Setting the search radius in an archaeological desk assessment

The archaeological desk based assessment search radius is the first decision in the report and the one that determines whether the rest of it is worth reading. Choose 250 m on a site where the relevant evidence sits 800 m away and the assessment will conclude that potential is low, the county archaeologist will disagree, and the application will attract a condition requiring evaluation that could have been scoped out at the start.

There is no statutory radius. The institute standard for desk based assessment expects a study area proportionate to the site and the questions being asked, and expects the choice to be justified. That justification is a paragraph most reports leave out.

Why the radius decides the quality of the assessment

A desk based assessment is an argument about potential built from the pattern of known evidence around a site. The radius defines the evidence base, so it defines the argument.

Two failure modes follow from getting it wrong. A radius that is too tight produces a record of silence, which reads as low potential and is actually an absence of investigation. A radius that is too wide produces 400 records, most of them irrelevant, and a report that nobody can use to make a decision.

The right radius is the one that captures the archaeological landscape the site sits in. On a river terrace that means the terrace. In a historic town centre it means the burgage plot pattern and the line of the defences. On a chalk ridge it means the barrow cemetery two fields away.

Typical radii and when to widen them

A radius of 1 km from the site boundary is the working default for most applications, and it is what most Historic Environment Records expect when you request a search.

Five circumstances justify going wider. A scheduled monument or registered park within reach, because the setting assessment needs a different geography from the buried archaeology assessment. A known archaeological landscape that extends beyond the default, such as a field system or a linear earthwork. Alluvial or peat deposits where the palaeoenvironmental sequence is regional rather than local. A site in an area with very little previous investigation, where the nearest comparable excavation may be some distance away. And a scheme large enough that the site itself spans more than one landscape character area.

Going narrower needs more justification than going wider, and in dense urban areas it is often right, because the density of records within 250 m exceeds what a 1 km search would produce in the countryside.

State the radius, give the reason, and show the search area on a figure with the site outlined. An assessment that reports results without showing the search area is asking the reader to take the evidence base on trust.

Historic Environment Record searches and lead times

The Historic Environment Record held by the county or unitary authority is the primary source and it is the one with a lead time.

Most records charge for a commercial search and most take a couple of weeks or more, with longer waits in summer and before the end of a financial year. A few are available online, and several publish their data through national platforms, but the curated search usually returns more than a self-service query, including grey literature reports that are not indexed anywhere else.

Four things to ask for when you commission the search. Monument records, event records and find spots within the radius. Copies of unpublished reports for interventions on or adjoining the site. Any historic landscape characterisation data for the area. And the record's own view on whether the site falls within a locally defined area of archaeological interest, which in London means an Archaeological Priority Area and elsewhere may be a local plan designation.

Commission it on the day the site is instructed. The search is the item most likely to delay the report, and a planning programme that assumes a two week assessment will slip by a month if the request goes in late.

National datasets, maps and aerial imagery

Around the record search sit sources that cost nothing and are frequently underused.

The national heritage list gives the statutory designations, with list entries whose descriptions often contain archaeological detail that the record summary omits. Historic mapping, from the first edition Ordnance Survey back through tithe, enclosure and estate maps, establishes land use, field boundaries, former buildings and industrial activity, and it is the source that most often explains a cropmark.

Aerial photography is where real discoveries still happen on desk based assessments. Vertical coverage from the 1940s onwards shows field boundaries since removed, wartime activity and earthworks later ploughed out. Oblique photography taken for archaeological purposes shows cropmarks under the right conditions. Airborne laser survey data, published openly for much of England, shows earthworks under woodland and slight relief invisible on the ground.

Geological mapping matters more than people expect. Archaeological potential correlates with drift geology, and a site on gravel behaves differently from the same site on clay. The national geological survey's mapping gives the solid and superficial geology and is the basis for any deposit model.

National planning data platforms carry designation boundaries in machine readable form, which is useful for figure production and for checking that a designation has not changed since the record search.

Borehole data as archaeological evidence

On urban and alluvial sites, geotechnical data is archaeological data and it is usually already in the project files.

Borehole and trial pit logs record the thickness of made ground, the level of the natural surface, and the presence of peat, alluvium and buried channels. From those a deposit model can be built: a set of sections showing where the archaeological horizon is likely to sit and how deeply it is buried.

That model does two things for the application. It establishes whether the proposed foundations will reach the archaeological horizon at all, which on a site with 4 m of made ground and piled foundations may mean very limited impact. And it identifies where organic deposits with palaeoenvironmental potential survive, which is often the most significant resource on a floodplain site and the one least visible in the record.

Ask the geotechnical consultant for the logs rather than the summary report, and ask early, because the two investigations are usually commissioned by different people. A Geotechnical Assessment and an archaeological assessment that share a deposit model produce a far stronger submission than two documents written in ignorance of each other.

The site visit and current land use

A desk based assessment includes a site visit, and reports that skip it are visibly weaker.

What the visit establishes is the condition of the resource. Visible earthworks. Evidence of truncation from previous development, quarrying or landscaping. Current land use and ground cover. Surviving historic structures, boundaries and surfaces. Standing buildings that may contain earlier fabric, which may need separate recording work.

Photograph systematically and reference the photographs to a plan. Where previous development has removed the archaeological horizon across part of the site, the visit is what allows the report to say so, and that single observation can remove a condition.

Concluding on potential rather than on records

The conclusion is the part of the report that gets read, and it should characterise potential by period with a stated level of confidence, not list what the record search returned.

A conclusion that works says, for each period, whether potential is low, moderate or high, on what basis, and what the likely character and significance of any remains would be. It separates the question of whether remains survive from the question of how important they would be, because those drive different responses. And it states what further work, if any, is proportionate: geophysical survey, trial trenching, a watching brief, or nothing.

Say plainly where the evidence is thin. An assessment that acknowledges that no archaeological intervention has ever taken place within 1 km, and that potential is therefore uncertain rather than low, is more credible than one that treats silence as reassurance. That honesty is also what makes an Archaeological Desk-Based Assessment useful for scoping the next stage rather than something to be argued over.

Where the conclusion points to fieldwork, agree the scope with the county archaeologist before commissioning it, since an Archaeological Field Evaluation carried out to the wrong specification has to be repeated. And where designated assets are in play, the setting assessment belongs in a Heritage Statement rather than being folded into the archaeological conclusion.

Councils expect a suitably qualified author for this work and professional review is strongly advised before submission.

Browse all 66 PlanScope report types to see which heritage and archaeology documents your site is likely to need.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional 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.