Preservation in situ as a design constraint

Preservation in situ means leaving archaeological remains in the ground, undisturbed, and designing the development around them. On sites where the remains are of the highest importance that is the outcome national policy expects, and it arrives as a design constraint rather than as a cost line. The consequence is usually a change to the foundation strategy, sometimes a change to the footprint, and occasionally the loss of a basement or a core position.

Developers who treat archaeology as a programme item to be discharged by condition find this out late. The layout is fixed, the engineer has priced a piled raft, and the county archaeologist asks for a scheme that disturbs a fraction of the deposit.

When in situ preservation is expected

The heritage chapter of the National Planning Policy Framework, August 2026 edition, treats remains of archaeological interest in two groups. Scheduled monuments and non-designated remains of demonstrably equivalent importance are handled as designated assets of the highest significance, where loss should be permitted only in the rarest cases. Everything else is weighed in a balance, and recording followed by removal is a legitimate outcome.

So the practical question on any site is which group the remains fall into. For a scheduled monument the answer is settled: works affecting it need scheduled monument consent from the Secretary of State under the Ancient Monuments and Archaeological Areas Act 1979, separately from planning permission, and refusal is likely where the works would damage the monument.

For undesignated remains, the judgement is made by the local planning authority on the advice of its archaeological adviser, usually a county archaeologist or a Historic Environment Record officer. That advice turns on the evaluation results, not on the desk based assessment alone.

Significance drives everything downstream

Two sites with identical Roman pottery scatters can produce opposite advice. What separates them is the condition and rarity of the deposits, their capacity to answer research questions, and whether comparable evidence has already been excavated nearby.

Waterlogged organic remains, stratified urban sequences with surviving floor levels, industrial structures with surviving process evidence and human burials all tend towards in situ preservation. Truncated features in a field that has been ploughed for two centuries usually do not.

The evaluation is what establishes this. A desk based assessment sets out what is known and predicts what might survive, and it is written to Chartered Institute for Archaeologists standards by a suitably qualified author. But it cannot tell you the depth of the surviving horizon or whether the deposits are waterlogged. Trial trenching or geoarchaeological boreholes do that, and those results are what the engineer needs.

Foundation strategies that avoid disturbance

The aim is to transfer building loads to competent ground without excavating through the archaeological horizon and without changing the conditions that have preserved it. In rough order of increasing intervention:

1. Raise the development on imported fill above the deposits, with a geotextile separation layer and a spread foundation bearing on the new material.
2. Use a ground bearing raft cast on a capping layer above the horizon, with services run within the build up rather than trenched through it.
3. Use widely spaced piles with ground beams spanning between them, so that the disturbed area is limited to the pile positions.
4. Excavate and record the affected area, which is preservation by record rather than in situ preservation.

The first option is attractive and frequently impossible, because raising levels conflicts with access gradients, flood levels or the height of neighbouring property. It works best on brownfield sites where a demolition platform already sits above the horizon.

Piling layouts and load transfer

Where piling is the answer, the negotiation is about the proportion of the deposit area disturbed and the position of that disturbance. Both matter. A pile that clips the edge of a known structure is a worse outcome than one placed in a location already truncated by a Victorian cellar.

Practical moves that reduce impact:

• Move to fewer, larger diameter piles rather than many small ones, then compare the total plan area displaced.
• Shift the column grid so that piles fall in areas already disturbed, which requires the evaluation plan and the structural grid to be overlaid at an early stage.
• Use ground beams or transfer slabs to span sensitive zones instead of adding pile positions within them.
• Avoid pile caps that need excavation below the horizon; design for a cap within the capping layer where the pile head can be raised.

The displacement method is relevant too. Driven piles cause vibration and lateral displacement into adjacent deposits, while bored piles remove material but disturb a narrower zone. Continuous flight auger piles are common on urban sites for this reason, though the arisings themselves can require archaeological monitoring.

Pile testing needs thinking about as well. A preliminary test pile in a sensitive area, installed before the mitigation strategy is agreed, has occasionally caused the damage the strategy was written to avoid. Agree the test pile position with the archaeological adviser at the same time as the layout.

Services, drainage and the access road are the parts everyone forgets. A soakaway or an attenuation tank will usually disturb more of the horizon than the building does. So will a new foul connection dug at 1.2 metres across the site.

Monitoring and long term protection

In situ preservation only works if conditions stay stable. The risk is not the construction moment but the decades afterwards. Dewatering during construction, permanent drainage that lowers the water table, oxygenation of previously anaerobic deposits, and chemical changes from concrete or made ground can all destroy remains that were left in place to protect them.

Authorities respond with conditions. Expect a written scheme of investigation approved before work starts on site, monitoring during groundworks, and on sensitive sites a burial environment monitoring regime with dip wells and an agreed reporting schedule. Some conditions bite after occupation, which means a management obligation that runs with the land.

Where any part of the resource is lost, the recording and publication obligation follows, and the post-excavation assessment, archive and publication stage is the part most often under budgeted.

Testing feasibility before the layout is fixed

The sequence that works is unglamorous. Commission the desk based assessment at the option stage. Agree an evaluation scope with the archaeological adviser before you submit, and get trenches or boreholes in while the layout is still a sketch. Overlay the results on the structural grid and the drainage strategy. Then ask the engineer for two foundation options with the disturbed area quantified for each.

Doing that gives you something to negotiate with, which is worth more than an argument about policy. A scheme that arrives with a quantified impact and a stated alternative is in a different position from one that arrives with an offer to agree mitigation later.

One more practical point. Where the deposits sit under the proposed access road rather than under the building, the answer may be a no dig construction using a cellular confinement system above a geotextile, which spreads the load without excavation. Highway adoption standards may then be the obstacle instead, because an adopting authority will want a specification it recognises.

The honest uncertainty is cost. Pile redesign, transfer structures and imported fill all carry a price, and on a marginal site the archaeology can be the thing that makes the scheme unviable. Better to find that out from an evaluation before exchange than from a condition after permission.

Archaeological reports are documents councils expect from a suitably qualified author, and professional review before submission is strongly advised. PlanScope can draft the assessment structure, the policy framework and the significance narrative, then flag the site specific evidence that only an evaluation can supply, including where an archaeological field evaluation is needed. Where harm cannot be avoided, the tests explained in the note on the substantial harm test and the justification it demands apply.

Enter your postcode on PlanScope to see which reports a site with known archaeology is likely to need.

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.