What an archaeological desk-based assessment must cover

A desk-based assessment answers three questions in order. What is already recorded on and around the site. What is likely to be present that has never been recorded. And how much of it can realistically have survived. The third question is the one that decides most applications, and it is the one thinnest in most submitted reports. If you are commissioning an archaeological desk based assessment, judge the draft on whether it engages with survival rather than on how many records it lists.

National policy requires an applicant to describe the significance of any heritage assets affected, including any contribution made by setting, and where a site has potential for assets with archaeological interest, to provide an assessment and, where necessary, a field evaluation. The assessment is the first half of that requirement.

The record search

The Historic Environment Record held by the county council or its equivalent is the core source. It holds monument records, find spots, event records for previous archaeological work, and often the grey literature reports from those events.

Agree the search radius with the curator first. Radii of 500 metres and one kilometre are both common, and the right figure depends on site size, topography and whether setting is in play. Then supplement the record search with the national list of designated assets, recorded finds from the public finds scheme, the national aerial photograph and imagery collections, and publicly available terrain data.

Event records are worth more than monument records on urban sites. A watching brief carried out two streets away in 2008 that found nothing tells you about survival conditions across the block, which no monument record will.

Read the grey literature rather than the summary line. Record entries compress a 60-page excavation report into a sentence, and the sentence rarely mentions the depth at which natural was reached, which is the number the rest of your assessment depends on.

Map regression and documents

Map regression is not a gallery of old maps. It is an argument about how the ground has been used, built on and disturbed, read forward in time.

Work through the sources that exist for the parish: enclosure awards, tithe maps and apportionments, estate plans, early town plans, successive Ordnance Survey editions, wartime and post-war aerial photography, and industrial or railway company plans where relevant. Each edition is compared with the last, and what matters is the change between them, not the content of any single sheet.

Documentary sources fill the gaps. Trade directories, parish registers, published county histories and antiquarian accounts all locate activity, and on some sites they explain features that otherwise look like noise in the geophysics.

Potential, period by period

The assessment should state potential separately for each period rather than giving a single overall rating. A site can have low prehistoric potential and high post-medieval industrial potential, and a single word cannot carry both.

Base the judgement on evidence rather than on convention. Topography, drift and solid geology, proximity to water, the pattern of known sites in the surrounding area and the character of recorded finds all bear on where activity is likely. Well-drained gravel terraces near a river carry different prehistoric potential from heavy clay on a north-facing slope, and the report should say why.

Be careful with absence. A blank area on the record often means nobody has looked there, not that nothing is present, and reports that treat an empty search result as evidence of low potential are the ones curators push back on most often. Say which it is.

Where the site lies within a defined archaeological priority or notification area, say so and explain what the designation reflects, since those areas were drawn for a reason that is usually documented.

Truncation, and why it decides the answer

This is the section that separates a useful assessment from a literature review.

Previous ground disturbance removes archaeology. Victorian cellars across a street frontage, a 1960s basement car park, quarrying, railway cuttings, levelling for a factory floor, and deep services in a back lane all destroy deposits. Where the evidence shows the site was reduced below the level at which remains would survive, the conclusion of low surviving potential follows, and a curator will accept it.

Evidence that supports a truncation argument includes historic mapping showing building footprints, earlier ground investigation logs showing made ground depth, photographs from previous demolition, and the results of nearby archaeological events. Borehole logs from a geotechnical investigation are frequently the most useful single source and are often already available on the project.

Illustrative example: a town-centre site with high medieval potential on the record, where the desk-based assessment used geotechnical logs showing two metres of modern made ground over natural across the whole footprint, together with 1930s photographs of the cellared building that had occupied it. The curator agreed no evaluation was required outside a narrow strip at the rear. That example is illustrative.

Conclusions a curator can use

Vague conclusions generate conditions that are worse for the applicant than precise ones. Write the final section so the curator can lift a recommendation into a condition.

State the potential by period. State the significance of any identified asset in the terms national policy uses. Assess the impact of the specific proposal, including foundation type, piling, basements and service runs, because impact depends on what is being built and not just on where. Then recommend, in order of preference:

1. No further work, where truncation or absence of potential supports it.
2. Targeted evaluation, specifying the technique and the area.
3. Preservation in situ through design, where deposits can be left undisturbed beneath a raft or a piling layout with a low ground disturbance ratio.
4. Preservation by record, where excavation in advance of construction is the proportionate response.

An archaeological desk-based assessment that stops at a statement of potential leaves the curator to invent the mitigation, and the invented version is usually broader than one you would have proposed.

Where it leads next

Field evaluation follows where potential is real and survival is plausible. Geophysical survey suits open ground and struggles on urban sites full of services and rubble. Trial trenching gives direct evidence and is usually sampled at a percentage agreed with the curator. Borehole or test pit survey suits deep urban stratigraphy and waterlogged deposits. All of it runs under a written scheme of investigation agreed in advance, and an archaeological field evaluation carried out without that agreement will not discharge anything.

Two adjacent workstreams are worth coordinating. Standing structures on the site may need historic building recording before demolition, at a level Historic England's guidance defines. And ground disturbance for habitat creation can conflict with buried archaeology, which is why archaeology and biodiversity net gain on the same site need to be planned together rather than in sequence.

Where the site contains a scheduled monument, scheduled monument consent under the Ancient Monuments and Archaeological Areas Act 1979 is a separate consent from planning permission and cannot be conditioned into existence. Finally, the archaeological work should feed a heritage statement that addresses the whole historic environment rather than sitting beside it as an unconnected annexe.

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.