Trees in Biodiversity Net Gain

Connection Between BNG and Trees

In modern UK planning, instead of just having landscape features or constraints to design around, there are also legally protected and quantifiable assets.

Under the UK government framework, most planning applications must show measurable improvements to the natural environment before being able to receive planning permission.

For most developments including commercial, residential and even qualifying small sites, it means that achieving a 10% uplift in the standard of the environment is a strict legal requirement, as part of biodiversity net gain (BNG).

Every single tree on your development site carries a specific biodiversity value within the statutory biodiversity metric. Whether you retain or remove them will single-handedly dictate your baseline calculation, your post-development BNG obligations, and your overall budget.

More than anything, understanding the role trees found on your site play in the planning process is essential to avoiding significant issues and costly delays.

Working Trees into the Biodiversity Metric Calculation

The statutory framework treats individual trees differently from full woodlands and other habitats.

Instead of measuring them strictly by land area, the process of recording individual trees involves submitting physical measurements into the DEFRA Tree Helper tool. It calculates a standardised area equivalent in hectares to generate baseline units.

A tree is categorised based on the stem diameter at breast height (DBH) and the canopy radius:

The Line of Trees Exception

If your site features a continuous row of trees with overlapping canopies, the biodiversity net gain (BNG) framework calculates them as a linear feature rather than isolated individual trees.

Doing this alters their unit output and must be mapped precisely during your baseline assessment to avoid data errors.

Three Pillars of Tree Unit Valuation

An ecologist or arboriculturist determines exactly how many units a tree is worth by running it through the BNG calculation process based on three distinct statutory variables:

Pillar 1: Distinctiveness

The metric rewards ecological complexity based on the specific habitat type.

Native species (English oak, field maple or silver birch, for example) automatically carry a higher distinctiveness score and yield significantly more units than non-native, invasive or ornamental species (like sycamore or leylandii) that offer less value to local biodiversity.

Pillar 2: Condition Assessment

Every tree is graded as ‘Poor’, ‘Moderate’ or ‘Good’ by running it through Natural England’s strict six-point criteria. Higher grades then heavily multiply the tree’s unit value.

The assessment looks for continuous healthy canopy cover, an absence of severe human damage, and the presence of natural microhabitats like deadwood or cavities.

Pillar 3: Strategic Significance

Location heavily influences the unit value.

If your site’s trees sit within a zone targeted for ecological enhancement by the local nature recovery strategy (LNRS), the unit value receives an automatic multiplier boost, helping you to meet the mandatory BNG requirements more efficiently.

The Cost of Tree Loss

Axing mature trees on a development site is one of the fastest ways to break a planning project’s budget.

The BNG rules are explicitly designed to disincentivise tree removal through heavy mathematical penalties, framing clear-cutting as a primary driver of local biodiversity loss.

Ancient and Veteran Trees

Often known as the ultimate red line in the planning system.

Much like ancient woodland, ancient and veteran trees are classed as an irreplaceable habitat.

If your new development design impacts an irreplaceable tree, you cannot use the standard 10% metric calculation.

Your planning project automatically bypasses the standard BNG process and triggers strict legal compensation strategies with the local planning authorities.

Apart from in exceptional circumstances, impacting the trees will result in severe design delays or outright planning refusals.

Private Garden Caveat

If you’re developing a residential scheme, trees planted or retained within proposed private gardens are generally excluded from your post-development BNG scoring.

The exception is that any tree with a DBH of 30 centimeres of greater (‘Medium’ to ‘Very Large’) must be accounted for in the baseline, even if it ends up inside a private plot boundary.

Temporal Multiplier Penalty

Due to freshly planted trees taking decades to replicate the ecological benefits of a mature tree, the metric heavily devalues new canopy growth.

With the time-lag penalty in place, removing just one mature, healthy native tree can require you to plant between 10 and 30 new trees on-site just to break even on your baseline.

Maximising Biodiversity Units for Trees

While trees present risks, smart architectural design can leverage them to achieve your net gain target much faster and much cheaper on-site, making them a crucial component of the site’s green infrastructure.

Double-Counting Canopy Benefit

As tree canopies sit above the ground, the metric classifies them as non-ground-based habitats.

It means that you can score the habitat on the ground (modified grassland, rain gardens or structural paving, for example), and score the tree canopy sitting directly above it at the same time.

With this, you will be able to effectively squeeze double the biodiversity value out of a single square metre of urban land.

Urban Street Trees as Unit Engines

If you are working with a tight footprint in urban environments where space for large-scale habitat creation is limited, integrating structural, high-distinctiveness urban street trees into your highways layout is one way to bridge a unit deficit.

Combined with other features like green roofs on nearby structures, they turn necessary hardstanding corridors into high-yielding BNG assets in crowded urban areas.

Process for Syncing BNG and Trees

The most efficient way to navigate the process is to refer to the expert guidance of an ecological and arboricultural team such as ours from day one, where we will follow the structure below:

1. Data Harmonisation

Including stem diameters, canopy spreads and retention categories, the standard data from a BS5837 tree survey is captured on the development site.

The data seamlessly populates the BNG Tree Helper tool, creating your accurate baseline without needing duplicate site visits.

2. Apply the Mitigation Hierarchy

Your design team uses the combined tree and ecology data to avoid harming high-value Category A and B trees.

In any areas where encroachment is tight, we aim to minimise impacts using specialised engineering, such as no-dig cell-web foundation systems within root protection areas (RPAs).

3. On-Site Enhancement

Instead of entering the market to buy credits, we look at enhancing what you already have.

Through targeted arboricultural management plans, we can upgrade an existing tree’s condition from ‘Poor’ to ‘Moderate’, instantly generating free on-site biodiversity units.

4. Secure the Deficit

If tree removal is entirely unavoidable and creates a unit deficit that cannot be resolved within your red-line boundary, you must look to the final tier of the hierarchy:

Purchasing off-site units from a validated habitat bank or choosing to buy credits via statutory BNG credits as an expensive last resort option.

Long-Term Liabilities

Getting your application for planning consent granted is only the first step in the process.

Whether it’s been retained or newly planted, any tree asset that contributes to your mandatory net gain of biodiversity comes with long-term legal strings attached.

Integration with the HMMP

According to the DEFRA user guide, trees need to be legally tied to a comprehensive Habitat Management and Monitoring Plan (HMMP) submitted to the local council.

The 30-Year Replacement Mandate

Secured via legal agreement or planning condition, any tree tied to your BNG commitment must be actively managed for a minimum of 30 years,

If a tree dies, fails or is severely damaged at any point within this mandatory lifecycle, the landowner is legally obligated to replace it with an equivalent specimen as a way of remaining compliant.

Local Council Auditing

Usually working in partnership with conservation charities and environmental bodies, the local planning authorities hold the power to audit site tree canopies and overall health throughout the 30-year window.

Failing an audit can result in code breaches, enforcement action and significant retrospective financial penalties.

Assistance with Trees and Biodiversity Net Gain

With the overlap between biodiversity net gain (BNG) and arboriculture, working with the statutory biodiversity metric correctly involves the guidance and instruction of experts.

Biodiversity unit values are dictated by tree constraints, and due to this, having two separate consultancies in charge of your tree survey and biodiversity net gain assessment can cause a lot of unnecessary risk and confusion.

Instead, it would be advisable to refer to a consultancy that specialises in both, like we do at Arbtech. Our dual-discipline means that our arboriculturists and ecologists can work together when it comes to solving any issues with delivering BNG in conjunction with trees on your site.

From a small sites project to a far larger new development, our focus is on protecting each irreplaceable habitat you already have before moving on to minimising biodiversity loss and producing ways of sourcing more biodiversity units.

After recording individual trees and establishing each tree’s value, we can fully understand your BNG requirements.

Contact Our Team for Professional Advice

Between an early BS5837 tree survey and the entire biodiversity net gain process, and even long-term management and monitoring, our team can do it all.

Get in touch with our team today on our website, via email or over the phone, explain your situation, and we can offer you a free quote based on your site and project.