Trial trench sampling percentages and how to justify them
There is no national standard for trial trench coverage. The curator sets it from the desk study and geophysics, which makes the sample and the layout negotiable.
Trial trench evaluation on development sites in England is normally specified as a percentage sample of the application area, and 2 to 5 per cent is the range most local authority archaeological advisers work within. There is no national standard setting that figure. It comes from the curator at the county or unitary authority, informed by what the desk study and any geophysics have shown, so the number is negotiable in a way that many developers never test.
Getting the sample and the trench layout agreed before mobilisation is worth more than arguing about the percentage afterwards. The cost driver is rarely the trenching itself.
What an evaluation is trying to establish
An evaluation answers five questions: is anything there, what is it, how old is it, how well preserved is it, and how far does it extend. From those the curator forms a view on significance, and from significance follows the mitigation strategy and the planning recommendation.
It is a sampling exercise and everyone involved knows it. Nobody expects a 4 per cent sample to find every feature. What it has to do is give a defensible basis for deciding between preservation in situ, strip map and record, open area excavation, a watching brief, or no further work at all.
That is why an evaluation report that says nothing was found is still a useful result. It converts an open risk into a closed one, which is the thing a programme needs.
Where coverage expectations come from
The professional standard for field evaluation is set by the Chartered Institute for Archaeologists, and it deals with methodology, competence and reporting rather than with a percentage. The percentage comes from local practice.
Two per cent tends to be applied where the desk study shows low potential and the geophysics is blank. Four or 5 per cent is common where there is known potential, a cropmark complex, or a site adjacent to a recorded settlement. Higher figures appear on sites with scheduled monuments nearby or where an earlier evaluation found remains.
Ask the curator to state the basis for the figure in writing. A requirement grounded in a named record from the Historic Environment Record is one you can plan around. A figure offered without a reason is one you can discuss, particularly where the Archaeological Desk-Based Assessment has already narrowed the area of potential to part of the site.
Targeted trenches against a grid layout
A grid or herringbone array spread evenly across the site gives an unbiased sample and supports a statement about the areas where nothing was found. Targeted trenches placed on geophysical anomalies give a much higher hit rate and tell you what the anomalies are, but they say nothing about the ground between them.
The usual answer is both. Lay a baseline array across the developable area, then add targeted trenches on anomalies, and describe the two components separately in the written scheme of investigation so that the curator can see which part of the sample supports which conclusion.
Two practical points. Trenches should be long enough to catch linear features running in any direction, which is why the conventional 30 metre by 1.8 metre trench persists. And the array should be oriented across the dominant alignment of any known field system rather than along it, or the sample will miss boundaries systematically.
Justifying a lower percentage
Constrained sites are common and curators know it. The argument works when it is specific.
Standing buildings, live operations, buried services, contamination, protected trees and their root protection areas, ecological constraints and seasonal restrictions all reduce what can safely be trenched. Set out the constrained area on a plan, calculate the achievable coverage of the accessible area rather than of the red line, and offer something in return: larger trenches where access exists, a staged approach with a second phase after demolition, or a watching brief over the constrained ground.
Where deep made ground means trenches must be stepped or shored, say so with the depths from the ground investigation attached. A 5 per cent sample of a site with 3 metres of overburden is a different exercise from a 5 per cent sample of a ploughsoil site, and the curator is generally receptive to that once it is evidenced.
Sequencing after the desk study and geophysics
Run the desk study first, then geophysics where the geology and land use suit it, then trenching. Geophysical survey gives poor results on igneous and some drift geologies, over made ground, close to services and fences, and on hard standing. Commissioning it anyway on such a site produces a report that says nothing and a curator who still wants the same trench sample.
Where geophysics does work, it usually reduces the trenching by focusing it, which is the main reason to do it. Say that in the brief so that the surveyor reports anomalies in a form the trenching contractor can set out from.
An illustrative example shows where the negotiation usually lands. A 6 hectare arable site has a desk study identifying a cropmark enclosure across the northern third and nothing recorded elsewhere. The curator opens at 4 per cent across the whole site. The counter proposal is 5 per cent across the northern third, 2 per cent across the rest, and an agreed contingency to expand any trench that finds structural remains. Total trench metreage falls slightly, the information value rises, and the curator accepts because the sample is now aimed at the question. This example is illustrative.
Access, services and standing crops
Practical constraints set the programme more often than archaeology does. Service drawings and a CAT scan before any machine moves. Landowner access agreements with notice periods, particularly on tenanted agricultural land where the tenant has rights over standing crops. Reinstatement specifications agreed in writing, including topsoil separation and compaction.
Standing crops cause the classic delay. Trenching between harvest and drilling is a narrow window in an arable rotation, and missing it can cost a season. Where the evaluation is a pre-determination requirement, that window should drive the planning programme rather than the other way round.
Reporting and what the result changes
The evaluation report describes each trench, with plans and sections, the deposits encountered, finds, environmental samples and a statement of significance. It should say what was not found as clearly as what was.
The result changes three things. It sets the mitigation, it sets the budget for that mitigation, and where significance is high it can change the layout, because avoiding remains through design is usually cheaper than excavating them. Where remains would be lost, the assessment of harm follows the ordinary heritage tests, and on a designated asset that means engaging with the substantial harm and justification test.
Budget for the whole sequence. Fieldwork is the visible cost, and post-excavation assessment, archive and publication is the one that sits unfunded at the end of a project and holds up condition discharge.
Negotiating the strategy before mobilising
Agree the written scheme of investigation with the curator in advance, and make sure it states the sample percentage, the trench positions, the method, the contingency arrangements for expanding a trench, and who decides. The contingency clause matters. Without it, a positive result stops the fieldwork while three parties exchange emails.
Set out the archaeological position in the Heritage Statement submitted with the application so that the evaluation result and the mitigation proposal are read together rather than separately.
This is general guidance, not legal advice. Check the requirement with your local planning authority and its archaeological adviser 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.
Frequently asked questions
What percentage of a site has to be trial trenched?
There is no national standard. Local authority archaeological advisers commonly specify between 2 and 5 per cent of the application area, with the lower end where potential is low and the higher end where the desk study or geophysics shows known remains. Ask for the basis of the figure in writing so that it can be discussed on evidence.
Can I refuse to trench part of the site?
You can argue for reduced coverage where access is constrained by standing buildings, live operations, services, contamination or protected trees. Show the constrained area on a plan, calculate coverage against the accessible area, and offer something in return such as a phased approach after demolition or a watching brief over the constrained ground.
Do I need geophysics before trial trenching?
Not always. Geophysical survey gives poor results over made ground, on some drift and igneous geologies, near services and fences, and on hard standing. Where the geology and land use suit it, it focuses the trenching and usually reduces the total cost. Where they do not, it produces a report that changes nothing.
How long are evaluation trenches and why?
The conventional trench is around 30 metres by 1.8 metres, which is long enough to intersect linear features running in any direction and wide enough for safe working at shallow depth. The array should be oriented across the dominant alignment of any known field system, because trenches laid along boundaries miss them systematically.
What happens if the evaluation finds something?
The result feeds the mitigation strategy, which may be preservation in situ through layout changes, strip map and record, open area excavation or a watching brief. Avoiding remains through design is usually cheaper than excavating them, so a positive result early enough to change the masterplan is worth more than one delivered late.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.