When a geotechnical investigation is needed for planning
Most small schemes need no ground investigation for planning. Mining legacy, made ground, sloping sites, shrinkable clay and infiltration drainage are the triggers that change that.
Most single-dwelling applications do not need a geotechnical investigation to get planning permission. Ground conditions are usually a building control matter, dealt with under Approved Document A when the foundation design is submitted. Planning asks a narrower question: is the site suitable for the proposed use given known ground risks. Where the answer depends on something the council cannot verify from a desk study, you will be asked for ground investigation at application stage, and on a handful of site types that request is predictable enough to budget for from the start.
What planning needs and what building control needs
Planning is concerned with whether the development is acceptable in principle and whether risks from land instability and contamination can be managed. The Planning Practice Guidance on land stability puts the responsibility for establishing ground conditions on the developer, and the local planning authority then decides whether the evidence is enough to grant permission, with or without conditions.
Building control is concerned with the foundation itself: bearing capacity, depth, heave protection, ground gas protection where needed. That work happens after permission, to Approved Document A and Eurocode 7, and it is the reason most householder schemes need nothing geotechnical at the planning stage.
The two overlap when the ground risk is large enough that no foundation solution might be acceptable, or when managing it would change the development in a way the council should know about before granting permission. A retaining wall two metres high on a boundary is a planning matter as well as a structural one.
The triggers that make it predictable
Six site characteristics account for most planning-stage investigation requests on small schemes.
- A Development High Risk Area for coal mining, where a coal mining risk assessment is expected with the application and the national mining body is consulted.
- Other mining and extraction legacy, including chalk and lime workings, brine extraction, tin and slate, and unrecorded shallow workings.
- Made ground and infilled land, particularly former quarries, ponds, watercourses and back-filled cellars, which show on historic Ordnance Survey editions.
- Sloping ground where the scheme needs cut, fill or retaining structures, or where there is any history of movement in the area.
- Shrinkable clay with existing or removed trees, because heave and desiccation control foundation depth and sometimes the footprint.
- Alluvial soils, peat and soft ground near watercourses, where settlement rather than bearing failure is the risk.
Add a seventh for schemes relying on infiltration drainage. If the surface water strategy proposes soakaways, the authority will want infiltration testing to BRE Digest 365 before it accepts the drainage principle, and that is ground investigation by another name.
Scoping a proportionate investigation
A single new dwelling does not need the investigation a nine-storey block needs. Scope in stages and stop when the question is answered.
- Desk study first: historic maps, geological mapping, mining records, the council's contaminated land register enquiry, well and borehole records and anything from neighbouring applications on the public file.
- A walkover, recording levels, seepages, existing retaining structures, cracking in nearby buildings and the position of trees.
- Intrusive work only where the desk study leaves a question open, sized to that question.
- Laboratory testing limited to what the design needs, with contamination testing scoped separately where a potentially contaminative former use is identified.
- Reporting that states what was found, what remains uncertain and what the design response is.
For a straightforward plot, trial pits and hand-dug or window-sampled holes answer most questions. Boreholes come in when depth matters, when groundwater has to be monitored over time, or when mining legacy has to be proved or disproved at depth. Ask the geotechnical adviser to write the scope around the specific planning question rather than ordering a standard package.
The standards that govern the work
| Document | What it covers |
|---|---|
| BS 5930 | Code of practice for ground investigations, planning and execution |
| BS EN 1997 (Eurocode 7) | Geotechnical design, including ground investigation reporting requirements |
| BS 10175 | Investigation of potentially contaminated sites, where contamination is in scope |
| BRE Digest 365 | Soakaway design and infiltration testing |
| Approved Document A | Structural requirements at building regulations stage |
Naming the standards in the report matters more than it sounds. An authority's technical consultee is looking for evidence the work was done to a recognised method by someone competent, and a report that cites nothing reads as an opinion.
Land contamination investigation is a separate regime with its own competency expectations, and a suitably qualified and experienced person is expected to prepare Phase 1 and Phase 2 reports. Do not let a geotechnical scope quietly absorb a contamination question.
Reporting for a planning audience
A planning officer is not going to read shear strength plots. Put a one-page summary at the front that answers four questions in plain terms: what the ground is, what the risks are, whether the development is feasible, and what has to happen next. Then the technical content behind it.
Where the answer is conditional, say what the condition should be. Officers are far more comfortable granting permission with a condition the applicant has proposed and justified than inventing one themselves, and a proposed condition that meets the six tests in the Planning Practice Guidance, that it is necessary, relevant to planning and to the development, enforceable, precise and reasonable, will usually survive into the decision notice.
The Geotechnical Assessment is the document that carries this, and on conversions where the existing structure is doing the work, a Structural Survey answers a different question about the building rather than the ground.
Conditions requiring later investigation
The common outcome on small sites is permission subject to a condition, to be discharged before development starts, requiring ground investigation, a remediation or foundation strategy where needed, and a verification report before occupation. That is a reasonable landing point, and it keeps the investigation cost in the construction budget rather than the planning budget.
Two cautions. Conditions of that kind now require the applicant's written agreement to the wording before they are imposed, so read the draft wording rather than discovering it on the decision notice. And a condition that defers the whole question is not available where the ground risk is the reason the site might be unsuitable. You cannot condition away an unanswered question about whether development is possible at all.
Where piling is likely, the vibration and noise effects come with their own conditions, as set out in piling vibration and the conditions that control it. Basements are a category of their own, with a separate audit process described in basement impact assessment: what the auditor checks.
Costs and programme
Fees vary widely with access, depth, the number of exploratory holes and laboratory scope, so get two or three quotes against a written scope rather than a description of the site. What catches self-builders out is programme rather than price.
Rig availability, laboratory turnaround, and any groundwater monitoring period all take real time, and monitoring in particular cannot be compressed: a series of readings over several weeks is a series of readings over several weeks. Infiltration testing has to happen before the drainage strategy is finalised, which means before the layout is finalised, which means earlier in the programme than most people plan for.
So on any of the trigger site types, commission the desk study while the design is still on paper. It costs little, it is quick, and it tells you whether you are budgeting for trial pits or for boreholes before you have committed to a layout that depends on the answer.
PlanScope drafts a Geotechnical Assessment structured to the sections a planning authority looks for and flags every site-specific figure it cannot verify rather than filling it in, which is essential when the whole point of the document is measured ground data. For specialist technical reports, qualified professional input may be required.
Enter your postcode on PlanScope to see which ground and structural documents your site is likely to need.
This is general guidance, not legal advice, so 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.
Frequently asked questions
Is ground investigation a planning requirement or a building control one?
Usually building control. Foundation design is assessed under Approved Document A and Eurocode 7 after permission. Planning asks whether the site is suitable for the proposed use given known ground risks, so investigation is requested at application stage only where the desk study leaves that question open or where managing the risk would change the development.
What is a coal mining risk assessment?
A report submitted with applications in a Development High Risk Area, setting out recorded mine entries, shallow workings and other coal mining features affecting the site, and the design response. The national mining body is consulted on such applications, so check whether the site falls within a high risk area before assuming nothing is needed.
Do soakaways need testing before permission?
Where the surface water strategy relies on infiltration, expect the authority or the lead local flood authority to want infiltration testing to BRE Digest 365 before accepting the drainage principle. That testing is ground investigation, it has to happen before the layout is finalised, and a failed infiltration rate can change the whole drainage design.
Can the council defer investigation to a condition?
Often yes, through a condition discharged before development starts, requiring investigation, a remediation or foundation strategy and a verification report. Conditions of that kind need the applicant's written agreement to the wording. What cannot be deferred is whether the site is developable at all, because that goes to the principle of permission.
How early should the desk study be commissioned?
While the design is still on paper. It is quick and inexpensive relative to intrusive work, and it tells you whether to budget for trial pits or boreholes before the layout is committed. Groundwater monitoring and laboratory turnaround cannot be compressed, so late commissioning delays the application rather than saving money.
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.