Archaeological trial trenching: what evaluation proves
Trial trenching converts an unknown archaeological liability into a costed mitigation scheme. The sample, the layout and the timing all need agreeing before the machine arrives.
Archaeological trial trenching tells you three things a desk study cannot: whether archaeological remains survive, how deep they sit, and how good they are. It does not tell you the site is clear. A negative evaluation on a 2 per cent sample means nothing was found in the 2 per cent that was dug, and every planning archaeologist reading your report knows that.
For a developer, trenching is a risk pricing exercise as much as a heritage one. The evaluation converts an unknown liability into a costed mitigation scheme, which is why it belongs in the acquisition programme rather than in the run-up to submission.
When an authority asks for field evaluation
National policy requires an applicant to describe the significance of any heritage assets affected, including those with archaeological interest, and to submit a desk-based assessment and, where that is not enough, a field evaluation. Cite the current National Planning Policy Framework edition when you quote that requirement, since the August 2026 edition uses policy codes rather than the paragraph numbers carried in older reports.
In practice the trigger is the local authority's archaeological advisor, often a county archaeologist or a shared service. They read the Historic Environment Record entry for the site and its surroundings, consider the geology and the previous land use, and form a view on potential. Sites near known settlement, along river terraces, inside an Archaeological Priority Area or on well-drained gravels attract attention quickly.
Scheduled monuments are a separate regime entirely. Works affecting a scheduled monument need scheduled monument consent under the Ancient Monuments and Archaeological Areas Act 1979, and that is not something a planning condition can resolve.
Start with an Archaeological Desk-Based Assessment prepared by a suitably qualified archaeologist to CIfA standards. Councils expect a competent author for this work, and professional review is strongly advised. The assessment is what the archaeological advisor uses to decide whether evaluation is needed at all, and a good one sometimes closes the question.
Geophysics, fieldwalking and trenching compared
Each technique answers a different question, and using the wrong one wastes a season.
Geophysical survey, usually magnetometry on open ground, maps buried features across large areas quickly. It works well on gravels and chalk where ditches and pits contrast with the natural. It performs poorly on clay with flints, on igneous geology, near overhead lines and fences, and on made ground. It shows anomalies rather than archaeology, and it cannot date anything.
Fieldwalking recovers artefact scatters from ploughsoil and needs recently ploughed, weathered arable. It costs little, it identifies broad activity areas, and it is useless on pasture or a site under stubble.
Deposit modelling using boreholes or test pits is the urban and alluvial equivalent, mapping made ground, buried land surfaces and waterlogged deposits where organic survival may be exceptional.
Trial trenching is the only one that gives you stratigraphy, dating evidence and a view on condition. It is also the only one that damages what it finds, which is why the sample is negotiated rather than assumed.
The usual sequence on a greenfield site is desk study, then geophysics, then targeted and blank-area trenching. Skipping geophysics to save time normally costs more, because untargeted trenching over a large site is a poor use of machine hours.
Agreeing the sample and the layout
Sampling is expressed as a percentage of the developable area. Two per cent is a common starting point on lower potential sites, rising to four or five per cent where the desk study or the geophysics suggests more. There is no national figure, and the archaeological advisor sets the level for the site.
The layout matters as much as the percentage. A typical evaluation trench is 30 metres long and about 1.8 to 2 metres wide, laid out either on a regular grid or a staggered herringbone pattern so that linear features running in any direction are intercepted. Targeted trenches sit over geophysical anomalies. Blank-area trenches test the ground where the survey showed nothing, and they are the ones developers try to delete.
All of this is set out in a written scheme of investigation approved by the archaeological advisor before work starts. Get an Archaeological Field Evaluation scoped against that document rather than against a contractor's standard method statement, and agree the sample in writing. A dispute about whether the sample was adequate, raised after the trenches are backfilled, cannot be resolved without digging again.
| Technique | What it gives you | Where it fails |
|---|---|---|
| Desk-based assessment | Known record, land use history, potential | No direct evidence of survival |
| Magnetometry | Rapid plan of anomalies over large areas | Unsuitable geology, made ground, interference |
| Fieldwalking | Surface artefact scatters | Needs ploughed arable, no depth information |
| Trial trenching | Stratigraphy, dating, condition | Samples a small percentage, destructive |
Reading the evaluation report
Skip to the conclusions last. The parts that carry the commercial information are the trench descriptions and the deposit sequence.
Look for the depth of the natural geology below existing ground level, because that number decides whether your foundations, drainage and level changes will reach archaeology at all. A site with 1.2 metres of colluvium over features is a very different proposition from one where features are cut into the surface at 0.3 metres.
Then read the finds and environmental assessment. Pottery that dates features tightly, waterlogged deposits with preserved organics, or human remains all change the mitigation. Note the phrase used about significance, since a report describing remains as being of more than local importance is telling you something about the mitigation to come.
Finally, check what the report does not cover. Access constraints, standing crops, service easements and a refused trench position all leave gaps, and those gaps come back later as a condition.
When remains are found
Two routes exist and the choice is not always yours.
Preservation in situ means the remains stay in the ground and the development is designed around them. That can mean moving a building, raising levels with a geotextile separation layer, using a piling matrix designed to minimise disturbance, or leaving an area as open space. Where remains are of national importance, whether scheduled or not, national policy expects preservation in situ, and that is a design constraint rather than a negotiation.
Preservation by record means excavation before construction, followed by post-excavation assessment, analysis, publication and archive deposition. Budget for the whole sequence. The fieldwork is visible and the post-excavation programme is the part that surprises people, because analysis, specialist reporting and publication can run to a substantial share of the total and continue long after the site is built.
A Heritage Statement should carry the archaeological conclusions into the planning case, setting out significance, the effect of the proposal and the mitigation, rather than leaving the evaluation report to speak for itself in an appendix.
Programming it before you commit
Evaluation takes longer than the fieldwork suggests. You need landowner access, a written scheme approved by the archaeological advisor, a contractor with a slot, ground conditions that allow machine access, and then four to eight weeks or more for reporting depending on the finds.
Crops constrain the window. Geophysics and trenching on arable land are far easier between harvest and drilling, and a scheme that misses that window waits a year on a farm that will not accept standing crop damage.
The practical advice is to run evaluation during the option or due diligence period, not after exchange. The result changes what the land is worth, and it changes the layout. Where a pre-commencement archaeological condition is proposed instead, remember that section 100ZA of the Town and Country Planning Act 1990 requires your written agreement to a pre-commencement condition, so read the wording before agreeing rather than after.
This is general guidance, not legal advice. Archaeological evaluation should be designed and carried out by suitably qualified professionals, and for specialist technical reports qualified professional input may be required.
If you are scoping the heritage evidence for a site, enter the postcode on PlanScope to see which reports the constraints are likely to trigger.
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Frequently asked questions
What percentage of a site is usually trenched?
Two per cent of the developable area is a common starting point on lower potential sites, rising to four or five per cent where the desk study or geophysical survey suggests more. There is no national figure. The local authority's archaeological advisor sets the level for the site and records it in the written scheme of investigation.
Does a negative evaluation mean the site is clear?
No. A negative result means nothing was found within the sampled area, which is a small fraction of the site. Archaeological advisors normally still recommend a condition requiring a watching brief or further work during groundworks, particularly where access constraints or standing crops left parts of the site untested.
Is geophysical survey enough on its own?
Rarely. Geophysics maps anomalies rather than archaeology, cannot date anything, and performs poorly on clay geology, made ground and near fences or overhead lines. It is most useful as a targeting tool that makes trenching efficient, so the usual sequence is desk study, then geophysics, then targeted and blank-area trenches.
What does preservation in situ mean for my layout?
It means the remains stay in the ground and the development works around them. In practice that can mean relocating buildings, raising levels over a separation layer, adopting a piling matrix that limits disturbance, or leaving an area as open space. Where remains are of national importance, national policy expects this outcome.
When should archaeological evaluation be programmed?
During the option or due diligence period rather than after exchange, because the result affects land value and layout. Allow time for a written scheme of investigation to be approved, for contractor availability, and for four to eight weeks or more of reporting. On arable land the practical window falls between harvest and drilling.
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.