ᐅ Which is more suitable for the site conditions: a basement or a slab foundation?

Hello everyone,
I am new to the forum and have purchased a plot of land from the municipality in a new development area, where a single-family house is planned to be built within the next 1.5 years (1 or 2 stories).
For some time now, I have been struggling with the site conditions (slight slope and below street level) and the associated question of whether a basement might actually be the smarter/more cost-effective (or at least cost-neutral) solution in this situation. To answer this question properly, I will probably need quotes from civil contractors and, above all, a geotechnical soil report. Regarding the soil report, I seem to be going in circles because all geotechnical engineers want very precise information about the planned construction, which in turn depends on the decision between a basement or slab-on-grade foundation.

I am hoping to get advice on the following questions:

• Is it possible, when building with a basement, to use the excavated soil to fill the plot up to street level (no costly disposal required)?
• What advice can you give me about the soil report (the exact future construction is not yet fixed)?
• I am torn between the following options (which do you think is more sensible?):
• Build with a basement and fill the plot to street level using the excavated soil (hold back the northeast property boundary with a 1 m high retaining wall).
Allow a partial slope to create a daylight basement room facing northeast, which could possibly be used as an office. The utility room would naturally also be in the basement, but I would prefer to avoid a sump pump system if possible.
• Fill the plot (up to street level or slightly below) and build with a slab-on-grade foundation (hold back the northeast property boundary with a 1 m high retaining wall). However, I am concerned about the costs for fill material and necessary compaction. The utility room would be on the ground floor or upper floor, no sump pump system, but probably with a backwater valve.

All prefab house suppliers, etc., I have contacted so far have really not addressed these conditions. They just ask for a budget range and then assume a flat fee of €25,000 for earthworks, etc. I suspect this will not be enough, and I want to make these costs tangible for my financial planning! When the basement topic comes up, even architects generally say a basement is always expensive. If the budget is tight, better to do without a basement...

Here are some details and premises I want to share with you:

• The plot is below street level. The adjacent neighboring plot (southeast) has been raised up to street level (top of finished floor level +156.14 m).
• Because of the filling up to street level, no backwater valve was needed there. On the opposite plot (northwest, behind the sidewalk), the fill was not raised as high, so a backwater valve was required. If I build with a basement, I would probably have to plan for a sump pump system even if I fill up to street level.
• According to the development plan, a retaining wall up to one meter high may be built to compensate for the slope; beyond that, slopes with an angle of 30° must be used (see attachment). Of course, both neighboring properties have taken advantage of this and installed suitable L-shaped retaining walls on the northeast side.
• Basically, I would not have necessarily filled the plot, but since the southeast side was raised and that is where most of the natural light comes from, I feel almost compelled to at least partially fill.
• Unlike the neighboring development, I am considering positioning the house (gable roof) with the gable perpendicular to the street (see sketch). This would improve lighting and usability of the plot.
• There is a castle to the northeast at some distance, which should also be taken into account when planning the house (view connection).
• The plot is 464 m² (about 5,000 sq ft) and located 800 m (0.5 miles) from a river (about 15 m (50 feet) above the water surface, no flood risk). Additional waterproofing measures for a basement are to be expected (most likely a waterproof concrete shell). A neighbor who built with a basement had to seal it from the outside (apparently afterwards).
• The development plan is generally quite generous: 2 full floors allowed, max ridge height = 6.5 m (21 ft 4 in), max eaves height = 10.5 m (34 ft 5 in), floor area ratio: 0.4
• Surveying has been done (see attachment)

Thank you very much, and I look forward to your suggestions and advice!

Note: Your wish for anonymity is perfectly fine. On the other hand, good help is often only possible when all facts are known. Your post is somewhere in between. Street names are disguised but still searchable if you know how to use search engines. Maybe you could bring yourself to share the name of the development plan here. (I won’t.)

Regarding the decision on whether to have a basement or not, I can’t provide any professional guidance. I would personally prefer to build with a basement, but sometimes that simply doesn’t fit the budget.

Best regards

PS: Castle view – I’m a bit jealous.

At first glance, I would estimate that you can still build well without a basement. You won’t be able to avoid terrain modeling either way. However, adding a basement is another significant expense.
The question you should really ask yourself first is: What do you need, and what is your budget?
Aside from that, a sewage lifting station (sump pump) is not a disaster. If you are still worried about it, no one will force you to install one even if you have a basement.
If I interpret your sketch correctly, you are positioning your house mostly along the slope. Perhaps a more square shape would be somewhat more economical.

More to come tomorrow. For now, briefly: with the "(11ant) basement rule" / "basement question," you can find my posts here on the topic of whether your plot is suitable for a basement, yes or no, or possibly a partial basement. Whether the excavated soil is suitable for backfilling depends on its quality. In any case, it will be looser when excavated than in its original compacted state, so it requires compaction. Backfilled soil therefore does not have the same volume as when it was removed.

TobsBAU schrieb:

Regarding the soil survey, I find myself going in circles, since all geotechnical engineers want very precise information about the planned construction, which itself depends on the decision for a basement or slab foundation.

That’s nonsense. The plot—and especially the building area—is not large enough to require numerous sampling points. I would simply select the inner area of the building footprint.

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Basically, with the slope you have the problem of being caught in between. It’s too steep for a flatland plan but not steep enough for a hillside house. When planning, you should also consider the entire outdoor area. Where will the terrace be located, how will you get from the house to the garden (stairs?), and where would the light well for the basement office be? What makes the most sense also depends somewhat on your specific needs (e.g., how often do you use the office?). In the end, two very different floor plans result from these options. Therefore, I will only address one detailed question here.

TobsBAU schrieb:

Is it possible to use the excavation material from building a basement to level the property up to street level without incurring disposal costs?

Excavated soil can of course be used for backfilling, but most soils cannot be compacted reliably, and you must expect some settling. You can create lawns and flower beds on it, but a driveway should be built on gravel.

WilderSueden schrieb:

Basically, with that slope, you're caught between two options. For a purely flat land design, the slope is too steep; for a hillside house, it's too shallow.

Which brings us to a split-level as another option. It doesn’t have to be a full half-story. Sometimes just 2 to 3 steps within the ground floor are enough to create a noticeable effect. This often works well for zoning within the space, too.