Writing the archaeological desk-based assessment baseline

The baseline is the part of an archaeological desk-based assessment that a county archaeologist actually reads. Everything downstream, the statement of potential, the significance assessment, the recommendation for evaluation or a watching brief, rests on whether you gathered the right sources and read them honestly. A weak baseline produces a document that gets a holding objection and a request for more work, which costs the client another month.

CIfA's standard for historic environment desk-based assessment sets the frame. What follows is how that frame is filled for an English planning application.

What the standard expects you to collect

A desk-based assessment is defined by its sources, and the sources are not optional. At minimum you are pulling from the Historic Environment Record held by the county or unitary archaeological adviser, the National Heritage List for England for designated assets, historic mapping from the earliest available county surveys through the Ordnance Survey series, and the record of previous archaeological interventions on and around the site.

Add to that the geological and soils mapping that tells you what deposits could survive, aerial photographic and LiDAR coverage, and the grey literature from any nearby excavation, evaluation or watching brief. Where the site is in London, the borough's Archaeological Priority Area mapping is a source in its own right.

State in the report which sources you consulted, when, and what each returned, including nil returns. An HER search that produced nothing within 200 m is a finding, and writing it down stops a reviewer assuming you skipped the search.

Setting the search radius

One kilometre from the site boundary is the usual radius for the HER search on a small or medium site. It is a convention rather than a rule, and it should be justified rather than asserted.

Widen it where the archaeology of the area is dispersed, where the site sits in a landscape of known linear features such as trackways or field systems, or where the setting of a designated asset is engaged. Narrow it only with agreement, and record that agreement.

The safest approach on anything above a householder scale is to write to the county archaeologist before starting, describe the site and the proposal, and ask what radius and what sources they expect. Ten minutes of correspondence at the outset removes the most common reason these assessments come back.

Reading survival and disturbance

Potential means nothing without survival. A site with strong Roman potential and a 1970s basement car park across two-thirds of its footprint has almost no deposits left under the car park, and saying so plainly is more useful than a general statement of high potential.

Work through the disturbance history:

1. Previous buildings and their foundation types, from historic OS sheets and any available drawings, with particular attention to cellars and basements.
2. Made ground, from geotechnical logs and borehole records where a site investigation exists.
3. Services and drainage runs, from utility searches and from the site plan.
4. Quarrying, landfill and ground raising, visible on historic mapping and in the geology.
5. Cultivation, terracing and levelling, which truncate shallow features without removing deeper ones such as pits, ditches and wells.

Then say what that means for each period represented in the baseline. Deep features often survive under truncation that has removed everything above them, and a report that concludes "low potential due to modern development" without asking whether cut features survive is the sort of thing an evaluation trench later contradicts.

Turning data into potential and significance

Two different statements are needed and they are frequently muddled.

Potential is a judgement about what might be present, expressed period by period, with a stated confidence and the reasoning behind it. Significance is a judgement about the heritage interest of assets that are present or likely to be present, in the terms the national policy framework uses, covering archaeological, architectural, artistic and historic interest and, where relevant, the contribution of setting.

The historic environment chapter of the National Planning Policy Framework, in the edition published on 17 August 2026, requires an applicant to describe the significance of any heritage assets affected, including any contribution made by their setting, and to use a level of detail proportionate to the assets' importance and no more than is sufficient to understand the potential impact. Where a site includes or has the potential to include assets with archaeological interest, a desk-based assessment and, where necessary, a field evaluation are the expected route. Historic England's advice on managing significance in decision-taking is the practical companion to that.

Write the potential statement so it can be tested. "Moderate potential for later prehistoric activity in the western half of the site, based on two cropmark enclosures 400 m to the north-west and an absence of recorded post-medieval disturbance in that area" is testable. "The site has archaeological potential" is not.

Illustrative example: a paddock with cropmarks nearby

Take a 0.8 hectare paddock on the edge of a village, proposed for eight dwellings. The HER records two cropmark enclosures 400 m to the north-west, undated, identified from 1970s aerial photographs. The site itself has no HER entries. Historic mapping shows it as pasture on every edition back to the tithe survey. Geology is a river terrace gravel over clay.

That combination is a moderate potential case, not a low one. Undated cropmark enclosures on terrace gravels usually indicate later prehistoric or Roman settlement, the site has never been built on, and pasture is the land use most likely to leave sub-surface features intact. The absence of HER entries on the site reflects an absence of investigation rather than an absence of archaeology, and the assessment should say exactly that.

The recommendation writes itself: geophysical survey across the developable area, followed by trial trenching targeted on any anomalies and on a percentage sample of the blank areas, scope agreed with the archaeological adviser. This example is illustrative and the figures in it are not drawn from a real site.

Recommending evaluation and the pre-commencement condition

Most assessments end in one of three recommendations: no further work, pre-determination evaluation, or a condition requiring a programme of archaeological work.

Pre-determination evaluation is requested where the potential is high enough that the council cannot judge the acceptability of the scheme without knowing what is there. It is unpopular with clients because it costs money before permission exists, and it is the right answer more often than the industry admits, particularly where a scheme could be redesigned to preserve deposits in situ.

Where a condition is the outcome, expect it to require a written scheme of investigation approved in writing before works begin, followed by the fieldwork, then post-excavation assessment, archive deposition and publication. Draft the assessment so the recommendation matches the condition the archaeologist will suggest, because a mismatch invites a second round of consultation. If the scheme also affects standing fabric, the archaeological field evaluation stage may sit alongside recording work rather than replacing it.

Where drafts overstate and understate

Two failure modes recur.

Overstatement usually comes from listing every HER entry within the radius as though proximity were evidence. Twenty medieval find spots in a village core tell you the village is medieval. They do not, on their own, make a paddock on the edge of it high potential.

Understatement comes from treating the absence of records as the absence of archaeology, and from over-reading modern disturbance. Both produce a document the archaeological adviser will not accept, and the second is worse, because it collapses at the evaluation stage after the client has budgeted on a nil finding.

An archaeological desk-based assessment is a report councils expect to see authored by someone with the relevant competence, and CIfA membership is the usual marker. PlanScope drafts the structure, the policy framing and the baseline template, and inserts flags where site data is missing rather than filling gaps with invention, but the source searches, the professional judgement on potential and the recommendation need a qualified archaeologist. Professional review is strongly advised before submission.

Where the scheme also engages built heritage, the assessment should be read alongside the heritage statement rather than duplicating it, since the two documents answer different limbs of the same policy test.

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

Browse all 66 PlanScope report types to see how the archaeology documents fit into a full application pack. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.