Ground investigation categories under Eurocode 7
The geotechnical category classifies the design problem, not the building. Assign it from the desk study, then scope boreholes and testing to match rather than to a rule of thumb.
The Eurocode 7 geotechnical category decides how much ground investigation your project needs, and most self-builders are told a figure for boreholes before anyone has worked out which category applies. The categories are a risk framework: Category 1 covers small simple structures on ground already understood locally, Category 2 covers conventional foundations needing measured data, and Category 3 covers everything unusual or difficult. Get the category right and the scope follows from it.
The category is also provisional. You assign it at the start on the information available, then review it when the desk study and the first trial pits come back, because ground has a habit of being more interesting than expected.
What the geotechnical categories classify
The categories classify the design problem rather than the building. They combine the complexity of the ground, the consequence of getting it wrong and the loads involved, and they tell the engineer what kind of evidence is needed before a foundation can be designed.
That is why two identical houses can sit in different categories. A detached house on a well documented gravel terrace, next door to three others built the same way, is a different design problem from the same house on a sloping site over made ground with a stream at the bottom.
A note on versions. The second generation of the Eurocodes is being published, and the revised geotechnical standard restructures this framework around consequence and complexity classes rather than the familiar three categories. National annexes follow the base standards, and the transition runs over several years. Ask your engineer which version they are designing to, and expect both frameworks to be referred to during the changeover.
Category 1 and its limits
Category 1 is for small and relatively simple structures where the risk of unacceptable settlement or instability is negligible, where ground conditions are known from comparable local experience, and where design can be based on routine methods and qualitative investigation.
The standard gives indicative limits, and they are tighter than people assume. Simple one and two storey houses and agricultural buildings with a maximum design column load of around 250 kN and a maximum wall load of around 100 kN. Retaining walls and excavation support where the difference in ground level does not exceed about 2 m. Small excavations for drainage and pipework.
What a Category 1 investigation looks like in practice: a desk study pulling the geological map, historical maps, borehole records from the national archive and a coal mining report where relevant, a walkover, and hand-dug or machine-dug trial pits to foundation depth with a description of the strata and the groundwater observed.
Two traps catch self-builders here. Shrinkable clay with trees nearby takes foundation depths well beyond what a trial pit at 1.2 m will suggest, and warranty provider standards often drive the depth more than the Eurocode does. And a site that looks simple on the surface may sit on made ground from a demolished building, which takes you straight out of Category 1.
Category 2 and routine investigation
Category 2 covers conventional structures and foundations with no exceptional risk or difficult ground or loading conditions. It requires quantitative geotechnical data and analysis, which in practice means intrusive investigation and laboratory testing.
Most house plots, small housing schemes, basements of modest depth, retaining walls above the Category 1 level, spread and raft foundations, piles, embankments and ground anchors sit here.
A typical scope:
- Desk study and conceptual ground model, prepared before anyone drills anything.
- Windowless sampling or cable percussive boreholes positioned to cover the footprint and any change in ground conditions across the site.
- Trial pits for shallow information, foundation inspection and services.
- In situ testing, which usually means standard penetration tests in granular soils and hand vane or pressuremeter work depending on the ground.
- Laboratory testing on recovered samples: moisture content, plasticity, shear strength, compressibility, sulphate and pH for concrete class.
- Groundwater monitoring over more than one visit, because a single reading in August is not the winter level.
The number and depth of exploratory holes should come from the conceptual model, not from a rule of thumb. A geotechnical assessment prepared to the Eurocode framework and the British Standard code of practice for ground investigations should explain why the scope is what it is, and councils expect a suitably qualified author with professional review strongly advised.
Category 3 and specialist work
Category 3 is everything outside the other two. Very large or unusual structures, abnormal risks, very difficult ground, areas of probable site instability or persistent ground movement, and loading conditions outside normal experience.
For a self-builder the realistic triggers are specific rather than exotic:
- A site on or near a landslip, a quarry face or a degraded cliff.
- Deep basements, particularly below the water table or close to a neighbouring foundation.
- Former mineral extraction, shallow coal workings, mine entries or brine extraction areas.
- Peat, alluvium or soft organic deposits of significant thickness.
- Dissolution features in chalk or gypsum, including swallow holes.
- Made ground of unknown origin and depth, or a former landfill.
Category 3 means the standard's own rules are not enough on their own, and the design needs additional analysis, specialist investigation techniques and often monitoring during construction. It also means a longer programme. Geophysics, rotary cored boreholes, inclinometers and instrumented trial excavations take weeks rather than days.
| Category | Typical project | Investigation character |
|---|---|---|
| 1 | Simple house or farm building, known ground, shallow works | Desk study, walkover, trial pits, qualitative |
| 2 | Conventional foundations, piles, modest basements, retaining walls | Boreholes, in situ and laboratory testing, quantitative |
| 3 | Unstable ground, mining legacy, deep basements, unusual loads | Specialist techniques, additional analysis, monitoring |
Scoping the intrusive work to the category
The sequence that saves money is always the same. Desk study first, conceptual ground model second, intrusive work scoped against that model third.
Commissioning boreholes before the desk study is the most common waste on small sites. The desk study tells you where to look, how deep to go and what to test for, and it will sometimes tell you that trial pits are enough.
Combine the geotechnical and the contamination scopes in one visit. The same holes can serve a land contamination assessment under the investigation standard for potentially contaminated sites, which must be prepared by suitably qualified and experienced persons, and ground gas monitoring wells can be installed at the same time. Our note on characteristic situations in a ground gas assessment explains how that classification works alongside the geotechnical one.
And agree the laboratory schedule before the rig leaves site. Samples degrade, and a return visit for one triaxial test costs more than the test.
The ground investigation report and the design report
The Eurocode framework expects two documents, and conflating them is a common source of argument with building control.
The ground investigation report presents the factual data and its interpretation: the exploratory hole records, the test results, the groundwater observations, the ground model and the derived geotechnical parameters. It is the evidence.
The geotechnical design report takes those parameters and sets out the design: the foundation solution, the assumptions, the design values used, the calculations, the construction requirements, and what has to be inspected or monitored on site. It is the decision.
For a single house the two are often bound together, which is fine, as long as the factual data and the design assumptions are separately identifiable. What is not fine is a four page letter giving a bearing capacity with no borehole logs behind it, and a building control officer is entitled to ask for the evidence.
Where planning needs ground information and where building control does
This trips up almost every first-time self-builder. Foundation design is a building control matter under the Building Regulations, and the planning authority has no interest in your bearing pressures.
Planning does need ground information in defined circumstances. Where the land is unstable or potentially unstable, national policy expects the stability issue to be addressed and, if necessary, a site stability report submitted. Where the land may be contaminated, a Phase 1 desk study and often Phase 2 investigation are validation requirements. Where the site is in a coal mining referral area, a mining risk assessment is normally required. Where a basement is proposed in one of the London boroughs with a basement policy, a basement impact assessment covering hydrology, hydrogeology and ground stability is expected, and those authorities audit them.
Drainage is the other overlap. Infiltration testing to establish whether a soakaway will work belongs in the drainage strategy at planning stage, and it uses the same trial pits.
So the honest answer for a self-build is that you commission the ground investigation once and use it twice. Scope it to the geotechnical category, add the contamination and infiltration testing the planning authority will ask for, and you avoid paying for two mobilisations.
This is general guidance, not legal advice; geotechnical design needs a qualified engineer, and ground conditions are site specific, so check the position with your local planning authority or a qualified professional before relying on it.
You can browse all 66 report types to see which ground related documents a scheme usually needs, or enter your postcode on PlanScope to see the constraints already recorded for your site. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
What is a geotechnical category?
A risk classification in the Eurocode for geotechnical design that combines ground complexity, the consequence of failure and the loads involved. Category 1 covers simple structures on well understood ground, Category 2 conventional foundations needing measured data, and Category 3 unusual or difficult cases requiring specialist investigation and additional analysis.
Does a single house need boreholes?
Not always. A simple house on ground well understood from local experience can sit in Category 1, where a desk study, a walkover and trial pits to foundation depth may be sufficient. Made ground, shrinkable clay with trees, sloping sites or mining legacy will take the project into Category 2 or 3.
Why do the desk study first?
Because it tells you where to put the holes, how deep to go and what to test for. Commissioning intrusive work before the geological maps, historical maps, archived borehole records and any mining report have been reviewed is the most common waste of money on small sites, and it often leads to a second visit.
What is the difference between the two reports?
The ground investigation report presents the factual data and its interpretation, including hole records, test results, groundwater observations and derived parameters. The geotechnical design report sets out the design itself: the foundation solution, the design values, the calculations and what must be inspected on site. They can be bound together but must remain distinguishable.
Will the planning authority ask for ground investigation?
In defined circumstances. Land stability, potential contamination, coal mining referral areas, basements in boroughs with a basement policy, and infiltration testing for drainage all generate planning requirements. Foundation design itself is a building control matter, so scope one investigation that serves both and avoid paying for two mobilisations.
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.