Archaeology evidence on a site already disturbed

Previous disturbance reduces archaeological potential. It rarely removes it. A desk-based assessment on a site with a documented industrial or basement history has to answer three questions. How deep the truncation went, across how much of the footprint, and what survives beneath it. Authorities refuse to drop an evaluation condition on an assertion that the site is made ground, and they are right to.

Get the depth evidence into the assessment. A county archaeologist presented with borehole logs, a historic basement plan and a record of a neighbouring excavation will engage with a reduced scope. One presented with a paragraph about twentieth-century development will recommend trial trenching.

Why previous disturbance reduces but rarely removes potential

Truncation is uneven. Nineteenth and twentieth-century development removed topsoil and upper horizons over parts of a site and left others intact: beneath yards, under floor slabs laid directly on the natural, around foundations, in the gaps between basements, and inside backfilled features that themselves preserve earlier deposits.

Deep features survive shallow disturbance. Wells, cess pits, quarry pits, ditches and the lower fills of post-holes can extend well below the level that later construction reached. On urban sites the deposit sequence can be several metres thick, so a 1960s slab at 0.4 metres below present ground level may have cut nothing of consequence.

National policy treats non-designated archaeological remains of equivalent significance to scheduled monuments as subject to the same policy approach as designated assets, and for everything else requires a balanced judgement having regard to the scale of harm and the significance of the asset. The Ancient Monuments and Archaeological Areas Act 1979 governs scheduled monuments separately, and scheduled monument consent is a different process from planning permission.

So the correct conclusion on a disturbed site is usually that potential survives, confined to defined depths and defined parts of the footprint. That is a more useful conclusion anyway, because it is the one that supports a targeted condition.

Establishing the depth of truncation

This is the technical heart of the assessment, and it is a matter of records rather than opinion.

Five sources carry weight. Geotechnical borehole and trial pit logs from the site or immediately adjacent, which give you the depth to natural and the thickness of made ground. Historic Ordnance Survey mapping and large-scale town plans showing building footprints, basements and yards across successive editions. Original building records, including basement and foundation drawings held in a local archive or with the freeholder. Records of services and drainage, since a Victorian sewer trench is a deep, narrow truncation. And the local Historic Environment Record, which holds the results of previous investigations.

Present the evidence as a plan and a section, not as prose. Overlay the historic footprints on the proposed layout, shade the areas of known basement, and show the depths you have evidence for. State the datum. Where you have borehole logs, tabulate depth to made ground, depth to natural and the ground level at each location.

Then say honestly where the evidence runs out. A site with three boreholes in one corner and nothing across the rest of the footprint has partial coverage, and claiming uniform truncation from that is the error an archaeological officer will find.

Made ground and what it can preserve

Made ground is not a sterile layer. It is a deposit with its own date and its own content, and on some sites it is the archaeology.

Dumped material can contain redeposited artefacts that indicate what was removed from the site or brought onto it. Waterlogged made ground preserves organic material, including timber, leather and environmental samples, better than free-draining natural subsoil. Where a site was raised rather than cut, the original ground surface may sit intact beneath the imported material, which is the most favourable scenario of all and the one most often missed in a quick assessment.

Industrial made ground has its own interest. Nineteenth-century foundry waste, tannery deposits and gasworks residues are archaeological evidence of the process that produced them, and the same deposits usually drive a contaminated land investigation, so the two site investigations should be programmed together rather than sequentially.

Distinguish levelling from truncation in the text. They are opposite processes and a report that uses made ground as shorthand for both will be challenged.

Evidence from neighbouring investigations

The most useful evidence on a disturbed urban site is somebody else's trench.

Query the Historic Environment Record for a defined radius and list every investigation it holds: watching briefs, evaluations, excavations and geotechnical monitoring. For each, record the depth reached, whether natural was encountered, what survived, and the distance and direction from your site. A watching brief 30 metres away that found the natural at 1.8 metres below ground level and intact medieval deposits at 1.2 metres tells you a great deal about what to expect.

Grey literature reports are the useful layer. Many are held by the record office or the county archaeologist and are not published anywhere else, so ask specifically rather than relying on the summary entries.

Be careful with negative results. An evaluation that found nothing may have been in the wrong place, may have stopped above the deposits of interest, or may have been small. Report the sample size and the depth reached alongside the conclusion, and resist the temptation to present a single blank trench as proof of absence across a street block.

Targeted evaluation on disturbed sites

Where evaluation is still needed, the argument is about scope rather than principle, and that is a much better argument to be having.

Options that authorities accept on truncated sites include trenches positioned only in the areas with no recorded basement, evaluation by test pit rather than machine trench where access is constrained, a geoarchaeological deposit model built from boreholes before any intrusive work, and a watching brief during the geotechnical investigation so that both investigations use one set of holes. Each of these cuts cost and programme without pretending the potential is nil.

Agree the scope with the county archaeologist through a written scheme of investigation prepared to the relevant professional standards, and get the authority's agreement in writing before mobilising. An evaluation carried out to a scope the curator did not approve may have to be repeated.

Timing matters on constrained sites. Where demolition has to happen before trenching is possible, the condition should be worded to allow a phased approach, and the site set-up and demolition content of a construction management plan should reflect the archaeological sequence.

Negotiating a lighter condition

The usual starting point is a pre-start condition requiring a written scheme of investigation and a programme of archaeological work. On a well-evidenced disturbed site there is room to shape that.

Ask for the condition to be framed around a defined area rather than the whole site, and for the trigger to be demolition and slab removal rather than any development at all, so that enabling works are not blocked. Where the evidence supports it, ask for a watching brief rather than a prior evaluation. Where deep foundations are proposed, offer a foundation design that minimises intrusion into surviving deposits, which is the mitigation curators most often accept in place of excavation.

What rarely works is asking for no condition. Even a thoroughly truncated site can produce something, and an authority that discharges its duty by imposing a proportionate condition has little reason to take the risk of imposing none.

Reporting the conclusion defensibly

Write the conclusion so that someone can disagree with the reasoning rather than only with the answer.

State the baseline: the recorded assets within the study area and their periods. State the disturbance evidence and its coverage, with the plan and section. State what the truncation means for each period of potential separately, because a site can have low Roman potential and moderate post-medieval potential at a shallower depth. Give the residual potential by depth band and by area. Then set out the recommended mitigation and why it is proportionate.

Label any quantified estimate of survival as illustrative. Illustrative example: a city centre plot with a 1930s building over a single basement across 60 per cent of the footprint, boreholes showing natural at 2.4 metres and made ground from 0.3 metres, and a neighbouring evaluation that found intact deposits from 1.1 metres. The defensible conclusion is low potential beneath the basement and moderate potential in the remaining 40 per cent, with evaluation targeted on that area. Those figures are illustrative.

An archaeological desk-based assessment is a specialist document and councils expect a suitably qualified author, so professional review before submission is strongly advised. Browse the 66 report types by category to see what else a site with a complex ground history engages, and enter your postcode on PlanScope to start the supporting statements.

This is general guidance, not legal advice. Check the position with your local planning authority, the county archaeologist or a qualified professional before relying on it.

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.