The deposit model in an archaeological desk-based assessment

An archaeological deposit model is the part of a desk-based assessment that answers the question a planning archaeologist actually has: at what depth, and in what condition, would archaeological remains survive on this site. Everything else in the assessment supports it. A list of nearby findspots without a deposit model tells the county archaeologist that the area is interesting, which they already knew, and tells them nothing about whether your basement will destroy anything.

The model is a reconstruction of the ground, built from records rather than from excavation.

What a deposit model shows

At its simplest it is a sequence: modern made ground over earlier made ground, over whatever natural deposits lie beneath, with the horizons at which archaeological material would be expected marked against measured levels.

A usable model states four things. The level of the current ground surface, in metres above ordnance datum rather than in depths below an unspecified datum. The thickness and character of made ground. The level and nature of the natural deposits, whether terrace gravel, alluvium, brickearth or bedrock. And the levels at which any archaeological horizon is predicted, with the basis for that prediction.

On a riverside or valley-floor site the model carries more weight than anywhere else, because deep alluvial sequences can preserve waterlogged organic remains and palaeoenvironmental evidence that dry sites do not. Those deposits are also the ones most often sliced through by a piled foundation without anyone realising.

The sources that build it

Desk-based work draws on a defined set of sources, and an assessment that has used fewer than these will be sent back.

• The Historic Environment Record for the study area, with a stated search radius and a reason for it.
• The National Heritage List for England for designated assets, including scheduled monuments and listed buildings with below-ground interest.
• Historic mapping: Ordnance Survey editions, tithe and enclosure awards where they exist, and estate plans.
• Geological mapping and borehole records held by the British Geological Survey.
• Previous archaeological investigations on and around the site, including unpublished grey literature.
• Aerial photography and, where available, lidar data.
• Geotechnical investigation reports for the site itself, which are usually the most valuable source and the one most often overlooked.

That last point is worth dwelling on. Site investigation boreholes drilled for foundation design record the stratigraphy an archaeologist needs, with levels. Asking the geotechnical engineer for the logs costs nothing and transforms the assessment.

Modelling survival against previous disturbance

Potential and survival are different questions, and conflating them is the commonest weakness in these documents.

A site can sit in the middle of a Roman town and retain nothing, because a 1960s building with a full-footprint basement removed the deposits. A site with no recorded finds at all can retain everything, because it has been a garden since the medieval period.

Mapping truncation means identifying what has already cut into the ground. Former basements and their extent. Service trenches, which are narrow but numerous. Previous foundations, with their type and depth. Quarrying, ponds and infilled ground. Wartime damage and subsequent clearance.

The output is usually a plan showing areas of likely good survival, partial survival and total removal, with the evidence for each. That plan is what allows a foundation design conversation to happen, because it shows where piles can go without harm and where they cannot.

Expressing potential by period

State potential period by period rather than as a single verdict, and say what the potential is for.

Attach a confidence statement to each. A high potential asserted on the basis of a single findspot 400 metres away is weaker than a moderate potential supported by an excavation on the adjoining plot, and saying so is better practice than hiding the difference behind the same word.

Avoid the trap of writing potential as negligible because nothing is recorded. Absence of record is often absence of investigation, particularly on sites that have never been developed in the modern era.

Where field evaluation becomes unavoidable

The desk-based assessment is a first step, and national policy expects field evaluation where the desk-based work cannot establish the significance of what may be present.

Three situations usually trigger it. A site of any size with predicted good survival in an area of known archaeological activity. A site where the deposit model itself is uncertain because no subsurface data exists. And a site where the proposed impact reaches below the modelled horizon across a large footprint.

Evaluation takes the form of trial trenching, geoarchaeological boreholes with core recovery, or both. Boreholes suit deep alluvial sequences where trenching is impractical; trenching suits shallow horizontal remains. Scoping the right method with the archaeological adviser before procuring it saves repeating the exercise.

A separate Archaeological Field Evaluation follows a written scheme of investigation agreed with the authority's archaeological adviser. Councils expect a suitably qualified author for both documents, working to the professional institute's standards, and professional review is strongly advised.

Mitigation the archaeological adviser will accept

Preservation in situ is the preferred outcome in national policy where remains are of sufficient significance. In practice that means designing the development around the deposits: raising levels, reducing basement extent, using a piled solution with a low ground displacement and a demonstrably small footprint of impact, or relocating the most intrusive elements.

Where in situ preservation is not achievable, preservation by record is the fallback, delivered through a written scheme of investigation secured by condition. That might be a watching brief on groundworks, a strip map and sample excavation, or full excavation of a defined area, with post-excavation analysis, archiving and publication funded.

The cost of the last stage surprises developers. Post-excavation assessment, analysis, specialist reporting and archive deposition routinely cost more than the digging, and a condition requiring them is enforceable.

How the assessment should conclude

End with a clear statement of significance, a clear statement of predicted impact, and a proposed mitigation strategy that the adviser can turn into a condition.

Do not end with a recommendation that the matter be dealt with by condition without saying what the condition should require. That reads as avoidance, and it produces a condition drafted by someone who has not read your evidence.

Where the site includes a scheduled monument, scheduled monument consent is a separate requirement under the Ancient Monuments and Archaeological Areas Act 1979, and planning permission does not substitute for it. The heritage case for standing buildings belongs in a Heritage Statement, and an Archaeological Desk-Based Assessment should cross-refer to it rather than repeat it.

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. For specialist technical reports, qualified professional input may be required.