Trial trenching sample rates and how they are agreed

Trial trenching on a development site is a sample, not a search. You dig a fraction of the ground, the county archaeologist decides from what you find whether anything more is needed, and the fraction you dig is agreed in advance. Two per cent of a site is a common starting figure and five per cent is common where the desk assessment has flagged real potential, but there is no national standard and the number is whatever the curator will accept for that site.

Getting that number and the trench layout agreed early is worth more than arguing about it later, because the evaluation is the step that decides the size of everything after it.

Why sampling rather than full excavation

Full excavation of a site before permission would cost more than most schemes are worth and would destroy the resource it was meant to assess. The evaluation exists to answer a narrower question: is there anything here, how well does it survive, how extensive is it, and what would development do to it.

The answer feeds a decision about mitigation. Preservation in situ through foundation design, a strip, map and sample exercise across the affected area, a targeted excavation of defined areas, or nothing at all beyond a watching brief.

Because the evaluation informs a decision rather than producing a final record, it is proportionate by design. A curator asking for a higher sample is usually saying that the desk assessment left too much uncertainty for a decision to be made.

Sample rates and what drives them

The percentage is calculated against the developable area rather than the red line, which matters on a site with large areas of retained open space.

Four things push the figure up. A desk assessment showing recorded remains on or immediately adjoining the site. Geophysical survey results with anomalies of archaeological character. A location within an area of known potential, such as a river terrace, a Roman road corridor or an urban historic core. And a scheme whose impact is total, such as one with a basement or extensive ground reduction.

Three things pull it down. Geophysics that has covered the site well and found nothing. Documented past disturbance that has truncated the deposits. And a scheme with a light foundation solution where preservation in situ is realistic.

Sample rate is not the only variable, and treating it as such is a mistake. Twenty trenches placed badly tell you less than twelve placed well.

Trench layout and targeting

The layout argument usually comes down to how much of the sample is targeted on geophysical anomalies and how much is laid out on a regular pattern to test the blank areas.

Curators generally want both. Targeted trenches confirm what the geophysics has found and characterise it. Pattern trenches test whether the geophysics missed anything, which it does routinely on certain geologies and for certain feature types. A layout that is entirely targeted invites the criticism that the negative areas were never tested.

Trench dimensions are usually standard for the machine and the recording method, commonly a long narrow trench excavated in spits under archaeological supervision down to the first archaeological horizon or to natural. Where deposits are deep, stepped or shored trenches are needed and the cost per trench rises sharply.

Submit the layout as a drawing with the geophysics underneath it, so the curator can see the reasoning rather than infer it. A layout drawing with the anomalies visible answers most of the questions a written strategy raises.

Access, services and what constrains the plan

A theoretically ideal layout is often undeliverable, and saying so early avoids a round of revisions.

The constraints that recur are underground services, which require scanning and hand-dug inspection before machine excavation and often force a trench to move. Then standing crops and tenancy arrangements on agricultural land, ground conditions that will not take a tracked machine in winter, trees with root protection areas, and site access for a machine and somewhere to put the spoil.

Deal with them in the written scheme rather than on the day. A strategy that identifies the constrained areas and proposes an alternative for each is approved far more readily than one that promises a grid and then delivers something else.

Where a trench cannot be dug where the curator wanted it, record why and propose a replacement position. Unexplained gaps in a trench plan are read as avoidance.

Reporting and the decision that follows

The evaluation report sets out what was found, its date, condition and extent, and assesses the effect of the proposed development on it. It should be written to the professional standard for field evaluation and submitted to the curator, who then advises the planning authority.

The outcomes divide roughly into four. Nothing found, and no further archaeological requirement. Limited remains, addressed by a watching brief condition. Significant remains across part of the site, addressed by targeted excavation or by redesign to preserve them. Or remains extensive and important enough that the scheme has to change substantially.

The report should say plainly which of those it supports. Evaluation reports that describe the findings without drawing the conclusion leave the curator to draw a more cautious one, and cautious is expensive.

Where the results point towards excavation, the analysis, archive and publication stage follows, and that is the part most often left out of the budget. The scale of it is set out in post-excavation assessment, archive and publication.

Escalation to full excavation

Curators can and do ask for additional evaluation where the first phase produces an ambiguous result. That is not a failure of the strategy: an evaluation that finds the edge of something has done its job and raised a question the sample cannot answer.

Build the possibility into the programme. A second phase of trenching, agreed and carried out promptly, is far less disruptive than one resisted for two months and then conceded.

Where excavation becomes necessary, negotiate its extent against the development's actual impact. Areas that will not be disturbed, or that will be covered by a foundation solution designed to preserve what is beneath, should be outside the excavation footprint. A Heritage Statement covering the whole site can carry that argument where the archaeological report focuses on the remains alone.

Agreeing the strategy early

The sequence that works is to complete the desk assessment and any geophysics, discuss the outline strategy with the curator informally, submit a written scheme of investigation for approval, and only then commission the fieldwork.

Doing it the other way round produces trenching that has to be repeated. A contractor who mobilises against an unapproved strategy risks a curator rejecting the results, and that is a cost nobody recovers.

Agree three things in writing before anyone digs: the sample rate and how it is calculated, the trench layout with the reasoning shown, and what happens if the results are ambiguous. Those three cover most of the disputes that arise later. An Archaeological Desk-Based Assessment that has already mapped the potential across the site makes that conversation much shorter.

Archaeological fieldwork should be carried out by a suitably qualified archaeologist working to recognised professional standards, and PlanScope's drafts support the written submission rather than the fieldwork.

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

Enter your postcode on PlanScope to see which reports a site with archaeological potential is likely to need.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.