How many boreholes does my plot need for the geotechnical study?
Discover how many boreholes your plot needs for the geotechnical study according to the CTE. Key factors: surface area, type of structure and ground complexity. Practical guide.
There is a question that almost all property owners ask at first contact: "How many boreholes are you going to drill in my plot?" The question is logical. Nobody wants more drilling than is necessary, but nor does anyone want to pay for an incorrectly designed foundation because the geotechnical study has come up short. The honest answer is that it depends, but not arbitrarily. It depends on specific technical criteria that the Building Technical Code (CTE) sets out with considerable precision.
What very few people know is that the number of boreholes is not decided by the geologist based on 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 building work envisaged, and the geotechnical complexity of the ground. Understanding those three factors lets you go into the meeting with the technician knowing exactly what is being discussed, and to spot if someone is offering you less than your plot actually needs.
The CTE leaves no room for improvisation
The CTE Basic Document for Structural Safety, in its Annex B, sets out a classification that every geotechnical undertaking 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 grounds are classified into three groups. Group T-1 corresponds to favourable ground, with low variability and no special problems: gravel, compact sands, cohesive soils in a firm state. Group T-2 are intermediate grounds, with some lateral variability or with materials whose behaviour is less predictable. Group T-3 covers unfavourable grounds: soft clays, expansive ground, anthropogenic made ground, karst, and the presence of water.
Constructions are also divided into three types. C-1 groups small-scale works: single-family dwellings of one or two storeys, and retaining walls of less than three metres. C-2 includes buildings 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 recognition points is not a recommendation: it is a regulatory requirement. A study that does not meet it is not valid to attach to the project."
The combination of both parameters creates a matrix of requirements. A single-family home on favourable ground (C-1 + T-1) needs a minimum of three recognition points. The same home on unfavourable ground (C-1 + T-3) may need double, with greater depth at each borehole.
👉 What is the foundation level and why does it matter?
Area, depth and spacing: the three numbers that matter
Beyond classification, there are geometric criteria that determine how boreholes are distributed over the plot.
01 — Minimum number of points For a standard single-family dwelling, the regulatory minimum is usually three recognition points. However, that minimum assumes a plot of moderate size and relatively homogeneous ground. If the plot exceeds 500 m² of built area, or if there are indications of lateral variability in the ground, that number increases.
02 — Spacing between points The recognition points must be distributed in such a way that they cover the entire footprint of the building. A common rule of thumb is that no foundation point should be more than 25-30 metres from a borehole. In elongated plots or with irregular geometry, this may force the addition of points even if the total area is small.
03 — Depth of each borehole The number alone is not enough: 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 is typically between 6 and 10 metres, but in ground with made ground or complex stratigraphy it can exceed 15.
KEY FACT: An additional borehole can cost between €400 and €800 depending on depth and the type of test. A poorly sized foundation due to insufficient data can lead to an additional cost of €15,000 or more on the works. The cost-benefit ratio speaks for itself.
The case of the “simple” building plot that wasn’t so simple
In a municipality in the Madrid metropolitan area, an owner commissioned the minimum study for a 300 m² plot: two recognition points, because the technician considered that the area was known and the ground was homogeneous. The two boreholes produced consistent results: compact gravel from 1.2 metres. Everything pointed to a straightforward shallow foundation.
During excavation, at the north-east end of the plot, a zone of anthropogenic made ground appeared, almost 4 metres deep, 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 extra 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 unforeseen events appear during the works?
"The most expensive borehole is always the one that wasn’t drilled: the one that would have detected the problem before the excavator discovered it."
Three questions you should ask before approving the number of boreholes
When the geologist presents their proposed recognition campaign, do not sign it without asking yourself these questions:
- Does it cover the complete building footprint? The boreholes must be arranged 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 dwelling’s floor plan.
- Does the proposed depth exceed by at least 1.5 times the width of the foundation? It is a basic technical criterion to ensure that the pressure bulb generated by the load lies within the ground that has been recognised. If the proposed depth seems too shallow, ask why.
- Have ground antecedentes (background information) been taken into account? Previous uses of the site, demolished structures, proximity to watercourses or flood-prone areas, or simply the regional geology can justify more points or greater depth. A good technician checks the IGME geological mapping before designing the campaign.
What prior information does the geologist consult before the study?
The conclusion that changes how you’ll 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 fewer. It is understandable. But now you know that 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 properly sized study is not the most expensive or the cheapest: it is the one with enough points, in the right locations, at the appropriate depth, so that your architect can calculate the foundations without uncertainty. That is exactly what you are paying for.
If you have doubts about whether the proposal they have made truly covers your case, the next step is to check it before signing, not after the excavator starts work.
