Floor plan: Building on a slight slope - not enough for a basement due to excavation?

Hello dear ones,

my partner and I looked at our dream plot yesterday. Price unbeatable, location great, here and there a few flaws, but nothing we couldn't live with - except for one thing. And that is the 'slope' - if it can even be called that. We are talking about a plot with a 20m street frontage to the south (valley) and a length of about 70m. The upper area is flat, but not building land, and borders another street. In my estimation, the height difference is at most 2m, spread over about 50m. I tried to attach some photos. We are building with a prefab house provider to KFW40 standard with a sewage pit, otherwise connected on the street side (south side). Now my concern is that planning for a plot 'on a slope' could plunge us into extra costs, which is why I would like to find out what possibilities for realization we have before signing the contract. My idea would be to remove soil in the lower half, since the house would per se have to stand in the lower half. Now I don't know how this behaves with the rainwater. Would simple retaining walls be enough? How much would something like that cost? Do you have any other ideas? It's just too little of a slope to build a basement (which would be super expensive anyway), but too much to simply ignore it.

Best regards

You want to catch the water with retaining walls, or am I misunderstanding something?

The decisive factor for the costs is how much slope you have in the building area. It’s best to measure that with a water level or something similar; eyeballing it is rubbish. Purely based on the numbers, I don’t see a reason for a basement either. You can probably do quite a bit against water running down by cleverly designing the outdoor area. Cross slopes, possibly a small (20-30cm) embankment with a hedge or a raised bed. The house itself can be built a bit higher, and you also have the drainage strip. If necessary, a channel can be added in front of the house.

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thank you very much for the answer! Well, I thought you could divert a little water with such a small wall. Seems I was wrong, but in my defense, I have absolutely no idea about house construction (more about heating, but that doesn’t matter).

I think in the area of the house we would have a maximum rise of 70-80 cm across the width of the house, that should be about 7% slope, if my math brain still works? What confused me was the statement from the sales rep of the house building company that we have to set the ground floor, or the top edge of the floor slab, exactly level with the top edge of the terrain (i.e. the highest point at the top), otherwise we would have problems with surface water or slope water. Otherwise, I would have said that you level the ground up to half of the plot (or a bit more), put the house there, and then lay a kind of drainage or diversion behind the house in front of the rest of the slope, so that no water can get to the house. I don’t know what to make of the house builder’s statement that it must be exactly flush so that water doesn’t damage the house.

That sounds relatively similar to my situation. We also have about ~70cm slope in the area of the base slab and even went down a bit so we don’t have to unnecessarily add fill. You can find a site plan here:

Currently, the base slab is in place and I have to say, the house extends much less at the bottom than feared. It doesn’t dig in at the top either, although the topsoil is missing there right now, so that certainly gives a bit of a false impression. My plan is to channel the water with a cross slope to the side and let it rise slightly again in front of the front door. For safety, a channel will be installed in front of the front door so that nothing pushes in during a thunderstorm.

I find the slope cute... if your estimate of 2m over 50m is correct and the house has a depth of 10m, that would be just 40cm. It could also be poorly estimated or irregular, then it's a bit more. But really nothing to be afraid of.

What the salesperson means is wood protection. There is a (wood) exterior wall with a (wood) threshold directly on the foundation slab. This wooden threshold must be at least 15cm above the ground so that water does not permanently soak into the wood. Whether you achieve these 15cm by placing the house on the higher ("slope" side) level, or if you lower this area a bit is, of course, up to you. If you lower the area (which is probably the better option), the property behind it will theoretically be steeper. In practice, however, you still have 40-50m to shape the terrain as you like and need. I really don’t see a problem there.

What you should consider from the start is access, entrance to the garden, terrace, etc. To the aforementioned 15cm, you also add the interior floor structure, meaning you have about 30-35cm difference between the floor height and the ground height. If that doesn’t bother you, you simply take two steps to get into the house, garden, or terrace. To achieve this without a threshold requires some effort (raising the terrace, ventilated drainage channel, etc.).

In summary: the "slope" you described is not a problem and should not stop you from buying the property. If you move forward with the realization, you should plan the outdoor area adjacent to the house right away (here or with a landscape gardener or similar).

We are building with a prefabricated house provider to KFW40 standard with a sewage pit, otherwise accessible from the street side (south side). Now my concern is that planning for a 'hillside' property might plunge us into extra costs, which is why I would like to find out before signing the contract what possibilities for realization we have.

At least you're asking before signing the contract, we've seen other cases here before. Show the plot (what do the red lines mean?) also in an aerial photo, or check a geoportal to see what you can find out about elevation points / contour lines of the property. And you should also look at the development plan (where there is none, there may be other local statutes), what it says about terrain changes, and so on. I see quite a bit of need for information before signing. By the way, KfW40 and a pit is a crazy combination, like Samba in Mettmann :)