Why the Netherlands has a foundation problem
Large parts of western and northern Netherlands sit on soil that is naturally soft, compressible and waterlogged: peat and soft clay. Cities like Amsterdam, Rotterdam, Gouda, Zaanstad and the peat meadow area (veenweidegebied) around Utrecht were built on subsoil that behaves fundamentally differently from the sand found in, say, the Veluwe. To build there — especially before roughly 1970 — homes were founded on wooden piles driven through the soft upper layers down into a load-bearing sand layer. As long as those pile heads remain permanently submerged, they last for centuries. Once they dry out, the problem begins.
Wood rot (paalrot): what actually happens
Wood that stays permanently submerged is barely attacked: there isn't enough oxygen for the fungi and bacteria that break wood down. Once the groundwater level drops below the pile heads, they come into periodic or permanent contact with oxygen. From that point, wood-rot fungi (mostly soft-rot fungi, unlike the brown-rot/white-rot you see above ground) can attack the wood. This process — pile rot, or paalrot — is insidious: it's invisible from the outside until the foundation is already weakened and the house shows visible tilting or cracks. By the time an owner notices, the rot has often been progressing for years.
Why does groundwater actually drop?
Groundwater level decline in Dutch cities has multiple, often compounding causes: active water-level management by regional water authorities (especially in peat meadow areas, to keep land drained for agriculture), increasing paved surface and sewage systems that drain rainwater away faster instead of letting it infiltrate, drinking water extraction, and climate change producing drier summers. In parts of, for instance, Zaanstad and the peat meadow area around Utrecht, the combination of peat oxidation and active water-level management is a structural, decades-long declining trend, not an incident.
Subsidence and peat: two coupled processes
Peat is largely made of organic material and water. As long as it stays permanently saturated, it remains roughly stable. Once the groundwater level drops and peat comes into contact with oxygen, it oxidizes: the organic material breaks down, its volume shrinks, and the ground subsides. This is called peat oxidation, and it's a self-reinforcing process — subsiding ground leads to lower surface elevation, which in turn calls for further water-level lowering to keep the area habitable and passable. The Dutch subsidence problem in peat areas is therefore not a single event but a slow, decades-long process.
For foundations, this matters in two ways. First, compressing peat causes uneven settlement: if one part of a parcel subsides faster than another (for example due to varying peat thickness), the house tilts — known as differential settlement. Second, the ground surface subsides relative to a groundwater level that doesn't drop as fast, meaning pile heads that once sat well below the groundwater can end up closer to it, or even above it.
Holocene peat versus deeper, older peat
Not all peat matters equally for a home's foundation. The BRO GeoTOP model — the Netherlands' national 3D soil model — distinguishes, among other things, Holocene peat (the relatively young, shallower peat layers formed during the current geological epoch) from deeper, older peat. Holocene peat is structurally the most risky: it often sits directly under or around building foundations, is less compacted than older layers, and is most sensitive to the combination of groundwater fluctuation and oxidation that leads to settlement. Deeper, older peat plays a smaller role for most foundations, simply because it sits farther from the pile heads.
PlotSense uses BRO GeoTOP to determine peat thickness and depth per parcel, at a resolution of roughly 100 metres. The measured accuracy of this model against AHN elevation data (the national elevation dataset) is within a median of about 7 centimetres — enough precision to distinguish a parcel with a thin peat layer from one sitting right in the middle of it.
GHG: the average highest groundwater level
For foundation risk, what matters is not the lowest but the average highest groundwater level (GHG, gemiddeld hoogste grondwaterstand) — the level groundwater reaches at its peak in a normal year. If even the GHG sits below the pile heads, those heads never sit permanently submerged again, and the pile-rot risk climbs structurally. PlotSense sources GHG data and subsidence figures (in mm per year) from Deltares, the Dutch research institute for water and soil issues.
Why deep groundwater isn't always a risk
A crucial point the PlotSense model deliberately accounts for: a deep groundwater level is not inherently a foundation risk. In higher, sandy areas such as the Veluwe, groundwater naturally sits much deeper than in the low, wet polders of western Netherlands — but homes there typically aren't founded on wooden piles, and the deep groundwater level is a normal, stable situation with no peat that can oxidize or compress. The model therefore doesn't look at groundwater depth in isolation, but at groundwater depth combined with soil context: only the combination of soft, peat-rich or low-lying subsoil with a declining or structurally low groundwater level constitutes a real risk for wooden pile foundations. Without that coupling, the model would incorrectly flag red flags on the Veluwe for foundations that were never at risk.
What this concretely means for your address
If you're considering a home in an area with known peat subsoil — large parts of Amsterdam, Rotterdam, Gouda, Zaanstad and the peat meadow area around Utrecht — the combination of construction year (see the previous article), peat thickness and groundwater trend is exactly the kind of soil and groundwater data PlotSense's Foundation Report shows for your address. The Foundation Report (€35) shows these figures per address, with source and date, so you don't have to guess whether your specific parcel falls in the risk zone. Check your address at plotsense.app/zoek.