Designing an archaeological field evaluation

An archaeological field evaluation exists to answer one question before the council decides: what is under this field, and how much does it matter? Get the design of that evaluation right and it costs a few weeks and produces a decision the officer can rely on. Get it wrong and you end up with a blanket condition requiring open-area excavation of a site whose extent nobody has measured, which is the worst outcome available to a developer.

The National Planning Policy Framework (August 2026 edition) expects an applicant to describe the significance of any heritage asset affected, including any contribution made by its setting, and expects a desk-based assessment followed where necessary by a field evaluation where a site may include assets with archaeological interest. Check the current policy code on gov.uk, because the Framework was re-issued on 17 August 2026 with policy-coded chapters. The practical effect has not changed: evaluation belongs before determination, not after.

Why evaluation comes before determination

A condition requiring archaeological work after consent works where the risk is known and bounded. It works badly where nobody knows whether the site contains a Romano-British settlement or nothing at all, because the developer has then bought land and secured a permission with an open-ended liability attached.

Curators know this, and so do lenders. An evaluated site with a defined mitigation strategy is a financeable site. An unevaluated site with a broadly worded condition is a discount on the land value and an argument at every stage.

Evaluation before determination also lets you change the layout. Once the permission is granted and the drawings are fixed, the only remaining response to a discovery is excavation, which is the most expensive option on the list.

Starting from the desk-based assessment

Nothing should be designed without the Archaeological Desk-Based Assessment in front of you. It draws on the Historic Environment Record, historic mapping, aerial photographs, lidar, geological mapping and previous investigations nearby, and it produces a statement of archaeological potential by period.

That potential statement is what the evaluation tests. A site with a known Iron Age enclosure cropmark on its southern half and alluvial deposits on its northern half needs two different techniques, not one grid of trenches spread evenly. Geoarchaeological boreholes answer the alluvium; trenches answer the enclosure.

The assessment should also flag the constraints that change the route entirely: a scheduled monument on or adjoining the site brings scheduled monument consent under the Ancient Monuments and Archaeological Areas Act 1979 into play, which is a separate consent from the Secretary of State advised by Historic England.

What geophysics can and cannot see

Magnetometry is fast, non-intrusive and low in cost relative to trenching, and it is the first technique most curators expect on a greenfield site. It detects contrasts in magnetic susceptibility, which means it sees ditches, pits and kilns well on chalk, gravel and limestone geologies.

It sees very little on clay with high background variation, on igneous or heavily magnetic geology, or on land with made ground, modern services, fencing, buried debris or pipelines. It does not reliably detect timber structures, ephemeral surfaces, stone walls without a contrasting fill, or burials. Earth resistance survey and ground penetrating radar answer some of those cases, and cost more per hectare.

The error to avoid is treating a blank magnetometer plot as a clean bill of health. Historic England's guidance on geophysical survey in archaeological field evaluation is explicit that geophysics informs where to trench rather than replacing trenching, and curators apply it that way.

Trench layout and sampling levels

Trial trenching is the technique that produces datable, stratified evidence, and its design comes down to coverage, layout and trench dimensions.

Coverage is expressed as a percentage of the developable area. In England, sampling levels commonly sit between 2 and 5 per cent, with higher levels where the desk-based assessment identifies strong potential or where the curator has local experience of a particular deposit type. There is no national rule, which is why the figure is negotiated rather than assumed.

Layout usually combines two approaches. Targeted trenches test specific anomalies from the geophysical survey or cropmarks. A systematic array, typically staggered or herringbone across the remainder, tests the blank ground so that a negative result carries weight. A scheme that only targets anomalies cannot say anything about the rest of the site.

Trench dimensions follow the machine. A 1.8m wide toothless ditching bucket and a 30m length is the common unit, stripped under archaeological supervision to the first archaeological horizon or to undisturbed natural, whichever comes first. Longer trenches give better stratigraphic context; more shorter trenches give better spatial coverage. Discuss the trade with the curator rather than presenting a fixed grid.

Agreeing the strategy with the curator

The evaluation is carried out under a written scheme of investigation approved by the local planning authority's archaeological advisor, usually the county archaeologist or an equivalent service. Get their agreement to the scheme in principle before the contractor prices it.

Bring three things to that conversation: the desk-based assessment, the geophysical results if you have them, and an outline layout with a stated sampling percentage. Ask what they expect on comparable sites in the area, and ask whether they have unpublished results from adjoining land. That last question saves more money than any other, because curators hold local knowledge that never reaches a published source.

Agree the reporting timetable and the standard at the same time. Work to the Chartered Institute for Archaeologists standard and guidance for field evaluation, and say so in the scheme, because it sets the expectations for recording, finds treatment and archiving that the curator will hold you to.

Reading a negative result

Trenches that find nothing are a result, not a failure. They reduce the probability of significant remains across the sampled area, and where the sampling was systematic and adequate, curators will usually accept that no further work is needed, or that a watching brief during groundworks is proportionate.

They do not prove absence. A 4 per cent sample leaves 96 per cent unexamined, and small, discrete features such as burials, corn dryers and isolated structures can sit between trenches. Say this plainly in the report rather than overclaiming, because an overclaimed negative is the thing that gets challenged.

Where the results are negative but the deposits are deeply buried under alluvium or colluvium, the evaluation may have stopped above the archaeology entirely. Depth of stripping should be recorded trench by trench for exactly this reason.

What the results do to the layout

Positive results give you choices, and the earlier they arrive the more choices there are.

• Preservation in situ by designing development away from the remains, leaving them under public open space, car parking on a raised build-up, or a landscaped buffer.
• Preservation in situ by foundation design, using a piled solution with a suspended slab, or raising site levels so that services and foundations sit above the archaeological horizon.
• Preservation by record, meaning open-area excavation of the affected part, with post-excavation assessment, analysis, publication and archive deposition.
• A combination, which is what most large sites end up with.

Open space and drainage basins are the obvious places to put remains, and they interact with other site constraints. Where a scheme is also delivering habitat, the overlap is worth planning deliberately rather than discovering late, and the note on archaeology and biodiversity net gain on the same site covers where the two regimes pull in different directions.

Programme and cost

Three parts of the programme are routinely underestimated.

Mobilisation takes longer than expected because contractors book out in the spring and autumn. Fieldwork itself is weather-dependent and slows sharply in a wet winter on clay. And post-excavation assessment, specialist analysis, publication and archive deposition run for months after the machines leave, with the archive deposition fee charged by the receiving museum.

Budget the whole sequence at the outset, not just the trenching. Where the evaluation leads to full excavation, the mitigation cost is an order of magnitude larger, and it belongs in the land appraisal rather than in a contingency. The scope of the Archaeological Field Evaluation and the way it is reported are what determine whether that later figure can be estimated with any confidence.

For the application itself, the evaluation results feed the Heritage Statement alongside any built heritage assessment, so that the officer reads one account of significance rather than two.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. Councils usually expect a suitably qualified author for archaeological work, and professional review is strongly advised. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.

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