The written scheme of investigation for field evaluation

A written scheme of investigation is the document that turns "there might be archaeology here" into a priced, programmed piece of fieldwork with a defined end point. For a trial trench evaluation it sets out where the trenches go, what percentage of the site they sample, how they will be dug and recorded, who is doing the work and what happens when something is found. The county archaeologist approves it before a machine moves, and nothing you agreed informally survives contact with that approval.

Developers underestimate it because it looks like a method statement. It is closer to a contract. Once approved, the scheme fixes your sampling obligation, and increasing the trench count later because the curator is unconvinced is a programme problem, not a paperwork one.

What the document commits you to

The scheme is prepared by the archaeological contractor, written to the Chartered Institute for Archaeologists standard and guidance for field evaluation, and issued to the local authority archaeological adviser, usually a county archaeologist or a Historic Environment Record officer.

It names the site, the planning reference and the condition it discharges. It summarises the results of the earlier archaeological desk-based assessment and states what those results predict. It defines the aims in terms an inspector could test: establish the presence or absence of remains, establish their date, extent, character, condition and significance, and gather enough evidence for a mitigation decision. Those aims matter because they set the threshold for when the evaluation has done its job.

It then binds you to specifics. A named project manager with the right level of institute membership. A stated staffing structure. A timetable. An account of how the archive will be deposited and with which museum, which sounds like a formality until you discover the local museum has closed to new deposits and a charge has to be negotiated.

Trench positions, sample percentage and rationale

The sample is the number that drives the budget.

A commonly applied starting point for open greenfield sites is a 2 to 5 per cent sample of the developable area by trenching, with the figure agreed case by case with the curator. Higher percentages appear where the desk-based assessment predicts dense remains or where a scheduled monument sits nearby. Lower figures appear on sites heavily truncated by previous development. There is no national statutory percentage, so the rationale in the scheme is what justifies the number.

Trench layout matters as much as the percentage. A grid gives even coverage and is defensible when the curator's concern is unknown archaeology. Targeted trenches over geophysical anomalies answer specific questions faster. Most schemes use both, with the plan showing each trench numbered, dimensioned and referenced to a site grid, plus a clear statement of which trenches are targeted and on what evidence.

Build in a contingency for trenches that cannot be dug. Live services, standing crops, tree root protection areas and ground conditions all move trenches on the day, and a scheme that allows the curator's agreement to a repositioned trench by email avoids a stop.

Approval by the county archaeologist before work

The approval step is where programmes slip.

The curator is reading for three things: whether the sample answers the questions the desk-based assessment raised, whether the methodology matches the deposit model, and whether the contractor is competent to do it. Comments usually come back within a fortnight on a straightforward site, longer on a complex one and longer again over the summer when field seasons are busy.

Two habits help. Send the draft scheme informally before the formal submission, so the substantive disagreement happens before dates are booked. And agree, in writing, what constitutes approval, because a condition that requires a scheme "approved in writing by the local planning authority" is not discharged by an email from the county archaeologist who advises that authority.

Monitoring, hold points and variation on site

The scheme should say when the curator will visit, what notice they get and what triggers a stop.

Monitoring visits are normally arranged at the start of machining, partway through and before backfilling. Give at least a week's notice of the start date. The scheme should also define hold points, which are the moments when work pauses for a decision: human remains, which bring in the Burial Act licensing route through the Ministry of Justice. Treasure, which triggers the reporting duty under the Treasure Act. Unexpected structural remains of a scale the deposit model did not predict. And contamination or unexploded ordnance, which are safety matters rather than archaeological ones.

Variation is the part most schemes handle badly. Write in the mechanism: a documented request to the curator, an agreed response time, and a note that the variation is appended to the scheme rather than replacing it. Fieldwork rarely matches the plan, and an auditable trail of agreed changes is what protects the condition discharge at the end.

Post-excavation assessment and publication

The trenches are the small part of the bill. The reporting and archiving that follow are where budgets run over, and a scheme that is vague about them is storing up a dispute.

The scheme should commit to a post-excavation assessment within a stated period of the fieldwork ending, specialist assessment of the finds and environmental samples, an updated project design if the results justify further analysis, and deposition of the archive. It should also commit to a summary entry for the Historic Environment Record and, where results warrant it, publication. Grey literature reports are the normal output for a negative or low-significance evaluation. A site producing a regionally important assemblage attracts a publication expectation that can run for years after the houses are sold.

Agree who pays for what at scheme stage. Specialist analysis of a large environmental assemblage is a real cost, and "reasonable" is not a figure.

Timing against the construction programme

These ranges are illustrative and vary by region and contractor. What they show is that an evaluation condition cannot be treated as a two-week item at the end of a pre-commencement list. Where possible, run the evaluation before determination rather than under a condition, because it converts an unknown into a known while the application is still being negotiated, and it removes the pre-commencement condition that would otherwise sit between you and a start on site. The same logic applies across a technical evidence base, as the piece on the ground gas section of a contaminated land assessment shows for a different discipline.

Moving from evaluation to a mitigation strategy

The evaluation ends with one of three outcomes, and the scheme should say what each triggers.

1. Nothing of significance, in which case the archaeological condition can usually be discharged on the report alone.
2. Remains that can be preserved in situ, which pushes the question into layout: foundation design, service routes, landscaping levels and a statement of how the design avoids the deposits. That often reshapes planting and levels, so the landscape work has to move in step, much as the lighting section of a landscape strategy has to follow the layout rather than lead it.
3. Remains that will be lost, which leads to preservation by record through an excavation or a watching brief, covered by a second written scheme of investigation prepared for that purpose.

That second scheme is a different document with a different sampling logic, and pricing the evaluation as if it were the whole archaeological cost is the most common budgeting error on a site with known potential. Where the results are extensive enough to reshape the layout, the design narrative in the planning statement has to be rewritten to explain the change.

Archaeology also interacts with everything else on a major site. Trenching cannot happen inside a tree root protection area without arboricultural sign-off, and it cannot happen during the breeding bird season in scrub without an ecological check first, which is why evaluation programming belongs in the same conversation as the rest of the survey calendar.

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