Cost estimate: New construction of a semi-detached house, hillside location (South Baden)

My wife and I are considering purchasing a plot of land for the construction of a semi-detached house in the south of Baden-Württemberg for our family (currently two, soon three children). Unfortunately, we are complete novices and are therefore very grateful for a rough (and conservative) estimate of the construction costs.

Framework conditions:
- The plot has 333 sqm (plot 17 on the plan) and is located on a slope (see photos).
- The semi-detached house should have three floors: basement/sockel level built into the slope (entrance area, garage, cellar), upper floor (kitchen, bathroom, living room, bedroom), attic floor (bathroom, three bedrooms). The upper and attic floors should together have about 150 sqm of floor space.
- We envision a "simple" building with a gable roof, probably a prefabricated house for cost reasons.
- We have (as far as I believe...) few special requests, except for photovoltaics, heat pump, and floor-to-ceiling windows on the upper floor.
- We do not plan any own work; the house should be turnkey.

Should we calculate with at least €3,000 per sqm (= €450,000) + €80,000 for the cellar + €80,000 incidental costs? That would be a total of at least €610,000 (excluding the plot).

Probably many details are missing for a serious estimate, which I will try to provide later. Many thanks for your assessments and also for further suggestions!

Humble question. Who, when and how will the second half be built? Much depends on that.

Regarding the price. Since the basement with the entrance must also be included in the thermal envelope and becomes [Sankt Wohnfläche], the budget will not be sufficient.

BNK, outdoor facilities etc. must not be forgotten

We are envisioning a "simple" building with a gable roof, probably a prefab house for cost reasons.

I understand that you want to minimize costs, but a prefab house is not cheaper than a brick house (often even more expensive). I have never owned a semi-detached house myself, but I keep reading about various problems here, especially if they are not built simultaneously and in mutual agreement.

We have (so I believe...) few special requests, except for photovoltaics, a heat pump, and floor-to-ceiling windows on the upper floor.

Almost everyone here believes that, myself included, yet every house is full of wishes.
There are many threads on the topic of basements and hills; the one in the picture can confidently be called that, and when I then read:

We are not planning any personal contributions; the house should be turnkey.

...I see high costs in addition to the basement for outdoor facilities, which are often carried out largely as personal contributions. "Turnkey" is usually not really "turnkey" either, so please read carefully what is offered or what is still added, which one believes to be included in the price.

An inexpensive house that can be built as smoothly as possible for.....

Unfortunately, we are absolute novices.

...I would probably avoid both the semi-detached house planned alone and a hill like that.
However, there are surely many people here who can report more concretely from their own experience or existing knowledge.

...I would probably keep my distance both from the semi-detached house planned alone and equally from such a slope. Surely, there are many people here who can report more specific information based on their own experience or existing knowledge.

One is actually enough, I recommend the Goalkeeper thread. You rightly advise the OP to keep their distance also from this "clearly not a beginner's plot" (which would best be handled as a package of two semi-detached houses and a single house by a local developer). Readers interested in this plot (but not also "novices") are advised to communicate with the neighbor (16). An advantage of this particular slope is that the neighboring half is excluded from the start from dreaming of a slab-on-grade house. Because of the earth-contact basement, at least for this one, a "stone" construction method is indicated. I have already said everything else here (see forum search in my posts with the keyword "semi-detached house" as well as in my external post "A semi-detached house has TWO halves").

Should we calculate with at least €3000 per sqm (= €450,000) + €80,000 for the basement + €80,000 ancillary costs? That would be a total of at least €610,000 (excluding land).

I can only say that the basement is not a cellar-basement, meaning only as a utility space, but should have an entrance, cloakroom, and probably a WC for residential purposes and therefore belong to the thermal envelope. You should then also include finished, heated living sqm in the costs. It may be that you come under €3000/sqm, but there are enough costs with a hillside that simply cannot be calculated. Therefore, calculate with €3000/sqm.

probably a prefabricated house.

Prefabricated house: You mean the timber frame prefab construction? That is then not feasible with a basement. Concrete belongs under the ground. Then you get a mixed construction method. You have to know that. It will not be cheaper. So I would not commit to that.

We are not planning any own work, the house should be turnkey.

Turnkey is not a registered term, thus it is interpretable and flexible. Usually not included are: floor covering work in the living and sleeping rooms, smooth walls, wallpapering and painting work. Kitchen. Most people do some of these themselves because it saves quite a bit.

We are envisioning a "simple" building with a gable roof

The simple building on a difficult plot. You usually have to shore up and heap embankments. That costs extra. What does the driveway to the basement look like? That would have to be excavated and secured?!

few special wishes, except photovoltaic, heat pump, and floor-to-ceiling windows on the upper floor.

The wishes may not be special, but they are expensive.
The floor-to-ceiling windows should be discouraged in tight developments. No one wants the neighbor to be able to look into the children's rooms at ground level.

The floor-to-ceiling windows should be talked out in such dense development. No one wants the neighbor to be able to look into the children's rooms at ground level.

I read the combination "nothing special, but floor-to-ceiling windows are a must" more as an (unintentional?) joke.