Made ground: what fill on your site means in planning

Made ground is soil that people put there. In a site investigation report it means material deposited by human activity rather than laid down by natural geological processes, and finding it changes three things at once: what might be in the ground, how the ground behaves under load, and whether surface water will soak away. On a small housing site that normally means one more round of investigation before the foundation design is fixed, and a line in the planning submission explaining how the risk has been dealt with.

The British Geological Survey maps it as artificial ground, split into made ground (material placed on the original surface), worked ground (cut away), infilled ground (cut away and then backfilled), landscaped ground and disturbed ground. Those categories come from map interpretation, not from a hole in the ground, so they tell you to go and look rather than telling you what is there.

What counts as made ground

Anything from a 300mm layer of imported topsoil in a garden to 8m of demolition rubble in a filled quarry void. The label says nothing about quality. Clean engineered fill placed and compacted in layers under supervision is made ground. So is an unrecorded tip of ash, clinker and household waste from the 1950s.

Age matters more than most people expect. Fill placed before the 1990s was rarely tested, rarely documented, and often contains whatever the site owner wanted to dispose of. Fill placed in the last 20 years under a materials management plan usually has validation records you can ask for, which can save a round of testing.

What you are trying to establish is three things: the thickness of the fill, what it is made of, and whether it is still settling.

Spotting it before you design anything

Start with the free desk sources, because they cost nothing and they tend to decide whether you need a full Phase 2 investigation.

1. Historical Ordnance Survey maps, which show ponds, pits, quarries, railway sidings, works and tanks that were later levelled. A pond shown in 1900 and gone by 1938 is an infilled void until proved otherwise.
2. The BGS GeoIndex, which shows the mapped artificial ground polygons and the location of any borehole or trial pit records already lodged for the area.
3. Neighbouring planning applications on the council's public register, which often carry someone else's site investigation for land 50m away.
4. The site itself. Brick, tile, clinker, glass and plastic in the topsoil, a mounded or unnaturally flat profile, differential settlement in an old boundary wall, or vegetation that dies back over one strip in a dry summer.

The pattern that should stop you is an obvious former void with no records at all. That is where the cost sits, and where the programme risk sits, because the council will usually condition intrusive investigation before any development starts.

Contamination and ground gas

Two separate mechanisms come out of fill, and they need different evidence.

The first is chemical contamination. Planning practice guidance on land affected by contamination puts responsibility for making a site safe on the developer, and the local planning authority will expect a preliminary risk assessment built around a conceptual site model: a source, a pathway and a receptor. Break any one of the three and the linkage is not complete. In practice, on a small residential site, the receptors that drive the assessment are the future occupiers and their garden soil, plus any controlled water beneath.

The second is ground gas. Anything with organic content in it (household waste, timber, peat that has been buried under fill, made ground over an old marsh) generates methane and carbon dioxide as it degrades, and carbon dioxide also comes from chalk and from carbonate-rich fill reacting with acidic groundwater. Gas monitoring is a series of visits over weeks, not a single reading, and it is meant to capture falling atmospheric pressure, because that is when gas is drawn out of the ground. Councils often ask for a minimum number of monitoring rounds before they will discharge the condition, so start it early.

Where a gas risk is confirmed, the answer is a designed protection system for the building rather than a statement of good intentions. A Contaminated Land Assessment sets out the Phase 1 findings and, where it is needed, the Phase 2 testing and remediation strategy behind that design. Land contamination work sits in the group of report types where a suitably qualified and experienced person has to do the assessment, and an AI-assisted draft is a starting point for that specialist rather than a replacement.

What fill does to foundations

Natural strata have been consolidating for thousands of years. Fill has not. Uncontrolled fill can carry on settling under its own weight for decades, it varies in density over a few metres, and it frequently sits on a buried soft horizon that was never stripped before tipping began.

That produces the failure most often seen on bargain plots: a slab or strip footing founded partly on fill and partly on natural ground, which then cracks along the join. Founding wholly on uncontrolled fill is not normally accepted for housing.

The realistic options are to dig through to competent strata with deep trench fill, to pile, to use a reinforced raft designed for the predicted movement, or to excavate and replace the fill as engineered fill with compaction testing. Which one costs least depends almost entirely on the depth of the fill, which is why the trial pit depths in the Phase 2 scope matter more than the number of holes. A Geotechnical Assessment draws the ground model and the bearing capacity together, and its conclusions feed the structural design that Building Control will check separately from planning.

Illustrative example: a plot behind a Victorian terrace where a 1960s garage was demolished and the hardstanding buried. Trial pits find 1.2m of brick rubble over stiff clay across two thirds of the footprint and 0.4m over the rest. Trench fill to the clay is straightforward on that depth. Had the pits found 3.5m of ash fill over soft alluvium, the same house would need piles, and the cost difference is the reason nobody should price a plot before the pits are dug.

Infiltration and drainage in fill

Made ground often looks permeable and behaves badly. Soakaway testing follows the BRE Digest 365 method, with three test cycles in each trial pit, and results in fill are commonly erratic between holes a few metres apart because the material is not uniform.

There is a second objection that catches people out. Even where the fill drains, infiltration through contaminated ground can mobilise contaminants towards groundwater, and in a source protection zone the Environment Agency will resist it. Discounting infiltration on those grounds is a legitimate position, but it has to be evidenced in the Drainage Strategy with the test results and the contamination findings, not asserted. Councils and lead local flood authorities see too many strategies that skip straight to a connection to the public sewer.

What the council actually asks for

Validation lists differ by authority, so check the local list for the application type. For a minor residential scheme on suspected fill, the request is usually a Phase 1 preliminary risk assessment with the application, then conditions requiring Phase 2 intrusive investigation, a remediation strategy, a verification report and a reporting-of-unexpected-contamination clause before occupation.

Write the Phase 1 so it answers the case officer's question directly. State the site history with map dates, name the potential sources, set out the conceptual site model, and say plainly whether intrusive work is needed. A desk study that ends without a recommendation gets a holding objection from environmental health, and that costs two weeks at validation stage for no reason at all.

And keep the records. The verification report is what a future buyer's solicitor asks for, and it is what stops the same testing being paid for twice.

Enter your postcode on PlanScope to see which of the reports a site with suspected fill is likely to need, and browse the full set of report types if you are scoping a submission from scratch.

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.