Expensive plot + single-family house 155 sqm + cellar KFW40+, financeable?

Discard KFW40. It never "pays off" during one's lifetime. Of course, the plot is not a bargain, but 300€/m2 is rather cheap in southern Germany. suggestion makes sense. Buy the land and amortize. Once the salary has increased, finance the house construction.

How good is Ärzte und Apothekerbank really when it comes to real estate financing?

2 of my senior physicians ultimately ended up with the Sparkasse because the ApoBank was not attractive enough.

and thanks for the hint. One more question regarding that. If we assume that we will only start building in the middle of next year and will need 9-12 months, then one could move in by mid-2022. From when exactly do I have to start servicing the loan?

It is important that you plan sufficient buffer time. Bad weather, force majeure, etc. can quickly extend the construction. When the loan must be repaid also depends on the payment milestones from the general contractor. For loans, is significantly more experienced.

I would also take another close look at the cost side. I don't necessarily share the assessment that it will probably be cheaper with the cellar + KfW 40. If you provide a more detailed calculation, we could see if anything is missing.

Sure, here is the calculation from the general contractor:

Land: 250k plus notary/court + property transfer tax = €271,300
House connection: €4,800
Earthworks: €3,000 (sewer works included)
Outdoor facilities: €15,000 (of which €7,500 for paving, €2,500 for terrace, €5,000 for garden)
Authorities/building supervision: €1,350 (inspections included, blower-door test etc.)
Construction supervision insurance: €200
Furnishing: €22,000 (kitchen + furniture)

(Sub)total €317,650

Residential building: €364,500 (€2,400 per sqm living space)
Cellar: €40,000
Additional windows: €3,700 (raffstores on a corner window in the cellar + anthracite-colored windows)
Carport: €13,000 (we will probably leave this out for now, I think)

Energy saving/sustainable building: here comes the interesting calculation, where the repayment subsidies are already deducted
Additional costs heat pump and ventilation system as an integral unit €12,000
Additional costs "screed drying": €2,000
Additional insulation: €4,000
Photovoltaic system: €12,000 (I know the managing director of a company in the next town through a friend, hoping it will be this price or cheaper)
Battery storage: €6,000
Expert: €4,000

Now come the subsidies:
BAFA: €9,000
Repayment subsidy KFW40+: €30,000
KFW subsidy energy consultant: €2,000
Progress subsidy storage + photovoltaic: €1,000
Ventilation system with heat recovery progress NRW: up to €1,000

This results in a total sum of: €735,850 (+ the advance provision of the subsidies)

The interesting thing in the calculation, if it is correct: supposedly you get €3,000 back if you build the heating that way.

Thanks! So for some items, I don’t necessarily find the calculation generous. Basement seems low to me. Incidental construction costs as well (or some things are missing there. Is that included in the house price then?) Kitchen and furniture can also be tight depending on the requirements. (But it can also work. But it adds up.) Exterior facilities not too generous. I would definitely plan with a buffer.