Trial trenching percentages and what triggers excavation

Archaeological trial trench sampling is normally specified as a percentage of the developable site area, and on greenfield sites in England that figure commonly sits between two and five per cent, with four per cent a frequent starting point. The percentage is set by the local authority's archaeological adviser in a written scheme of investigation, not by the applicant, and it is negotiable on evidence rather than on principle. What the trenches find then decides whether you leave site with a watching brief condition or with an open area excavation in the programme.

Why evaluation precedes mitigation

National policy expects an applicant to describe the significance of any heritage assets affected, including assets with archaeological interest, at a level proportionate to their importance. Where a site may contain such assets, the National Planning Policy Framework (August 2026 edition) expects a desk-based assessment and, where that is not sufficient, a field evaluation.

The logic is that a decision-maker cannot weigh harm against public benefit without knowing what is there. A desk-based assessment tells you what is recorded: the Historic Environment Record entries, historic mapping, aerial photography, geology, previous interventions and the pattern of finds in the surrounding area. It cannot tell you what survives below your topsoil.

Geophysical survey narrows the gap. Magnetometry across an arable field picks up ditches, pits and kilns quickly and at modest cost, and on the right geology it produces a plan good enough to target trenches. On clay with flint, on made ground, or over a site with modern ferrous disturbance, it produces noise. Curators know which is which locally, which is one reason the conversation starts with them.

Evaluation is the stage where assumptions turn into a plan. Skipping it and offering a condition instead works on small sites with low potential and fails on anything a county archaeologist considers sensitive.

Grid, targeted and hybrid trenching compared

A standard trench is around 30 metres long and 1.8 to 2 metres wide, excavated by machine with a toothless bucket under archaeological supervision down to the first archaeological horizon or to natural geology, then cleaned and recorded by hand. Trench lengths vary, and on constrained urban sites test pits replace trenches entirely.

The hybrid layout is worth arguing for. Purely targeted trenching confirms what geophysics already suggested and tells the curator nothing about the rest of the field, which usually produces a request for more work later. A background array answers the absence question at the same visit.

The sampling percentages commonly required

Two per cent is the lightest sample a curator will usually accept on a site with genuine potential, and it is more common on urban or previously developed land where access and services constrain the work. Four per cent is the figure most often written into a written scheme of investigation for a greenfield residential site. Five per cent appears where potential is high, and higher samples are specified over known features or where the first phase produced ambiguous results.

The percentage is calculated on the area to be disturbed rather than the red line, so excluding retained woodland, existing hardstanding and land outside the developable area from the denominator is a legitimate and frequently overlooked saving. Put the calculation in the written scheme of investigation and show the excluded areas on the trench plan.

Sample size is not the only variable. Trench distribution, machine time, the proportion of features excavated by hand, environmental sampling, specialist assessment and radiocarbon dating all sit in the same document and all affect the cost more than one percentage point either way.

Both the evaluation and the Archaeological Field Evaluation report that follows should be prepared by a suitably qualified archaeologist working to the Chartered Institute for Archaeologists standard and guidance. Councils expect that, and professional review of any draft is strongly advised.

What results push a site into full excavation

Curators are not looking for finds. They are looking for coherent, datable, interpretable deposits that would be destroyed by the development.

The results that escalate a site:

1. Structural evidence. Beam slots, post holes in patterns, walls, floors, hearths and industrial features such as corn dryers or kilns.
2. Human remains, which carry their own licence and ethical requirements and almost always extend the work.
3. Waterlogged deposits, because organic preservation is rare and the environmental evidence is scientifically valuable.
4. Well-preserved stratigraphy in an urban context, where successive occupation levels survive rather than a single truncated horizon.
5. Remains of demonstrably national importance, which shifts the discussion from recording to preservation in situ and can put the principle of development in doubt.

Conversely, a scatter of undated ditches on a heavily ploughed field, modern field drains and residual pottery in the topsoil usually produce no further requirement beyond a watching brief during groundworks.

Where remains are nationally important and would be substantially harmed, the tests described in our note on substantial harm and the justification required apply, and a scheduled monument brings a separate consent under the Ancient Monuments and Archaeological Areas Act 1979.

Strip, map and sample as a middle option

Between a watching brief and an open area excavation sits strip, map and sample. The topsoil is removed mechanically under archaeological control across a defined area, the exposed features are planned, and an agreed proportion of them is excavated and recorded.

It suits sites with dispersed, low-density features where an open area excavation would waste effort on blank ground and a watching brief would miss the pattern. It is also programme-friendly, because it can run alongside the early groundworks contract rather than blocking it, provided the contract allows the archaeologists room and the machine time is theirs to direct.

The trap is an open-ended sampling proportion. Agree the percentage of features to be excavated and the criteria for increasing it before anyone strips anything, and write a variation mechanism into the scheme rather than leaving it to a site meeting.

Cost and programme consequences of each outcome

Fieldwork is the visible cost and rarely the largest one. An evaluation is measured in weeks. An open area excavation on a multi-hectare site is measured in months and is weather-dependent. Post-excavation assessment, specialist analysis, conservation, archive deposition and publication follow, and that tail is often longer than the digging, as our note on post-excavation assessment, archive and publication explains.

Three programme risks deserve naming in the appraisal. Human remains stop the clock while a licence is obtained. Waterlogged deposits require specialist recovery and conservation that cannot be rushed. And archive deposition has become harder as museum stores fill, so the receiving body and its charges need confirming early rather than at the end.

Budget the whole sequence, not the trenches. A scheme that priced an evaluation and ignored publication is the one that stalls two years after practical completion.

Negotiating the strategy before trenches are opened

Talk to the county archaeologist or the authority's archaeological adviser before the written scheme of investigation is drafted. Bring the desk-based assessment, the geophysics and the developable area plan, and ask three questions: what sample percentage are you looking for, what would change your mind about it, and what result would you treat as triggering mitigation.

Getting an answer to the third question in writing is the valuable part. A scheme that says in advance what counts as significant gives the appraisal a defined risk rather than an open one, and it stops a disagreement at the end of the evaluation about whether four undated ditches justify an excavation.

Where evaluation before determination is not practical, ask for a phased condition: approval of a written scheme, implementation of the evaluation, then approval of a mitigation strategy based on the results. That structure is well established and keeps the determination on programme.

Start from a properly evidenced Archaeological Desk-Based Assessment, because everything downstream is priced off it. To see which supporting documents your site is likely to need, enter your postcode on PlanScope and run the constraint check.

This is general guidance, not legal advice. Archaeological work should be commissioned from a suitably qualified professional and any draft report reviewed by one before submission. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.