Soil Study G2 AVP in France: Why It Is Essential
What Is a Soil Study For?
Before laying the first stone, you need to know what lies beneath your feet. A soil study — also called a geotechnical survey — analyses the ground conditions to determine what type of foundations will safely support your house. Without one, you are building blind.
Clay soils, a shallow water table or old backfill can cause cracking, differential settlement or even structural failure. A soil study is the only reliable way to anticipate these risks.
Georisques?} B -->|High / Medium| C[G2 AVP MANDATORY] B -->|Low| D[G1 recommended] A -->|By project phase| E{Project phase?} E -->|Before purchase| F[G1 ES / PGC] E -->|Design| G[G2 AVP] E -->|Execution| H[G2 PRO] style A fill:#0F4C81,stroke:#0F4C81,color:#fff style B fill:#FDFCF9,stroke:#C67A3C,color:#0F4C81 style E fill:#FDFCF9,stroke:#C67A3C,color:#0F4C81 style C fill:#CD212A,stroke:#CD212A,color:#fff style D fill:#F58220,stroke:#F58220,color:#fff style F fill:#6B5876,stroke:#6B5876,color:#fff style G fill:#56C6A9,stroke:#56C6A9,color:#fff style H fill:#0F4C81,stroke:#0F4C81,color:#fff
The Different Types of Geotechnical Studies
The French standard NF P 94-500 defines several missions, classified by project phase. This framework is widely referenced even outside France:
| Mission | Full Name | When? | Purpose |
|---|---|---|---|
| G1 ES | Site study | Before purchase | Identify general geological risks for the plot |
| G1 PGC | General construction principles | Preliminary design | Initial guidance on adapting to soil conditions |
| G2 AVP | Preliminary design | Design phase | Define geotechnical assumptions and possible foundation types |
| G2 PRO | Detailed design | Execution | Precisely size foundations and geotechnical structures |
| G3 | Execution study & monitoring | Construction | Verify compliance during works |
| G4 | Geotechnical supervision | Construction | Independent external review |
💡 Tip — For a single-family home, the G2 AVP is the most common and most useful mission. It will tell you concretely which foundation type to plan and at what depth.
G1: The Preliminary Study
The G1 mission is a first-level diagnosis. It relies on:
- Analysis of geological maps
- Consultation of public databases (in France: Géorisques, InfoTerre; in other countries: national geological surveys)
- Possibly one or two light boreholes
It gives an overview but is not sufficient to size foundations.
G2 AVP: The Design-Phase Study
This is the key study for a builder or project manager. The G2 AVP includes:
- Deep boreholes (core sampling, pressuremeter tests, penetrometer tests)
- Laboratory tests on collected samples
- A hydrogeological analysis (water table level)
- Foundation recommendations with several costed options
G2 PRO: The Final Sizing
The G2 PRO extends the G2 AVP by providing the definitive sizing of foundations. It is carried out by the structural engineer in coordination with the geotechnical specialist. It is essential for complex projects (very heterogeneous soil, sloping site, basement).
When Is a Soil Study Mandatory?
The ELAN Law (France, 2018 — enforced from 2020)
Since 1 January 2020, French law (loi ELAN) requires a geotechnical study for:
- The sale of buildable land located in a zone with medium or high exposure to clay shrink-swell → the seller must provide a G1 PGC
- The construction of a single-family home in those same zones → the builder must carry out at least a G2 AVP
⚠️ Warning — Even if your plot is not in a clay zone, a soil study is strongly recommended. Foundation-related damage accounts for over 50% of claims reported to ten-year warranty insurers.
How Do You Know If Your Plot Is Affected?
In France, visit the Géorisques website (georisques.gouv.fr) and enter your plot address. The clay shrink-swell exposure map will show the risk level:
- High exposure → study mandatory
- Medium exposure → study mandatory
- Low exposure → no legal obligation, but recommended
In other countries, check with your national geological survey or local building authority for equivalent soil risk maps.
How Much Does a Soil Study Cost?
| Study Type | Price Range | Average Timeframe |
|---|---|---|
| G1 ES / PGC | €500 – €1,500 | 2 to 3 weeks |
| G2 AVP | €1,500 – €4,000 | 3 to 5 weeks |
| G2 PRO | €3,000 – €8,000 | 4 to 6 weeks |
The cost depends on the plot size, location, number of boreholes required and the complexity of the subsoil.
💡 Tip — Relative to the total cost of a house, the soil study represents less than 1% of the budget. It is a negligible investment compared to the risk of unsuitable foundations, which could cost tens of thousands of euros in remedial work.
What Does the Soil Study Report Contain?
A G2 AVP report typically includes:
- Geological context — description of the formations present, site history
- Borehole results — soil profiles, mechanical resistance, water levels
- Risk analysis — shrink-swell, liquefaction, landslide, cavities
- Geotechnical assumptions — bearing capacity, expected settlement
- Foundation recommendations — type (shallow, semi-deep, deep), depth of embedment, construction measures
- Additional requirements — drainage, rainwater management, earthworks
The Direct Impact on Your Foundations
The soil type directly determines the foundation type — and therefore a significant part of the structural budget:
| Soil Type | Recommended Foundation | Estimated Extra Cost |
|---|---|---|
| Rock or compact gravel | Standard strip footings | Baseline (no extra cost) |
| Stable clay | Reinforced strip footings with tie beams | + €5,000 to €10,000 |
| Sensitive clay (shrink-swell) | Raft foundation or micropiles | + €10,000 to €25,000 |
| Backfill or heterogeneous soil | Piles or micropiles | + €15,000 to €40,000 |
| High water table | Waterproof tanking + perimeter drainage | + €10,000 to €30,000 |
⚠️ Warning — A builder who quotes you a price without a prior soil study is taking a major risk. If the foundations specified in the contract do not match the actual ground conditions, the additional costs will fall on you — or result in a costly contract amendment.
How Does a Soil Study Work in Practice?
- Initial contact — You contact a certified geotechnical consultancy. Provide them with the site plan and the proposed building footprint.
- Site investigation — A team arrives with a drilling rig. They carry out several boreholes (typically 3 to 5 for a single-family home) spread across the building footprint.
- Laboratory testing — Samples are analysed: particle size distribution, Atterberg limits, moisture content, strength tests.
- Report writing — The geotechnical engineer compiles the data and formulates recommendations.
- Delivery — The report is issued to the client, the project manager and the structural engineer.
Checklist Before Ordering Your Soil Study
- Check clay shrink-swell exposure on Géorisques (or your national equivalent)
- Request several quotes from certified geotechnical firms
- Provide the site plan with the proposed building footprint
- Ensure the commissioned mission is a G2 AVP (not just a G1)
- Verify that the report includes foundation recommendations
- Share the report with your builder or project manager BEFORE signing the contract
- Factor any foundation surcharges into your overall budget
- Keep the report: it will be requested by your building warranty insurer
Key Takeaway
A soil study is not an administrative formality — it is a technical safeguard that protects your project and your investment. Mandatory in clay zones under French law since 2020, it remains indispensable everywhere else. For a few thousand euros, you avoid tens of thousands in remedial costs and you start your build on solid ground — literally.