations.
Although, it should be noted that culverts do not have a riparian zone and culverts recorded within the site boundary do not require a River Condition Assessment.
3. Finalising your site boundary before trying to calculate your baseline for BNG is critical for your schedule.
Site boundary changes or inconsistencies in your plans can upend your schedule for development. They can mean a complete resurvey or recalculation. Any changes, however minor, to the proposed plans trigger a need for the metric calculations to be be re-done.
Changing or inconsistently mapping boundaries (for example, between a landscape design and a detailed BNG plan) can alter your baselines, so your various maps must match up. Inconsistencies between different plans or realising part way through the project that the intended boundary is different to what you have on your plans can trigger the need to resurvey. This applies even with a change of only 1m in the boundary outlined.
This is because BNG assessments (using tools like the Defra Metric) rely on accurate site boundaries to calculate baseline biodiversity units. If your boundary changes later, the baseline data becomes invalid and your results must be recalculated. This can cause delays or extra costs if the revised area alters your results from the Metric.
4. Hedgerows are an easy win for generating net gain
Looking at what you can use to your advantage, hedgerows can be used very effectively for the creation of biodiversity net gain. It can also be a cost effective and relatively easy means of helping to reach your net gain, and it doesn’t need to be a huge amount of hedgerow creation to generate this.
Hedgerow creation is often underestimated and an under-utilised but powerful tool to generate biodiversity. According to The Woodland Trust, “Hedges … are much more than useful dividers. … they are teeming with life, provide landscape-scale connectivity and make an immense contribution to halting biodiversity decline.”
Hedgerows are made up of a mix of trees, shrubs, herbs, and grasses. Common native shrub species in hedgerows include hawthorn, blackthorn, hazel, and dog rose.
Birds also use hedgerows for safe feeding, nesting, shelter and corridors. Bees and butterflies feed on hedgerow flowers, so they provide excellent support for pollinators. The Wildlife Trusts note that: “Thick, flora-rich hedges provide valuable nesting and foraging opportunities for a huge range of wildlife.” And hedgerows give benefits to even small mammals and bats.
In addition to all of this, hedgerows act as vital wildlife corridors, connecting isolated habitats such as woods, ponds, and meadows. The connectivity reduces habitat fragmentation and allows healthy gene flow between populations.
5. Hedgerow only can be replaced by hedgerow
Leading on from the above point, you should be aware that you can only turn hedgerow into hedgerow under BNG rules. Hedgerows are valued for their unique contribution to biodiversity, and their loss must be offset by providing a comparable or greater hedgerow value in return. This applies to many habitat types, not just hedgerows. In many cases you cannot simply substitute one habitat type, such as hedgerow, for a different habitat type to achieve your net gain, because the BNG metric treats hedgerows as a distinct category and rewards their specific ecological functions. If your development impacts hedgerows, you must create or improve hedgerows to achieve the required net gain, rather than providing a different habitat like a tree or a field. However, this doesn’t necessarily take a lot of time with expert advice and guidance.
6. Standalone trees are assessed individually and generate their own units.
When it comes to trees, how they are categorised makes a big difference to your biodiversity unit calculations and this can be used to maximise your BNG outcome.
If a tree stands alone or in a small group that doesn’t qualify as woodland or part of a hedgerow, each one is treated as an individual habitat feature in the BNG metric. That means that each tree has its own row in the BNG spreadsheet (Defra Metric) and each one is assessed separately for condition, distinctiveness, and strategic significance. Each tree generates its own biodiversity units. This can be beneficial to your project because every single tree counts toward your baseline and potential gain.
In short, every individual tree that is not in hedgerows or woodland is assessed separately – each one its own row in the metric. Each merits its own assessment and needs to be looked at separately to others, which amounts to a significant gain if they are all counted individually. Trees are a great way to increase BNG post-development.
7. Veteran trees, an irreplaceable habitat that you can’t offset, don’t even have to be old or ancient
You should know that “veteran” doesn’t necessarily refer to the age of a tree.
A veteran tree is not defined strictly by age, but by its physical characteristics, habitat value, and ecological importance. If a tree that shows features of maturity and long ecological continuity, such as hollowing of the trunk or branches, decay holes, dead wood or rot pockets, crevices in the bark or roots, it may be considered a veteran tree. Further features may include fungal fruiting bodies or the presence of specialist wildlife such as lichens, bats, invertebrates, or fungi.
In contrast, an ancient tree is one that is exceptionally old for its species, usually both large and ecologically significant. Both ancient and veteran trees are classed as irreplaceable habitats.
Ancient trees, according to The Royal Forestry Society, have one or more of the following criteria:
- Biological, aesthetic or cultural interest due to its old age
- A growth rate that is ancient or post-mature
- Chronological age that is old in relation to surrounding trees of the same species
Under the Environment Act 2021 and the Statutory Biodiversity Metric (Defra Metric 4.0 and later versions), veteran and ancient trees are recognised as irreplaceable habitats, meaning they cannot be measured within standard Biodiversity Net Gain (BNG) unit calculations. The BNG metric explicitly excludes irreplaceable habitats from the biodiversity unit process, and the loss of a veteran tree cannot be offset by creating or enhancing other habitats.
BNG rules require avoidance of loss or damage to these trees as the primary approach and replacement planting or off-site unit generation is not an appropriate mitigation because the ecological and cultural value of veteran trees cannot realistically be replaced.
Development should avoid direct or indirect impacts on veteran or ancient trees and their root protection areas. A root protection area is usually a calculated area of ground that lies immediately under a tree and just beyond the extent of its crown, intended to help avoid damage to the tree’s rooting system.
Local planning authorities are likely to refuse applications where such loss or harm would occur unless wholly exceptional reasons apply and a suitable compensation strategy for other habitats is provided. (gov.uk)
8. Irreplaceable habitats are exempt from standard BNG process.
As touched on above, irreplaceable habitats are exempt from standard BNG process – any impacts to these habitats will be flagged as unacceptable in the metric and they will require bespoke compensation to be agreed with the relevant Local Planning Authority (LPA).
Irreplaceable habitats in the context of BNG include (but are not limited to):
- Ancient woodlands,
- Ancient and veteran trees,
- Blanket bog,
- Limestone pavements,
- Coastal sand dunes,
- Spartina saltmarsh swards (coastlines of southern and western Britain, particularly in the estuaries of the Solent, Poole Harbour, and the Essex coasts)
- Mediterranean saltmarsh scrub (found in the UK primarily along the North Norfolk Coast),
- Lowland fens.
These habitats are exempt from the standard 10% BNG requirement because they are immensely difficult or nearly impossible to recreate once lost and are a crucial part of our natural heritage. Developers can only get planning permission for development resulting in their loss in exceptional circumstances.
We discuss irreplacable habitats in more detail here: What are the irreplaceable habitats under BNG?
And a full list of the habitats can be downloaded here: List of irreplaceable habitats
9. Open Mosaic Habitat on Previously Developed Land is highly difficult to recreate
This is a common urban habitat that is listed as a Habitat of Principal Importance (HPI) under the NERC Act, and can occur when a development site has been left over time between developments, for example, or where previous buildings have been demolished or left abandoned before the site is redeveloped.
Any removal of this habitat type requires compensation via ‘like for like’ replacement, i.e. creation of new ‘Open mosaic habitats on previously developed land’. But, this is nearly impossible to do due to the nature of the habitat, and the issue therefore becomes very expensive to solve. The main lesson here is to avoid leaving sites long term in this kind of state.
How can Cura Terrae help you achieve BNG targets?
- Our teams guide clients through every phase of BNG, from initial surveys to the long-term stewardship of habitats to achieve BNG targets.
- Plan and achieve a quantifiable net gain in biodiversity with our teams through the Statutory Biodiversity Metric. We provide integrated Biodiversity Net Gain Services, from baseline assessments to creation and maintenance of habitats, and advice and recommendations, delivered by seasoned, personable experts.
- Our team of qualified ecologists skilfully conduct your BNG baseline habitat type and condition surveys, as well as River Condition Assessments in line with the latest guidance. Using cutting-edge tools like Coreo and QGIS, we accurately map baseline habitats and export the data into the Statutory Biodiversity Metric.
- Receive ongoing support from preliminary design to post-development stages, ensuring habitats are mapped and recorded to BNG standards. Our experts conduct options appraisals and incorporate BNG into project designs, enabling clients to achieve their net gain. We offer off-site compensation surveys, liaise with local planning authority (LPA) ecologists, and work closely with our Landscape and Habitats teams to deliver habitat creation, enhancement, and maintenance.
See our results for clients in detail here or get in touch with us with any queries.
