Guest blog – Biodiversity Net Gain: for good or ill, by Dominic Woodfield

Dominic Woodfield is the Managing Director of Bioscan, a long established and well-respected consultancy specialising in applied ecology.

He is a life-long birder, a specialist in botany, habitat restoration and creation and in protected fauna including bats, herpetofauna and other species. He is also a highly experienced practitioner in Environmental Impact Assessment and Habitats Regulations Assessment. Most of his work is for the development sector, but he has also undertaken commissions for Natural England, the RSPB, Wildlife Trusts and campaign groups. He once mounted an independent legal challenge in defence of an important site for butterflies in Bicester, Oxfordshire, which resulted in planning permission for a five-hundred unit housing development being overturned. He lives in Oxford with his partner and family.

After around a decade in gestation, and with some significant hiccups and bumps in the road even since the primary legislation that forms its foundation stone was passed in 2021, Biodiversity Net Gain (“BNG”) is, as of 12th February 2024, finally and fully ‘here’.

For those still not quite sure what it is, BNG is the Government’s answer to making development (defined for this purpose as a land-use change requiring formal planning consent) play its part in arresting biodiversity decline. More broadly, it is one of the Government’s various mechanisms intended to achieve ‘sustainable development’ – i.e. ensuring the natural environment is left in a better place for future generations than ours has found it.

BNG works by imposing a standing (mandatory) requirement for all developments (with a few defined exceptions) to not just fully compensate for habitat losses (a principle that has arguably been enshrined in national planning policy since 2012) but to go further and deliver a 10% overall uplift in biodiversity value. Inevitably, a precise figure such as 10% requires that the biodiversity value of an area of land before and after development be quantified and counted, and this is where BNG begins to polarise opinion. How can you quantify biodiversity value? Should you even try? Is it morally bankrupt to reduce our stewardship of the natural world to numbers on a balance sheet? Do we diminish nature, and ourselves, by so doing? Those are valid and important questions, but they have not stopped BNG becoming a mainstream part of development management and policy.

The way the statutory BNG system measures biodiversity loss and gain is by a collection of simple (even simplistic) arithmetic formulae collectively known as ‘The Biodiversity Metric’ (see here). Now on its umpteenth iteration, the Metric remains essentially a system based around attributing numerical values to intrinsic habitat quality (a function of factors such as rarity and fragility – here termed ‘distinctiveness’) qualified by the more obviously self-explanatory variable of ‘condition’. Thus, a high value habitat such as lowland heathland will be high scoring on ‘distinctiveness’ but its total score potentially suppressed if it is neglected and in ‘poor’ condition. Getting it into ‘good’ condition provides an enhancement opportunity that can be quantified, and that quantity then monetised as credits or units which can be bought by a developer who cannot achieve net gain on their development site.

Although called the ‘Biodiversity’ metric, the system only deals with habitats. It pays no heed to individual flora or fauna (at least terrestrial fauna, as opposed to biota that are prescriptive of certain intertidal habitat types). Habitat quality and condition is instead used as a proxy for biodiversity value generally. The potential pitfalls of this approach are not hard to spot – take species-poor grasslands or beet fields essential for the survival of wintering pink-footed geese or whooper swans for example, or a scrappy strip of bramble like any other, but which is home to dormice. The rationale for this simplistic approach, at least originally, was a sense that for the system to work and be adopted, it must be easy to use and accessible – including by non-experts. Those faced with the current spreadsheet version of the Metric calculator tool for the first time may wonder whether it has fallen between two stools in this respect.

Having now become unavoidable, BNG is exercising the minds of developers, local authorities and landowners throughout England (there’s no mandatory equivalent in Wales and Scotland as yet). A nascent market bubble is slowly ballooning, as developers realise that the Metric may not allow them to get away with the traditional approach of a few trees and a balancing pond at the edge of their housing or warehousing development, but requires something more. Local Authorities are increasingly facing requests from developers to offer up or suggest places where such deficits can be met, and they are also waking up to the fact that they are not exempt themselves and that sites on their own land portfolios are also bitten by the 10% gain requirement.  At the same time, farmers and other landowners are realising that BNG represents a new and potentially lucrative source of income for land of marginal agricultural value (and even of land that is suitable and profitable for food-growing: another source for potential controversy around food security concerns).

The transformation of BNG from concept to mandatory reality has sparked renewed debate about the rights and wrongs of using a simplified metric system to measure biodiversity gain and loss, about whether BNG provides a ‘licence to trash’ high value habitats, about how easily the system can be gamed or fiddled, and around whether it has any genuine prospect of slowing the seemingly inexorable cycle of UK biodiversity decline. I have weighed into this debate on occasion over the last ten years or so, and have not been slow to criticise the Metric-based approach, nor the motives of some of those who have most enthusiastically promoted it (Woodfield 2013, Woodfield 2018). However, in 2022 I agreed to put on a second hat alongside my consultant ecologist’s one, and became a non-executive Director of a company specifically set up to service those needing BNG solutions. Why?

I could rationalise this apparent volte face with a treatise along the lines of ‘it’s here, I might as well try and make sure it’s as good as it can be’. But I am not writing this piece to seek redemption or acceptance from peers. This is not a holy confession. It is, rather, a call to all involved in land-use and land-use planning to grasp the opportunity that BNG – for all its flaws – genuinely offers to do things better.

Much of my public and professional ire about the system in the past has been due to the ease with which I have seen it abused by unscrupulous developers and consultants, and the failure of such abuse to be spotted, arrested and called-out by those in a position to do so. Quite simply, BNG will fail if that is allowed to continue and to become the norm. There are already grounds for concern that this is the way things are slipping. With my ecological consultancy hat back on, I am increasingly being called upon to independently review other consultants’ BNG assessments and the calculations that flow from them. I would estimate the proportion of assessments that I would class as accurate, broadly accurate or adequately representative to be less than 10%, perhaps less than 5%. The errors I find are not trivial – they have real-world consequences for biodiversity. It is a notable feature of my experiences that it is almost unknown for an error to be in the favour of biodiversity. In practically every case the error is an under-estimate of a development site’s value – sometimes a gross under-estimate – or a wildly optimistic and at times scientifically nonsensical or even risible positive assessment of the success of compensatory habitat creation. Very often, it is both.

But, to return to my main theme here, this is not (entirely) the fault of the Metric, as lumpen and flawed a tool as it is, nor is it the fault of the principles and objectives behind it. BNG can still be a force for good – a step change to deliver better outcomes from development, but it cannot be stressed enough that for it to realise its latent potential will require vigilance, professionalism and close scrutiny. So, what can those who deal with BNG, whether daily or occasionally, do to ensure it becomes a force for good, not ill? Well, we might start with the following:

  1. LPAs need to be sufficiently resourced (with appropriately trained and experienced ecological staff) and empowered to conduct the independent reviews necessary to discourage ‘gaming’ of the system, or – worse – even outright figure fiddling. This means staff. This means money. If this is not going to come from Central Government any time soon, I suggest a % levy on application fees should be explored.
  1. It is important, even essential, that those completing BNG Metrics for developers, landowners or LPAs have been to the site, and are sufficiently competent to be able to classify and assess the baseline condition of the relevant habitats to a high degree of accuracy. In a recent major infrastructure case I was involved in scrutinising, it was clear that the Metric calculation had been put together by desk-based minions with scant ecological knowledge attempting to interpret field data collected by a party that had since departed the organisation.
  1. The provisions within the Metric guidance put in to prevent sites being trashed – subtly or more overtly – in order to deliver a lower baseline need to be a) enforceable and b) enforced.
  1. I and many others are of the opinion that the Metric needs significant revision to encapsulate more realistic timescales for the delivery of certain high value habitats, such as good condition Lowland Meadow. The defaults given – as little as 15 years – are entirely at odds with the weight of scientific research in this area.
  1. On a related point, any habitat creation and enhancement objectives that stray beyond ‘high ambition’ and into the realms of fantasy – in order to deliver a positive score – need to be questioned and where appropriate tempered through challenge. It’s quite simple really – if there are no clearly documented and scientifically robust precedents for habitat X being delivered to condition Y within timescale Z then it shouldn’t be taken as something that’s going to happen, and should carry no weight in planning judgments. You are simply not going to get a species-rich semi-natural grassland on improved soils in the space of a few years by chucking on a proprietary seed mix and rewarding people for making throwaway comments to that effect has to stop.
  1. In due course, I believe the Metric really has to be revised to take into some sort of account species interest. At the moment this is a major hole in its ability to deliver net gain even where the will is there. A simple multiplier to account for an ostensibly low value habitat supporting an important species – something that happens surprisingly often – will do much to combat the current very high risk of on-paper Metric outputs masking what will in reality prove to be significant biodiversity loss.

There you go – a six-point plan for making the best of what the Biodiversity Metric and mandatory 10% Biodiversity Net Gain presents us with as an opportunity. Six points that are merely a starter for ten – there’s bound to be lots of others, even before we get into the debates around BNG as a driver for taking land out of agricultural production and risking our food security, or the ethical reductionism of giving wildlife a ‘score’. But BNG is here and here now, for better or worse, and in the absence of anything better, who wouldn’t want it to work?



Woodfield, D.M.G (2013) Biodiversity offsetting – setting off on the wrong foot? British Wildlife 25.1 October 2013.

Woodfield D.M.G (2018) ‘Biodiversity Accounting’ – a tool for transparency or for dumbing down? British Wildlife 30.2 December 2018.


8 Replies to “Guest blog – Biodiversity Net Gain: for good or ill, by Dominic Woodfield”

  1. I would endorse everything that Dominic says here and have to say his journey to the dark side is full of risk. One good example of the apparent BNG abuses relates to the recently approved Sizewell nuclear power plant plans on the Suffolk coast, fast tracked in response to energy security worries as the war in Ukraine began. And despite legal challenge over several other major issues such as an eroding coastline and lack of freshwater supply. The case features a series of large, ugly, out of character car parks and main roads between the A12 and the current Sizewell B plant. These will literally divide the Area of Outstanding beauty in half from the skylark filled fields inland to the special wetlands, marsh and sandy land habitats that form designated interest for which this area, next door to RSPB Minsmere, is famous. Has an AONB been split in half before with a brand-new road? Astonishingly, EDF proudly proclaim a net gain benefit of 19% but any careful evaluation of the kind Dominic describes show this to be a paper tiger and any gain at all seems unlikely wit a net loss more than possible. Success or failure has therefore become an argument not a checkable fact because so much, hangs on the aptitude of those responsible for the detailed advice and work over the next twenty years. The immediate losses stack up and how long will it take for any benefit to be made, if it even happens? Anyone’s guess. Is there a generous contingency fund in case things go wrong due to unforeseen events including the project ebign cancelled half-done?

    Early on we objected to the so-called ‘compensation’ act prior to permission being given for habitat creation at a place called Aldhurst farm, a generally useful bit of habitat reversion but using dirty water source of the local town of Leiston and heathland regeneration done on the cheap. This involved digging out peat from the river valley and spreading it on the battered agricultural soils but without prior nutrient stripping over 5 years or so. The result not surprisingly is rich soil with scrub trying to establish, now requiring costly management with autumn failing to make it look ok but not the target habitat. No doubt scrub and slightly polluted reed bed is of some value to bitterns and marsh harriers (try to keep the bird people sweet) but how does it relate to the habitats being lost with their aquatic invertebrate interest – not too much, partly because they weren’t studied in any special detail to see exactly what will be lost.

    The lost wet woodland and other delicate micro habitats from just one of the SSSI/s involved are being parcelled off to floodplains around the county, none of them contiguous with the general areas and each of the three being within metres of sewage treatment plants and we all know what that means. Floating brown objects were seen not that long ago at one of them and they weren’t water voles. Equally marshland to be dug up to try to make pools are already used by birds like barn owls to hunt, so knock on habitat loss effects continue. Designated sites with similar interest to those being lost that could/should be regenerated given a bit of blue sky thinking don’t seem to be part of the plan. IUCN guidelines on translocation may risk being broken too.

    With Aldhurst Farm, during examination so called offsite compensation was often linked to this area. Skylarks to be lost from road building would resettle there for example. However, with one area scrubbing up and patrolled by cats and foxes form the local urban edge there seems little chance and on the quieter side the land has been designated as a dog walking area -so a massive hostage given the unpoliced poaching and other activities rife in the area that will soon swell with thousands of ‘blokes’ brought in as labourers to a massive temporary dormitory complex (think Hinkley point), to this remote location. What else will they be doing?

    The roads have modest speed limits yet no specialised fencing and underpasses to mitigate the enormous severance effects roads bring. The area is renowned for reptiles and bats and in an ugly skirmish early on land was cleared inside Sizewell B in secret with Suffolk police backing EDF’s approach to reptile mitigation. Which left us all breathless with the set of rules currently being allowed, that cut across printed guidance. A single probable bat roost tree was left exposed overwinter to freezing coastal winds, with its surrounding woodland removed, like the lonesome pine. The large area of heathland restoration like Aldhurst farm was not nutrient stripped, (saving around £5 million) and with ditches dug and filled with hay (for the local rats?) and many loose woodpiles with eroding soil in them, made that no respectable lizard or snake would bask on were soon looking hopeless and in need or re-doing. The bleak landscape will be feeding grounds for birds of prey picking off animals fleeing from the development areas, but what else? Are specialist teams doing careful monitoring or will it be a token gesture? Where is the publicly available overview that is not just a short greenwash?

    This is paperwork spreadsheet biodiversity, and the reality is that land is being cleared with the expectation that snakes and lizards and everything else will naturally migrate away from their diggers and setup in the new habitats. But the new habitats are homes to nutrient loving species, they can’t deliver the promise. Much as the pilot projects done since 2015 have proved – so where is the excuse? Even if the promised dedicated Trust was given an extra annual budget of £4 million (with probably more needed in early years) to manage, monitor and police it with the intensity it is going to need. What is the bet it will be stocked with livestock to graze it and we end up with an agri-mess. Everything is secretive and local stakeholders excluded bar the RSPB And Wildlife Trust who must cooperate to keep existing agreements and obligations in place, and mind how they go.

    So here is a classic example on a massive scale of where the new system fails. Fails yet has a happy face, saying it is working/will work at some time in the future. When those in charge now no doubt will have passed it on to someone else, in the manner of the promises made with Sizewell B. And what was Natural England’s part? Well, they did stand up quite well until the inquiry and then backed down as they must, piece by piece. But they have now binned their national Standard examining uncertainty in nature decisions in favour of evidence reviews. Looking forward to seeing those.

    The issue with Net Gain and DLL is all about policing. In fact, it was introduced because mitigation efforts under previous legislation needed to be better. They needed the hand of guidance and authority that English Nature were never likely to be allowed to give, and better policing and enforcement. And we see where that has gone too with the wild wildlife crime enforcement fiasco in play. CIEEM can try and train and give guidance, but this is too big for them. The facts are that old mechanisms could have worked given a proper steer from proper ecologists acting for government and with enforcement powers. Instead, it is a move death by form and spreadsheet and developer experts have the upper hand to please their clients. Meanwhile small household developers are being gouged to prop up a deficit in wider countryside resourcing. Computer says ‘Yes’ so, it’s alright. Complex systems have been developed as a fix that can’t work because developers and vested interests can afford to bend and steer to their advantage. The fake outrage from senior politicians over the last ten years about delay and costs were just wrong and spin to enable the Gove-plan. Nature doesn’t hold you up any more than archaeology or heritage does, and the new systems in very many instances weakens protection at all levels of rarity and vulnerability and often in unmeasured ways.

    So good luck Dominic. Give it your best and lets hope the main NGO’s and government que up to support you.

  2. It is a fair point about needing to establish realistic timescales for achievement of biodiversity gain but once a project is given the go ahead what are the mechanisms for ensuring the promised NG is achieved and what are the sanctions if it is not? It is easy to envisage a situation in which, after multiple changes of ownership, the original developer no longer existing as a legal entity and so forth, it might be difficult to find who should bear the liability for any such sanctions. Of course, it should be possible verify achievement of various milestones prior to the full, long term fulfilment of the commitment but I’d be interested to know how this aspect is managed. Without effective enforcement of the actual achievement of net gain there will always be a strong incentive to over-promise and we risk a fantasy future full of all sorts of miraculous biodiversity improvements.
    With respect to the resourcing of LAs to manage BNG, it is hard to feel optimistic as they are unlikely to get adequate funding for this from central government and, though a levy charged on planning applications is a good idea, we can expect it to be vigorously contested by developers. Hopefully, a change of government will mean that the bosses of large developers do not have the ear of ministers to quite the extent that they presently do…

  3. The simple answer is that assesments should be made by neutral – probably Government employed – ecologists. My experience certainly confirms the bias of ecological consultancies employed by developers – probably not surprising as if they don’t give the answer the developer wants they won’t get future work but the abuses can be so blatant as to be laughable. Hopefully a change of Government might be the chance to put things right.

  4. I review BNG for LPA’s and some of the surveys to inform the Metric are undertaken from space (Google Earth) and we are told that software will assist in determining habitats, so not much ecology ever involved.

    Challenge the developers multiple ecologists (and they ship in additional ones for the meetings) and they roll their eyes to make you feel inadequate .

    By planting ‘woodland meadow’ seeds around boundary trees a 78% Net Gain can be achieved even though those trees probably had their own associates beneath the canopy that enabled them to source nutrients etc. The newly created ‘habitat’ will mean competition for resources especially water, but not all ecologists have the required landscaping skills.

    Every green space seems up for grabs now; we are losing priority habitat and there is no arbiter.

    The Design Review Panels are not for profit charitable trusts (now known as placemaking panels). This would be a better model for truly independent ecologists. Or where appropriate attached to the Local Records Centres, which would be Greenspace Information for Greater London in my area.

  5. Biodiversity Numbers Game

    It is good to be open and honest. So when looking back to see where BNG came from it is refreshing to see the intentions of fixing a planning system that does not deliver for biodiversity in Professor David Hill’s on-line “Interview with an Influencer” (google that wording). Some may question whether it was a conflict of interest to be a founding member of Natural England (2006) and it’s Deputy Chair (2011-16) whilst also setting up a lobbying organisation in 2007 (The Environment Bank Ltd.) to push Government in the direction of biodiversity off-setting – but shining a light on the inadequacies of the development industry and the planning system to deliver long-term infrastructure for biodiversity we would all agree is a fight worth fighting for so many thanks to Professor David Hill CBE for that.

    Yet there is a real fear that the way BNG has been set up we may not see the meaningful change we know is needed. Lets look at an example elsewhere in this country where we rely on the private market to deliver long-term infrastructure for the public good, in this case the privatised water companies. We know that climate change means that droughts will be more common and therefore we need more reservoirs to satisfy local water demand, but do we get adequate investment in the long term solutions? The last reservoir was built 35 years ago (1989) which was way before climate change was such a widely accepted concept. And when were the publicly-owned water companies privatised? – yep you guessed it, the same year 1989. Private water companies have to juggle the demands of share-holders with short-term financial interests against investing in long-term expensive new infrastructure.

    A similar fate may threaten BNG before it has really begun. We are starting from a point of BNG relying on an unregulated private market to deliver long-term infrastructure for biodiversity. At least the private water companies have a regulatory body in OFWAT who can decide to issue fines for bad practice, but BNG has no such regulatory body. It seems that the “BNG Police” are the local planning authorities and the expectation is that their Enforcement teams (where they exist) will keep an eye on delivery both on-site and off-site and it will all work even without the threat of financial penalties. I don’t think I need to say any more on this issue other than to say does that system of carrot and stick work as effectively as we would like in the planning system at the moment even before BNG?

    BNG seems to be a world of missed opportunities. It did a useful thing in creating a measuring tool (the Metric) that can assess the on-site impacts from development and turn this into numbers than can in turn be translated into a financial sum for compensation off-site. The Metric has delivered the “measurable” aspect of the wording in the NPPF (para. 185b) around delivering net gains for biodiversity.

    The arising financial penalty for any residual short-falls in Biodiversity Units on-site is effectively a biodiversity tax on developers and we should all be honest about this. And developers are satisfied to pay such a tax instead of their on-site development objectives being thrown into doubt. The publication of the national statutory biodiversity credit prices by the Government (to be sold by Natural England) was interesting and considered by most to be on the high side. But imagine for a moment that this was the only option available to developers to solve their on-site shortfalls – and also remember that two credits have to be purchased for every one biodiversity unit lost, and that VAT is charged on top.

    Such an approach outlined above would focus the minds of the development industry because it would be costly to have a short-fall in biodiversity units on-site, so more effort would be made to retain any scoring habitats on-site and also developers would only try to develop sites with low baseline biodiversity unit scores. Instead we will see private markets undercutting these national statutory credit prices so it will be a race to the bottom.

    Many of us working in nature conservation and planning have seen the flaws for many years so it is disappointing when a new major piece of legislation arrives which addresses part of the problem but not the solution. The Environment Act (2021) gave us two potentially exciting new opportunities for biodiversity: Local Nature Recovery Strategies and Biodiversity Net Gain but although arguments are stated that they are intertwined in reality this is only through a weak (15%) Metric multiplier that hopes to encourage off-site private-market delivery in strategic locations.

    Imagine again if instead of relying on private markets to deliver off-site BNG the only solution was through buying the national statutory biodiversity credits from Natural England. A small sum could be kept by Natural England to prop it up as an effective national nature conservation body and possibly fund local BNG officers to monitor and take action against poor practice by local consultants using the Metric (this would really help local planning authorities). And then (this is the important bit) all the rest of the funding arising from developments in a particular local planning authority (LPA) area is ring-fenced and passed back to the LNRS Responsible Authority where that LPA is located. Then the LNRS steering group has the say on where and how the money was used in order to meet their LNRS biodiversity priorities – which could be through buying land for new nature reserves, employing land management advisors for Local Wildlife Sites etc.

    Imagine the spending of the monies paid for off-site BNG did not have to be monitored in numbers of biodiversity units because it was simply part of a biodiversity tax paid by developers and used by trustworthy local decision-makers on the LNRS steering groups to agree the best way to use it – it could result in massive new nature reserves in every LNRS across England. Most developers don’t have much interest in what happens to their money after they have paid it, they just want certainty through the planning system that a problem can be paid to go away – and a simple biodiversity tax does this (albeit currently under the name of BNG – with it’s very complex, hard-to-enforce and monitor pieces of the jigsaw).

    With a bit more thought to the risks associated with allowing an unregulated private market to deliver off-site biodiversity gains we could have achieved something more aligned with public goods that are long-term and empowering (for the LNRSs) through the Environment Act (2021). Instead do we need to pin our hopes on the next Government nationalising our BNG? I also have significant doubt that would happen.

    What we all want to see from BNG is a solution and mechanisms that allow investment in long-term biodiversity infrastructure that benefits both people and wildlife.

  6. Hello Mark/Dominic – my post is a long one so may need formatting to read a bit better – or if you decide it is worthy of either of you using in some other capacity please do so. It may not get the full readership here perhaps? I wanted to put a hyperlink in to the Interview with an Influencer but this did not seem to work.

  7. Just discovered this via a link on a Facebook page about planning matters. It is bloody brilliant – thank you Dominic for your extremely well written and superbly put case for drastic improvements to the BNG system. I work in arboriculture and find, similarly to you, that under 10% of arboricultural impact assessments carried out for developers are remotely accurate. LPA staff generally don’t have the knowledge to even detect the inaccuracies, let along query them: the default scenario is that whatever the AIA says is taken as gospel and treated as fact for planning purposes. It drives me mad.

    I’m also an ecologist with special interest in ancient woodlands. I have fallen out with NE over the inadequacy of their AWI. I’ve fallen out with the Arb Assoc (and resigned from it) over their accredited contractor criteria, which admit contractors without any special affinity with or knowledge/experience of executing high quality, sensitive work to important (often ancient) trees, resulting in several I know of having been killed. I seem to fall out with just about everyone I have anything to do with, not because I’m obnoxious but because I spot flaws and can’t resist insisting that they are accounted for.

    Anyway, I won’t ramble on – what I want to know is, how do I subscribe to this blog? Have I done so simply by posting this reply or is there something else I need to do? Thanks.

  8. BNG is an obvious scam to enable developers to build wherever they want to build.
    A better plan is –
    a) If there’s already a wildlife habitat there, you CANNOT build on it.
    b) Have legislation in place to create more habitats.
    c) Remove the ridiculous BNG link between the two issues.

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