Mitigation Hierarchy: Explanation & Stages

The Mitigation Hierarchy (MH) shapes many of the policies, frameworks, legislations and guidance that we follow today. It’s a way of understanding how to best mitigate harm to the environment, which is more and more important as our biodiversity crisis worsens. 

The principle ensures that all potential avenues have been explored to prevent damage to our degraded environment, either to achieve NNL (No Net Loss) or a NPI (Net Positive Impact).

This blog post explains the Mitigation Hierarchy in great detail, including the 4 steps of the concept, key information, and key relevant policies shaped by it such as Biodiversity Net Gain (BNG).

What is the Mitigation Hierarchy?

The Mitigation Hierarchy is a conceptual framework and guiding principle that is central to many policies and practices that challenge the UK’s biodiversity crisis. It’s commonly applied in related fields like natural resource management, infrastructure development and urban planning.

Mitigation delineates actions taken to reduce the severity, seriousness or harmfulness of something, and subsequently, the Mitigation Hierarchy guides practices to avoid environmental harm in development, then to minimise, then to restore, or lastly to offset any impacts.

It originated within the field of Environmental Impact Assessment (EIA) and became widely used in the late 20th century as ongoing mass development heightened biodiversity concerns. It’s now central to a range of policies, such as Sustainable Development Goals (SDGs) around sustainability and nature and the National Planning Policy Framework (NPPF), a framework that local planning authorities follow when reviewing planning applications.

The concept has been integrated into the lives and work of many in 2024, with new Biodiversity Net Gain rules only allowing major and smaller developments to go ahead if they can over-compensate any harm to the environment to a degree of 10%.

How the Mitigation Hierarchy works

The Mitigation Hierarchy works by following a simple order so that environmental harm is mitigated in the most effective possible way. It either works as steps for a project to follow, to achieve maximum mitigation of harm to nature through 4 key stages, but also creates fall-back options for effective nature protection when the best options are not feasible. 

MH is widely used in the UK, as there have been significant declines in our biodiversity. The UK is significantly smaller in land area than countries like France, Italy, or Germany, but is one of the most populated countries in Europe. This is due to intensive urbanisation, consumerization and overconsumption, resource extraction, agriculture and industrialisation. For these practices to continue for the sake of our economy, the Mitigation Hierarchy guidelines must be followed.

A lake with mountains in the background

Mitigation Hierarchy is considered at the early stages of the planning and design of projects, where potential harms are measured and the process begins to stop them. Regulatory agencies, such as the Environment Agency, may review policies and make sure they are well in line with the framework.

For instance if a new development will destroy a habitat, it is nonsensical to fund off-shore wind projects to compensate for this harm, if the development’s plans could have been amended to avoid harm to the habitat in the first place.

4 stages of the Mitigation Hierarchy

1. Avoid

Avoidance is the first and most beneficial option for us and nature, so it stands at the very top of the hierarchy and must first be exhausted before considering the following options. 

Beyond our biodiversity crisis, development can impact the environment in many ways, such as water pollution from urban runoff, air pollution from emissions or soil degradation from intensive land conversion. The potential harms that have been identified might be avoided by conducting the planning stage through the lens of the Mitigation Hierarchy. 

An already degraded development location could be chosen, or one that doesn’t host sensitive ecosystems. Operations could be postponed in line with sensitive wildlife periods like migration or breeding. Projects should strictly comply with environmental laws, whereby pre-development ecological surveys could be conducted, and non-invasive construction techniques could be adopted. These are examples of ways to curb and directly avoid harm.

When the avoidance stage of the MH is effectively implemented, the developer reduces the need for mitigation measures and extra fees down the line. Avoidance alleviates the risk of irreversible losses of biodiversity and natural resources, enabling a sustainable model of development.

2. Minimise

Curbing environmental harm is ideal. When avoided, the process has been amended to not initiate any processes that will lead to ecological disruption. This, however, is not always possible—in which case we fall to the second rung of the hierarchy and must attempt to minimise the harm wherever possible. 

We must then reduce the severity of the impacts through refinement and management of the project’s techniques and operational aspects. Instead of planning pre-development to avoid harm, minimising harm is usually maintained during the design and operational phases. 

Examples of this can be low-noise equipment to decrease noise disturbance, water conservation measures, phased construction to limit disturbance for a lengthy period, speed limits or habitat barriers to shelter sensitive ecosystems. 

These methods encourage continuous improvement, with close monitoring and regular adaptation to measures, utilising environmental data to keep track of impact reduction and ensure targets are being hit.

While this may sound exhaustive, advanced technologies and refined procedures are in place to do this exactly, making it easier to lessen environmental harm and enact development in line with the Mitigation Hierarchy. 

3. Rehabilitation/Restoration

The Mitigation Hierarchy’s structure is in place to accommodate projects and developments of varying complexity and feasibility. 

Restoration intends to revert the damaged ecosystem to its natural state and rehabilitation restores key ecosystem services such as the stabilisation of soil. This stage includes steps to reconstruct, repair and enhance any ecosystems damaged by the development; with tangible and active intervention to ensure habitat restoration is wholly met. 

Examples of this are native revegetation or the planting of shrubs and trees to create new habitats, removal of invasive species like the harmful Japanese knotweed or grey squirrels, and cleaning up pollution like agricultural runoff in water bodies. Artificial habitats, like insect hotels, areas for bird nests and bat boxes, encourage a diverse ecosystem and resilient structures of nature. 

This means that biodiversity may be lost, but through meticulous measuring and environmental data collection it is ensured that it is made up for, and the local ecosystems and habitats have been strengthened by the addition of the development not deteriorated.

4. Offset or Compensate

To offset or compensate for harm is the last resort option. It’s for when practicality obstructs adequate measures of avoidance, minimisation or restoration.

For example a project like a highway, airport or dam, which undoubtedly will cause mass environmental harm to an area. Urban development may include green infrastructure, but usually transforms land into built environments and fundamentally converts the natural landscape. Further, resource extractions like oil, quarrying and mining have permanent substantial harm to the environment, as well as intensive agriculture, leading to hefty losses of biodiversity. 

To offset or compensate is to ensure a development project’s residual impact is balanced with external improvements to the environment. A prolific example of this is off-site BNG units, where the development site’s baseline study predicts the extent of impacts can not be made up for on-site, so the developer must source and purchase off-site BNG units in order to meet the mandated gain of 10%.

The irreversible impact on biological diversity and ecosystems must be made up. A great method is habitat construction, such as mass-scale tree planting to create forests; fantastic habitat providers and devout carbon sinks, or the creation of wetlands, by utilising water retention systems and planting aquatic wildlife to create flourishing marine habitats. 

Biodiversity conservation projects can compensate for harmed nature, or habitat enhancement of those partially degraded, as well as putting funding into protected areas to support ecological resilience.

The Mitigation Hierarchy is not an officialised law or legal requirement itself. It is a set of respected principles that have proven themselves effective in mitigating environmental harm via sustainable development practices. It ensures that all risks and harms have been identified, and all avenues have been considered to result in the least loss of biodiversity possible.

It is a governing principle that guides many other frameworks, policies and legislations, especially in the UK, such as major developments having to undergo an Environmental Impact Assessment (EIA) before any tangible progress can be made. Through thorough analysis, the EIA can reveal harms and threats a project will have on the environment, and halt its development or call for adequate amendments.

Purple flowers

After the initial assessment, the stages of avoidance, minimisation, restoration and offsetting can be explored to identify the best avenues the project may take to allow it to progress. Without clear routes for mitigating harm, regulatory bodies will not pass approval.

Beyond policies shaped by the framework, many industries and sectors will adopt the Mitigation Hierarchy voluntarily. It is a massive aid in corporate social responsibility and helps maintain a reputable public image, encouraging beneficial partnerships with stakeholders, environmental groups and investors, and harnessing overall growth.

The Mitigation Hierarchy in Biodiversity Net Gain

Biodiversity Net Gain (BNG) follows the Mitigation Hierarchy in an orderly fashion. In January 2024 the new law was officialised for all major developments, and in April 2024 it also became mandatory for small sites. This means that the large majority of developments in England will fail to obtain planning permission if they do not meet this new law.

The law states that the development must end in a 10% gain of biodiversity to the natural environment, including biodiversity lost in the process of development. Habitats and ecosystems must not only be repaired but strengthened and enhanced. This gives us a great shot at repairing our crisis of biodiversity, and large-scale habitat loss across the country.

A chosen development site begins with avoidance through a baseline assessment, assessing the site’s original ecological state in great detail, its potential for biodiversity gain, any specific habitats, species, and crucially how much biodiversity will be lost from the specific development planned. This is done using the latest Biodiversity Metric; the 4.0 as of May 2024. 

This discovers the potential routes to shape and mould the development’s plans and uncover any changes that can be made to avoid avenues of environmental harm. The harm will be minimised as much as possible.

Biodiversity Net Gain ensures that habitats will be restored and repaired, either through on-site BNG or off-site BNG. On-site BNG means that harm at the site of development will be thoroughly restored and repaired over a long-term 30 year period. Offsite BNG is when there are limitations preventing on-site BNG, so the developer has to source and purchase off-site BNG units.

These units should be as close to the development site as possible, to restore local habitats, and will follow a regimented plan over 30 years that will end in a gain of biodiversity resulting in the project’s overall environmental contribution as a 10% positive gain.

No Net Loss (NNL)

Like MH, No Net Loss (NNL) is a guiding principle that prioritises biodiversity and the function of ecosystems. It is simpler to achieve than NPI (Net Positive Impact) in the sense that no harm must be done to the environment, but it doesn’t have to be improved either.

The Mitigation Hierarchy aids NNL implementation by avoiding impacts, minimising harm, restoring harmed areas, and producing offsets in compensation when necessary.

Net Positive Impact (NPI)

One step further, NPI (Net Positive Impact) ensures an increase in biodiversity is achieved and ecosystem health is prioritised. Through structured systems, the ecological conditions will be improved, even when it has been harmed throughout the process.

When looking to have a strong sustainable image and corporate responsibility, NPI is a fantastic route to demonstrate commitment and forward-thinking.


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