How many boreholes does my plot need for a geotechnical study?

There is a question that almost all owners ask at the first contact: "How many holes are you going to make in the ground?" The question is logical. Nobody wants more drilling than is necessary, but nobody wants to pay for an incorrectly designed foundation because the investigation fell short. The honest answer is that it depends, but not in an arbitrary way. It depends on specific technical criteria that the Building Technical Code sets out with quite a lot of precision.

What very few people know is that the number of boreholes is not decided by the geologist based on their personal judgement, nor by the owner based on their budget. It is determined by a combination of three factors: the plot area, the type of structure being designed, and the geotechnical complexity of the ground. Understanding these three factors lets you go into the meeting with the technician knowing exactly what is being discussed, and to spot whether someone is offering you less than your plot really needs.

The CTE leaves no room for improvisation

The CTE's Basic Document for Structural Safety, in annex B, sets out a classification that every geotechnical investigation must follow. Before determining how many boreholes are necessary, the technician must assign two parameters to your specific case: the ground group and the type of construction.

The ground is classified into three groups. Group T-1 corresponds to favourable ground, with little variability and no special problems: compact gravels, sands, cohesive soils in a firm state. Group T-2 consists of intermediate ground, with some lateral variability or materials whose behaviour is less predictable. Group T-3 covers unfavourable ground: soft clays, expansive soils, anthropogenic fill, karst, and the presence of water. 

Constructions are also divided into three types. C-1 groups works of small scale: single-family houses of one or two storeys, retaining walls of less than three metres. C-2 includes buildings of between three and ten storeys and structures of moderate complexity. C-3 are large buildings, special works, or infrastructures with significant loads.

"The minimum number of investigation points is not a recommendation: it is a regulatory requirement. An investigation that does not meet it is not valid to attach to the project."

The combination of these two parameters produces a matrix of requirements. A single-family dwelling on favourable ground (C-1 + T-1) needs a minimum of three investigation points. The same house on unfavourable ground (C-1 + T-3) may require double, with greater depth in each borehole. 

👉 What is the foundation level and why does it matter? 

Area, depth and spacing: the three numbers that matter

Beyond the classification, there are geometric criteria that condition how the boreholes are distributed across the plot.

01 — Minimum number of points For a standard single-family dwelling, the regulatory minimum is usually three investigation points. But that minimum assumes a plot of moderate size and relatively homogeneous ground. If the plot exceeds 500 m² of built-up area, or if there are indications of lateral variability in the ground, that number increases.

02 — Spacing between points Investigation points must be distributed so that they cover the entire building footprint. A common practical rule is that no foundation point should be more than 25-30 metres from a borehole. On elongated plots or those with irregular geometry, this may require adding points even if the total area is small.

03 — Depth of each borehole It is not enough to consider the number: depth matters as much as quantity. An insufficiently deep borehole may fail to reach the competent stratum on which to found the foundation, or it may fail to detect a problematic layer that appears at greater depth. The minimum indicative depth for a single-family dwelling usually lies between 6 and 10 metres, but in ground with fill or complex stratigraphy it can exceed 15.

KEY DATA: An additional borehole may cost between €400 and €800 depending on the depth and the type of test. A foundation wrongly sized due to insufficient data can mean an overrun of €15,000 or more on site. The cost-benefit ratio speaks for itself.

The case of the "simple" plot that wasn’t so simple

In a municipality in the Madrid metropolitan area, an owner commissioned the minimum study for their 300 m² plot: two investigation points, because the technician considered the area to be known and the ground homogeneous. The two boreholes produced consistent results: compact gravels from 1.2 metres. Everything pointed to a simple shallow foundation.

During excavation, at the north-eastern end of the plot, a zone of anthropogenic fill of almost 4 metres depth appeared, probably from an old demolished structure. That point was not covered by any borehole. The result: redesign of the foundation in that area, unplanned piles, and an additional cost of €22,000 that nobody had budgeted.

A third borehole in that corner, for around €600, would have detected the anomaly before the excavator discovered it too late. 

What happens if geotechnical surprises appear during the works?

"The most expensive borehole is always the one that was not drilled: the one that would have detected the problem before the excavator found it."

Three questions you should ask before approving the number of boreholes

When the geologist presents their proposed recognition investigation campaign, do not sign it without asking yourself these questions:

1. Does it cover the complete building footprint? The boreholes must be distributed so that no corner or important load area is left without nearby recognition. Ask them to show you the location plan of the points overlaid on the house plan.
2. Does the proposed depth exceed, by at least 1.5 times, the width of the foundation? This is a basic technical criterion to ensure that the pressure bulb generated by the load remains within the ground that has been investigated. If the proposed depth seems insufficient, ask why.
3. Have ground background conditions been taken into account? Previous uses of the plot, demolished structures, proximity to watercourses or flood-prone areas, or simply the regional geology, may justify more points or greater depth. A good technician consults the IGME geological mapping before designing the campaign. 

 What prior information does the geologist consult before the study?

The conclusion that changes how you will see the budget

When you receive a budget for a geotechnical study and you see the number of boreholes, the temptation is to look for the one that proposes the fewest. That is understandable. But now you know that this number is not arbitrary: it responds to regulations, to the geometry of your plot, and to what the technician knows — or should know — about the ground in your area.

A well-sized study is not the most expensive and not the cheapest: it is the one that has enough points, in the right locations, with the appropriate depth, so that your architect can calculate the footings without uncertainty. That is exactly what you are buying.

If you have doubts about whether the proposal they have made really covers your case, the next step is to check it before signing, not after the excavator starts work.

To check your case without commitment, complete the contact form →