Archaeology and biodiversity net gain on the same site

Archaeology and biodiversity net gain collide on the same site in one specific way: trial trenching strips the habitat you are about to measure. Run the archaeological evaluation before the ecological baseline and you can find yourself calculating a metric on bare soil, or arguing with the council about which condition the site was in when the clock started. On a scheme of any size, the order in which those two surveys happen is worth more than the fee for either of them.

The two regimes are unconnected in law and share nothing except your programme and your site.

Two regimes, one programme

Archaeology reaches planning through the historic environment policies of the National Planning Policy Framework (current edition published 17 August 2026), the county archaeologist's advice and the Historic Environment Record. Where remains are suspected, the authority asks for an assessment, then usually for field evaluation before determination, then imposes a condition requiring a written scheme of investigation and a programme of work. Scheduled monuments sit outside planning altogether and need scheduled monument consent under the Ancient Monuments and Archaeological Areas Act 1979.

Biodiversity net gain reaches planning through Schedule 7A to the Town and Country Planning Act 1990, inserted by the Environment Act 2021. A deemed condition requires a biodiversity gain plan to be approved by the authority before development begins, showing at least a ten per cent uplift measured with the statutory metric against a pre-development baseline.

Neither regime knows the other exists. You are the only person joining them up.

Trial trenching and the habitat baseline

The metric baseline is the habitat present on the site, surveyed and condition-assessed by someone competent to do it. Where habitat has been degraded on or after 30 January 2020, the pre-development value can be assessed as it was immediately before that degradation, using the best available evidence.

Trial trenching is degradation. So is topsoil stripping for a strip, map and sample exercise, and so is the haul road you put in to get the machine to the trenches.

That leaves two ways to get it wrong. Trench first without recording the habitat, and you have no evidence of the baseline, which usually means arguing for a lower value than the site actually had. Or trench first and assume the post-trenching condition is the baseline, which understates the loss and produces a gain plan the council will not accept once the sequence becomes clear.

The fix is procedural and costs very little. Commission the Preliminary Ecological Appraisal and the habitat condition assessment before any intrusive archaeological work, and have the ecologist record the trench locations and the surrounding habitat on the same visit. Photographs with dates, a habitat plan and a condition assessment take a morning and settle the question permanently.

Where habitat creation and buried remains conflict

The second clash comes later, when you decide where the biodiversity units are going to be delivered.

Preservation in situ is the preferred outcome for archaeological remains wherever it can be achieved. That points towards low-intervention ground cover over the deposits: species-rich grassland, shallow-rooting meadow mixes, and management by cutting rather than by cultivation.

It points away from three things planners often like:

• Woodland and tree planting, because root penetration and windthrow disturb deposits
• Wetland features, ponds and attenuation basins, because they require excavation and change the hydrology that has preserved organic remains
• Deep soil placement or regrading, which changes burial depth and drainage

So the site plan that maximises biodiversity units and the site plan that preserves archaeology in situ are frequently not the same drawing. Resolve that at masterplan stage. Grassland over the archaeologically sensitive area, tree planting and wetland features where evaluation has shown the ground is clear, is the arrangement that satisfies both advisers.

Waterlogged deposits deserve particular care. Organic archaeology survives because it is wet, and a drainage scheme that lowers the water table can destroy remains that excavation would have recovered. Say so explicitly in the Archaeological Desk-Based Assessment if the historic environment record suggests waterlogging.

Sequencing that works

A workable order for a site with both constraints:

1. Desk-based assessment and Historic Environment Record search, plus a preliminary ecological appraisal, both early and both before any ground disturbance.
2. Habitat condition assessment and metric baseline, recorded with dated photographs and a habitat plan.
3. Non-intrusive archaeological survey where useful, such as geophysics, which does not damage habitat.
4. Intrusive Archaeological Field Evaluation with trench locations agreed against the habitat plan, avoiding the highest-value parcels where the archaeological questions allow.
5. Masterplanning that places habitat creation and archaeological mitigation in compatible parts of the site.
6. Metric calculation and biodiversity gain plan, informed by where mitigation excavation will happen.

Step three is the one most often skipped, and it is the one that saves the most. Geophysical survey narrows the trenching, which reduces both cost and habitat damage.

Season imposes its own order. Habitat condition assessment is best carried out in the growing season, roughly April to September, while trenching can happen at almost any time the ground is workable. That asymmetry favours the sequence above rather than fighting it: book the ecologist for the summer and the archaeologist for the autumn.

Conditions that both land before you start

Both regimes bite at the same moment. The biodiversity gain plan must be approved before development begins. The written scheme of investigation must normally be approved before development begins too, and the archaeological fieldwork itself often has to be completed or substantially advanced before other works start.

Two consequences follow. Programme them in parallel rather than in series, because each takes weeks of council time. And be careful about what counts as beginning development, because a material operation carried out to satisfy one condition can trigger the argument that development began before the other was discharged. Where archaeological mitigation involves stripping, get the position in writing from the authority first.

Illustrative example: a 1.8 hectare former orchard with a cropmark across the northern third. Evaluation is inevitable. The sequence that works is habitat survey in May, geophysics in June, trenching in July confined to the cropmark area with trench spoil returned and the orchard grassland elsewhere left intact, and a gain plan that puts new hedgerow and species-rich grassland over the northern third with tree planting confined to the south.

When to move gain off site

Sometimes the two cannot be reconciled on site. A dense urban plot with significant deposits under every part of it will not deliver meaningful on-site habitat without excavation.

The statutory hierarchy still expects on-site delivery to be considered first, but off-site units or statutory biodiversity credits exist for exactly this situation. Make the case explicitly in the gain plan: state what was tested, why in-situ preservation constrains on-site delivery, and what is being delivered instead. An unexplained jump to off-site units invites a refusal. A documented one usually does not.

The metric work itself is a competent person exercise under the Environment Act 2021 regime, and the archaeological work should follow the standards of the relevant professional institute. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required. This is general guidance, not legal advice; check the position with your local planning authority before relying on it.