Archaeological desk assessment or field evaluation
The desk study collates existing records and decides where trenches go. The evaluation puts holes in the ground. Most sites with potential need both, in sequence.
A desk-based assessment is a study of existing information. A field evaluation puts holes in the ground. The council's archaeological adviser decides which one your application needs, and the honest answer to the desk based assessment vs field evaluation question is that on many sites you will be asked for both, in sequence, with the first deciding the scope of the second.
The distinction matters for programme more than for cost. A desk study takes weeks and can be commissioned at any time of year. An evaluation needs access, machine hire, a team, a written scheme approved by the adviser and reinstatement afterwards, and none of that happens quickly.
What each one actually is
| Desk-based assessment | Field evaluation | |
|---|---|---|
| Method | Collation and analysis of existing records | Intrusive investigation on site |
| Sources | Historic Environment Record, historic maps, aerial photography, borehole logs, previous fieldwork | Trial trenches, test pits, sometimes geophysical survey first |
| Output | A statement of archaeological potential and likely survival | A record of what is actually present, with extent and condition |
| Timing | Any time of year, weeks | Requires access, approval and reinstatement, months |
| Decides | Whether evaluation is needed and where | What mitigation, if any, is required |
The Archaeological Desk-Based Assessment establishes potential. The Archaeological Field Evaluation establishes fact. Neither replaces the other, and an evaluation designed without a desk study behind it usually puts trenches in the wrong places.
The sources behind a desk study
The Historic Environment Record held by the county or unitary archaeological service is the starting point, and a search of it within a defined radius is the backbone of the report. Around it sit historic Ordnance Survey editions, tithe and enclosure maps, estate plans, aerial photographs including recent lidar coverage, geological mapping, and any borehole or trial pit logs from previous ground investigation on or near the site.
Two outputs earn the fee. A reasoned statement that parts of the site have been truncated by later development, which can remove those areas from any trench sample. And a deposit model that predicts what survives and at what depth, which lets the adviser scope trenching against evidence rather than against a guess.
A desk study that lists records without interpreting them is not doing the job, and advisers say so.
Geophysical survey sits awkwardly between the two stages. It is non-intrusive, so it is not evaluation in the strict sense, and on pasture over chalk or gravel it can map enclosures and ditches across several hectares in days. On clay, alluvium or a former industrial yard it returns noise. Ask the adviser whether a survey would be accepted as part of the evaluation sample before paying for one, because if it will not be, it is an extra cost rather than an alternative.
When trial trenching is required
The adviser asks for evaluation where the desk study identifies potential that cannot be resolved from existing information, and where the scale of the proposal means the answer must be known before the application is determined.
Two positions are common. On a site with high potential and a large footprint, evaluation is required before determination, because the authority cannot assess the impact on an unknown resource. On a site with moderate potential and a smaller footprint, the adviser may accept a condition requiring a written scheme of investigation, with evaluation and any mitigation carried out after permission.
That difference decides your programme entirely. Ask the adviser which position applies at pre-application stage, in writing, before you assume the condition route is available.
Written schemes for each stage
Both stages are carried out under a written scheme of investigation approved by the archaeological adviser, and the two schemes are different documents.
A scheme for evaluation sets out the trench layout and sample percentage, the methodology for machine excavation and hand cleaning, recording standards, environmental sampling strategy, the specialists to be retained, and the arrangements for reporting. A scheme for mitigation, prepared after evaluation, sets out what will be excavated, what will be preserved in situ, the post-excavation programme and the archive deposition arrangement.
Each needs formal approval. Building a fortnight into the programme for approval of each scheme is realistic, and longer where the adviser is a single officer covering several authorities.
Programme and cost, compared properly
The desk study is a fixed cost you can obtain a firm quotation for. Evaluation is not, because the trench sample is agreed with the adviser after the desk study and the ground conditions change the rate of work.
The drivers that move an evaluation budget are the sample percentage, the depth of overburden, whether shoring is needed, contamination on the site, and the season. Wet clay in February costs considerably more to work than the same trenches in June.
And the evaluation is not the end of the spend. A positive result leads to mitigation, and post-excavation analysis, publication and archive deposition can match the fieldwork cost on a site with well preserved remains.
What the adviser expects from each document
Advisers read a lot of these and they are quick to spot a template.
For a desk study: a clearly defined study area with a stated reason, sources listed with dates of consultation, a deposit model, and a conclusion that commits to a position on potential rather than hedging. For an evaluation report: trench locations as dug rather than as proposed, context records, dating evidence, an honest account of what was not reached and why, and a statement of the implications for the development.
Both should be written by someone whose competence the adviser recognises, normally a member of the relevant chartered institute working to its standards. Professional review before submission is strongly advised, and councils routinely reject reports where the author cannot be identified. The competence question is covered in more detail in archaeological competence and registration.
Choosing your next step
If you have nothing yet, commission the desk study. It is the least expensive document in the sequence and it is the one that can remove the rest.
If you have a desk study recommending evaluation, go back to the adviser with it and agree the scope before commissioning anyone. If you have permission with an archaeology condition, read the condition carefully, because it usually commits you to the full sequence through to archive deposition rather than to the evaluation alone.
One last practical point about land deals. Evaluation needs access, and an option agreement that does not grant it, or a tenancy that runs to Michaelmas, can stop the work for a full farming year. Check the access position before you agree a planning programme with anybody, because it is the constraint nobody writes into the appraisal.
For what an adviser looks for in the method statement itself, see written schemes of investigation and what gets approved, and for the decision about when evaluation happens, see archaeological evaluation before or after determination.
This is general guidance, not legal advice. Requirements are set by the local planning authority's archaeological adviser and differ between areas, so confirm the position before commissioning work. 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, and archaeological fieldwork must be carried out by a qualified archaeologist.
Frequently asked questions
Do I need both a desk study and an evaluation?
On a site with archaeological potential, usually yes and in that order. The desk study establishes what is likely to survive and where, which is what allows the trench sample to be scoped sensibly. An evaluation designed without one tends to put trenches in the wrong places, which costs money and answers the wrong question.
Can evaluation be left to a planning condition?
Sometimes. Where potential is moderate and the footprint is small, an adviser may accept a condition requiring a written scheme of investigation with evaluation and mitigation carried out after permission. On sites with high potential and a large footprint, evaluation is normally required before determination. Ask the adviser in writing at pre-application stage.
What is in a written scheme of investigation?
For evaluation: the trench layout and sample, machine and hand excavation methodology, recording standards, environmental sampling strategy, specialists to be retained and reporting arrangements. For mitigation: what will be excavated, what is preserved in situ, the post-excavation programme and archive deposition. Each needs formal approval, so allow time for it.
What makes a desk study worth its fee?
Two things. A reasoned case that parts of the site have been truncated by later development, supported by borehole logs or records of previous building, which can remove those areas from the trench sample. And a deposit model predicting what survives and at what depth, which lets the adviser scope trenching against evidence rather than assumption.
Who should write these reports?
Someone whose competence the archaeological adviser recognises, normally a member of the relevant chartered institute working to its published standards. Councils routinely reject reports where the author cannot be identified or where the work does not follow those standards. Professional review before submission is strongly advised for both document types.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.