Biodiversity net gain: the 10% you now have to plan for

Look down on the edge of Cardiff, where the city gives way to the older industrial land and the green margins beyond, and you are looking at the kind of ground where biodiversity net gain now bites. The mixed-use schemes, the residential blocks rising on former employment sites, the car parks and verges and scrub that surround them: these are no longer just development plots. They are sites where the habitat present before work starts, and the habitat present after, has become a measured, conditioned part of getting consent. The green you can see from above is now an input to the planning system, not a leftover.

Net gain has shifted from a thing good developers did to a thing every major scheme has to do. For property and land managers, that changes the job. The obligation does not end when the diggers leave. It runs for decades, and it has to be evidenced for decades, which makes it squarely a records-and-inspection problem rather than a planning footnote.

What the rule actually requires

Under the Environment Act 2021, most new development must deliver a minimum 10% biodiversity net gain, secured for at least 30 years, according to gov.uk and the NBN. The obligation became mandatory for major applications from 12 February 2024 and for small sites from 2 April 2024. Gain can be delivered on-site, off-site, or, as a last resort, through statutory biodiversity credits, per gov.uk.

Three numbers in that paragraph reshape how a site is managed. The 10% sets the target. The 30 years sets the commitment, which is far longer than most management appointments and most service-charge horizons. And the dates mean this is live now, not a future concern, for any scheme that has come through planning since early 2024.

The 30-year point deserves dwelling on. A net gain secured for at least 30 years is a habitat that must be maintained, monitored and proven across multiple changes of manager, owner and managing agent. That is a long time to keep a promise, and a long time to keep the evidence that the promise is being kept.

The duty that outlives the developer

The practical risk in net gain is not the planning condition itself. It is the gap that opens once the development is complete and the obligation passes from the people who designed it to the people who run the site. The habitat management and monitoring plan that justified the consent becomes someone's recurring responsibility, and if that responsibility is not written into the site's ongoing record, it quietly lapses.

What a managed net gain commitment typically needs held and dated over its life:

• The baseline habitat assessment and the post-development biodiversity metric that secured consent.
• The habitat management and monitoring plan, with its schedule of interventions.
• Records of each maintenance and monitoring visit, dated and attributed.
• Evidence that on-site features, or off-site units, are being delivered as agreed.
• A clear line of responsibility that survives a change of manager.

This is the same discipline that good compliance management already applies to buildings, pointed at the land. The habitat is an asset on the site with a recurring obligation attached, exactly like a lift with a LOLER examination or a high-rise with a monthly fire check. It belongs on the same record.

Why the boundary comes first, again

You cannot manage a net gain commitment you cannot locate. The on-site habitat areas, the retained features, the new planting all sit inside the site boundary, and they need to be drawn and held there so that thirty years of monitoring has somewhere to attach. A site whose boundary is defined can carry its habitat layer cleanly; a site managed only as a building has nowhere to put it. This is why we keep returning to the idea that you should draw the boundary once and let everything hang off it. Net gain is one of the clearest cases of a duty that lives on the land rather than in the building.

Cardiff and the Welsh context

A practical note for sites in and around Cardiff: biodiversity net gain as set out under the Environment Act 2021 is part of the planning regime in England, and the position in Wales is governed by Welsh planning policy and its own approach to biodiversity and ecosystem resilience. The principle that habitat is now a planned, evidenced part of development holds wherever you are, but the precise statutory mechanism and the metric used depend on the jurisdiction. Anyone managing a Cardiff site should confirm the applicable Welsh requirements rather than assume the English figures transfer. The honest answer is that the duty exists in both, and the evidence discipline is the same; the legal detail is not.

Treat it as a 30-year inspection regime

The most useful mental shift is to stop thinking of net gain as a planning hurdle and start thinking of it as a long-running inspection regime, like any other recurring compliance task. It has a schedule, it generates findings, and it has to be provable years after the people who set it up have moved on. Held that way, it stops being a forgotten condition and becomes a managed obligation with a clean trail. We have made the same argument for drainage and SuDS: the feature on the ground is only as defensible as the record that proves it was inspected.

Plan for the years you can see coming

The 10% is the headline, but the 30 years is the work. Capture the baseline and the management plan as part of the site record, schedule the monitoring like any other recurring duty, and make sure responsibility passes cleanly at every handover. Done that way, a net gain commitment is just another well-managed, well-evidenced line on the site. In SAMRISK, habitat areas and their monitoring schedule can sit on the site record alongside the building, so a thirty-year promise has a thirty-year home rather than a folder that outlives nobody's memory.