The Boundary Problem in Biodiversity Net Gain
Red line boundaries are one of the most deceptively simple elements in Biodiversity Net Gain planning, but they’re also one of the most consequential. Whether you're submitting a baseline habitat map or a post-development layout, the red line defines the legal and spatial extent of a project.
One of the biggest challenges is deciding which dataset to trust. OS MasterMap is widely used for digitising habitat features because of its spatial precision, but it doesn’t always match Land Registry boundaries. That’s a problem, because planning law recognises Land Registry as the definitive source of ownership. If your red line falls outside that boundary, you’re potentially claiming biodiversity units on land you don’t legally control. That’s not just a technical error, it’s a legal one.
Then there’s the question of what the red line should include. Should it cover all land under ownership, or just the area being developed? It’s not just a philosophical debate, it affects the maths. Say you own 10 hectares, but only 2 hectares are being developed. If you only draw the red line around the 2 hectares, you can’t claim any uplift from enhancements on the remaining 8. Even if you create 30 units of biodiversity gain on that land, it won’t count unless it’s inside the red line. That could mean the difference between passing or failing the 10 percent net gain requirement. You can still pass the net gain, but only if you bear in mind the penalty multiplier applied to off-site enhancements. These multipliers reduce the value of units delivered outside the red line, meaning you’ll need to create more units to compensate. In some cases, this can make off-site delivery less efficient or even unviable, especially if the red line could have been drawn to include the enhancement area in the first place.On the other hand, a smaller red line boundary also means a smaller uplift target. For example, if your baseline habitat value is 10 units, you only need to deliver 1 additional unit to meet the 10 percent gain. But if your baseline is 100 units, you’ll need to deliver 10 extra units on top of replacing any that are lost. That’s a much bigger ask, even if the development footprint is the same. So while a larger red line gives you more space for on-site enhancements, it also increases the scale of the uplift you need to achieve.
This is where the FRIDAS checklist becomes essential. It’s designed to catch these kinds of errors before they reach the Local Planning Authority. FRIDAS asks whether the same red line boundary has been used across all datasets, whether it aligns with Land Registry records, and whether it’s been applied consistently between baseline and post-development plans. These aren’t just good practice, they’re safeguards against rejection, fines, and long-term monitoring failures.
And yet, many of these errors go unnoticed. LPAs are underfunded for the task they’re now required to deliver. With reduced budgets and increasing workloads, there’s limited capacity to scrutinise every technical detail of a BNG submission. A mismatch between baseline and post-development boundaries should be an automatic rejection, but whether it’s picked up is another matter.
That’s why standardisation matters. The FRIDAS checklist isn’t just a technical tool, it’s a way to build trust in the data. It helps ensure that what’s submitted is legally sound, spatially accurate, and ecologically meaningful. And in a system where BNG plans are supposed to be monitored for 30 years, that kind of rigour isn’t optional, it’s essential.
