Preservation in situ of archaeological remains

Preservation in situ means leaving archaeological remains where they lie, undisturbed beneath a development, instead of excavating and recording them before they are destroyed. In England it is the outcome national policy prefers, and it is an architectural problem long before it is an archaeological one. Foundation type, slab level, drainage runs and tree pits all decide whether buried deposits survive. Settle the layout early and the archaeological condition on your permission may be a watching brief rather than a full excavation.

What preservation in situ means

The phrase describes a physical outcome. Remains stay in the ground, in their original stratigraphic relationships, with the overlying soil and the groundwater regime that has kept them stable for centuries.

Its opposite is preservation by record, sometimes called mitigation by excavation. There the remains are dug, drawn, photographed, sampled, analysed and published, and the ground is then cleared for construction. The knowledge survives. The resource does not.

Between the two sits a spectrum that most real schemes occupy. A site may keep its waterlogged timber structures under a raft while a shallow brick cellar in one corner is excavated and removed. The planning question is never simply which approach applies, but how much of each the scheme needs and whether the balance can be defended.

Why policy prefers it

Archaeological deposits are finite. Once a trench is dug the stratigraphy is gone, and no later technique can recover what was not recorded at the time. National policy treats that as a reason to avoid harm first and record only what cannot be saved.

The historic environment chapter of the National Planning Policy Framework (August 2026 edition) asks local planning authorities to weigh the significance of a heritage asset against the public benefits of the proposal, and treats non-designated assets of demonstrably equivalent significance to scheduled monuments as though they were designated. That matters practically. A buried Roman villa with no statutory designation can attract the same weight as a scheduled monument, which puts substantial harm on a very steep test.

Scheduled monuments themselves sit outside the planning system for consent purposes. Works affecting them need scheduled monument consent from the Secretary of State under the Ancient Monuments and Archaeological Areas Act 1979, and a planning permission does not authorise them.

Design responses that achieve it

Most of the useful moves cost almost nothing at concept stage and a great deal afterwards.

• Raise finished floor levels and build up rather than cut down, so the new load sits above the archaeological horizon on imported fill.
• Delete the basement, or shrink it to a footprint over ground already disturbed by a previous building.
• Keep drainage shallow, use pumped runs where gradients allow, and avoid deep soakaways over sensitive deposits.
• Route incoming services along an existing disturbed corridor, usually the old service trench or the line of a demolished wall.
• Place car parking, amenity space or attenuation at surface level over the most sensitive part of the site.
• Specify shallow-rooting planting, or raised planters, where deep tree pits would otherwise cut through deposits.

A capping layer of clean granular fill, sometimes with a geotextile separator, is a standard way of taking construction and landscaping loads off the archaeology below. The thickness is a design calculation, not a convention, and the county archaeologist will expect to see the load path explained.

Piling, rafts and ground beams

Piled foundations are often presented as the preservation solution, and sometimes they are. A small number of piles with a suspended slab can leave the great majority of a deposit untouched. But piling is still destruction, just distributed differently, and the honest way to describe it is as a measured percentage of deposit volume lost to pile shafts and pile caps.

That calculation is the heart of the negotiation. Reduce pile numbers and increase diameter, and you lose more volume per pile but hit fewer features. Use many small-diameter piles and you lose less volume while scattering damage across the whole site. Which is preferable depends on what is down there: a dense Roman masonry complex argues for fewer, carefully placed penetrations, while homogeneous waterlogged alluvium may tolerate a dispersed pattern.

Three details catch schemes out. Displacement piling drags material downwards and smears deposits far beyond the shaft. Ground beams between pile caps are usually excavated at depth and can remove more archaeology than the piles themselves. And the piling rig needs a working platform, which is itself an imported layer with its own impact on the ground beneath.

A raft avoids all of that at the cost of needing competent bearing material close to the surface. Where a Geotechnical Assessment shows a soft upper horizon, the raft option usually dies early and the argument moves back to pile layout.

Monitoring and long term protection

Remains left in place still change. Waterlogged organic material, the timber, leather and seeds that make urban and wetland sites nationally important, survives only while it stays saturated. Dewatering during construction, permanent drainage, or a new attenuation basin that alters local flow can desiccate deposits over years, and nobody notices until a later evaluation finds them collapsed.

Where that risk exists, expect a condition requiring groundwater monitoring before, during and after construction, with trigger levels and a remedial mechanism. On larger schemes the protection runs through a management regime secured by a section 106 obligation, covering who may dig where in future, how services are renewed and what happens if a later owner wants to add a basement.

Build the monitoring obligations into your programme as soon as the condition is drafted. Several archaeological requirements have to be satisfied before development begins, so they belong on the same list as the other conditions you must discharge before work starts.

Where excavation is accepted instead

Preservation in situ is a preference, not a rule. Authorities accept excavation where the remains are of lesser significance, where their character means that recording adds more to understanding than leaving them undisturbed, or where no viable design keeps them while delivering a development the plan supports.

Truncated urban deposits are a common example. If Victorian cellars and 1960s piling have already cut a medieval sequence into disconnected islands, a careful excavation may produce more knowledge than a preservation scheme that protects fragments with no stratigraphic relationship left between them.

The decision has to be evidenced. That means an Archaeological Desk-Based Assessment establishing what is likely to be present, then an Archaeological Field Evaluation with enough trenches to characterise depth, survival and extent. Guessing the answer and offering a watching brief is the single most common reason archaeology holds an application up.

An illustrative example

Take a four-storey block on a half-hectare plot in a historic city centre, where evaluation trenching finds medieval tenement foundations at 1.2m below existing ground level across the northern half of the site. The team raises the whole slab by 600mm, moves the undercroft parking to the southern half where a 1970s basement has already removed the sequence, and switches the northern foundations to a piled raft with pile positions plotted against the recorded wall lines. Deposit loss falls low enough that the county archaeologist accepts a watching brief and a targeted strip, map and sample in the south. This example is illustrative, and the numbers in it are not a benchmark for any real site.

Making the design case

Write the mitigation strategy as a design narrative, not an archaeological appendix. The officer and the county archaeologist want to see that the scheme was shaped by the remains, with the alternatives you rejected and why.

1. State the significance of the asset, using Historic England's approach to assessing heritage significance, and say which elements carry it.
2. Set out the evaluation evidence: trench locations, depths, what was found and what remains uncertain.
3. Quantify the impact of the proposal, with the deposit volume or area affected by each foundation element.
4. Describe the design changes made to reduce that impact, and the options discarded.
5. Explain the residual harm honestly, and where it is justified, say what public benefits weigh against it.
6. Propose the conditions and the written scheme of investigation you expect to be attached.

Most of that narrative belongs in your Heritage Statement, cross-referred to the evaluation report rather than repeating it. Agree the written scheme of investigation with the county archaeologist before you submit if you can. It converts a likely objection into a drafting exercise.

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, and for specialist technical reports, qualified professional input may be required.