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!