Land planning Single-family house on a north slope with basement

Basement on flat ground costs similarly to a basement on a hillside property. On the hillside, there is also the anchoring and leveling of the remaining property.

The planned floors on flat land will also exceed your budget. You have to build smaller.

If I had the choice, I would take the hillside property. You do not have the budget for it. If you have a debt burden that is crushing you, you have gained nothing.

Take a look at the model house Orlando from Rensch-Haus.
Is the office sufficient for your practice?
Storage will become the WC, the building services move to the utility room and WC.
You leave out the bay window in the dining area (cost driver).
With a carport and own work, consistent material selection within the framework, this should be feasible for your budget on flat land.

But can you sell the hillside property?

Ultimately, you have to decide.
Just consider that every excavator bucket costs,
every square meter also costs the enormous corridor area.
Does it have to be 3 floors?

Parking spaces and carport on the north side at the street
Practice and bedrooms in the basement
Above living rooms and terrace facing south.

It is a large 1250m² plot that would be divided. The other half would simply not be built on and might possibly revert to me if I were to become debt-free.

I don’t quite understand: someone is holding the other half for you and leaving it undeveloped until you can buy it back later, and what would it be good for then?

Can you explain to me what financial and technical difference it makes whether I build a cellar on a flat plot or the basement level on a sloping plot?

Technically, in the first case, the cellar is "underground" all around and a storage room — partial residential use is more complicated here; in the other case, the cellar is only such on the downhill side and partial residential use is quite obvious; financially, I don’t think this can be generalized so easily; a significant difference lies in the terrain itself: a slope typically has a steeper gradient than the groundwater can follow — you usually have pressing groundwater on a slope in a different magnitude. A slope also doesn’t hold naturally — whether walls or roots, something must stabilize it, and that costs.

Generally, I would say a slope always requires a budget surcharge; the desire for partial residential (/living) use of the "cellar" naturally offsets that. I could imagine that renting commercial space could be cheaper than insistently trying to place it under the apartment.

A slope is actually only nicer where it combines with a corresponding distant view — but then of course with a corresponding premium for that location. A slope because the plot is cheaper than flat usually balances out again with the terrain construction costs.