ᐅ Raising the ground level of a property in a flood-prone area

Hello everyone,

We are currently considering purchasing a plot of land and would like to hear your assessments and experiences—especially regarding additional costs or risks.

Key details:

• Plot size: approx. 576 m² (6,200 sq ft)
• Distance to the Danube River: approx. 150 m (490 ft)
• Location according to flood hazard map: HQextrem (information from the building plan)
• Price: approx. €180,000
• Region: Bavaria, postal code: 93*

Excerpt from the building plan:

“According to the flood hazard map by the Bavarian State Office for the Environment, the development area lies within the HQextrem flood hazard zone.
The soft clay soils of the cohesive upper layer are not suitable for foundation construction without additional measures, as they can cause twisting and tilting due to unfavorable load/settlement behavior.
The at least stiff clays of the cohesive upper layer are generally suitable for foundation construction.
If buildings are constructed without basements, only minor water retention measures are necessary (e.g., sump pumps, drainage pipes).
Waterproofing against groundwater under pressure according to DIN 18195-6 standard must be provided.
Basements are explicitly not recommended.”

Situation:

The plot currently lies about 1.5 m (5 ft) below street level, basically in a kind of “pit.” According to the building plan, the ground must be filled up to street level.

This issue affects all parcels along the planned road—some to a greater extent, some less. There might be an opportunity to collaborate with future homeowners here.

My questions:

• What additional costs could realistically be expected for the necessary ground filling and soil improvement (rough estimates)? Also, what about waterproofing against groundwater under pressure?
• Are there risks or disadvantages associated with the location in the HQextrem zone, even if no basement is planned? According to a neighbor (living there for about 20 years), floodwater was so far well controlled by a dam, but who knows how this will look in the coming years…
• Does anyone have experience with how building permits/planning permissions are handled in such areas (e.g., additional requirements or insurance issues)?
• Would you personally consider buying under these conditions at all?

Photos of the land-use plan, Google Maps for assessing proximity to the Danube, and a picture of the “pit.”

Thanks in advance for your opinions and experiences!

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Are there alternatives? Keep in mind the additional costs, which can add up, and the 180,000 may sound affordable probably because all other expenses have been deducted!

What about insurance over there? Are flood damage policies available, or are they all declined? That would be important information. I have to say that I personally probably wouldn't build there, but of course, that is a subjective risk assessment.
Unfortunately, I cannot provide any information regarding costs.

Are the geological surveys from the municipality available? Where exactly are the stiff clay layers of the cohesive topsoil located?

Construction was done directly by the Danube as well.

You should consult a civil earthworks contractor to find out the cost of suitable fill material (delivered and installed) in that area.

You always have the waterproofing against rising damp.

The construction method should be adapted to the situation. In my opinion, only solid construction is suitable.

Tom_bay schrieb:

• What additional costs should be realistically expected for the necessary soil fill and ground improvement (approximate scale)? Also for the “waterproofing against hydrostatic pressure”?

That is a major unknown, which I cannot answer either. In England and the Netherlands, I have seen houses built on pontoons inside containment basins, where their elevation on supports can be adjusted according to the water level. Similar construction methods have also been tested in the Emsland region. I don’t have exact prices, but I recall seeing an order of magnitude of around 200,000€ for a single-family house. These houses can be insured, although the costs are somewhat higher than for a standard house without risk. I would roughly estimate a 25% surcharge. The pontoon system requires maintenance, which I would estimate at about 1,500€ per year.
In any case, that would be the approach I would take. For unusual projects, there is sometimes available funding and similar support. Just be creative.

Tom_bay schrieb:

• Would you personally buy under such conditions at all?

With a pontoon: yes, without: rather not.