Is financing for house construction realistic?

Thank you for your effort and detailed answer.

Regarding the planning of the house and project:

It will be built and planned with an architect without being tied to a developer.. largely turnkey without own work (or only to an insignificant extent, possibly for outdoor facilities). Construction start this year is no longer possible, we are still in the preliminary planning and cost estimation,.. before October/November there will be no building permit, a positive preliminary building permit is available.

Flat roof house 150 sqm living space with attached double garage, gas heating and central ventilation system (additional costs ventilation system 10-12k), without fireplace, without basement, 35 sqm terrace, 40-50 sqm driveway.. no frills but also not the cheapest materials to be used.

Budgeted for house and garage medium standard: 330k (including interior finishing, floors, painters, etc..
Additional construction costs (architect, structural engineer...): 55k
Development costs: 10k
Outdoor facilities: 25k
Interior furnishings: 25k (we don't want a luxury kitchen and can use a lot from the old apartment)

Is this budget too tight?

What is currently a common variable interest rate for the plot?

Regarding the subject of commitment interest for understanding:

Assuming we would already take out the house loan this year but only fully need it at the beginning of 2020 (to complete the house construction), then commitment interest would be charged 12 months after conclusion? Can we avoid that by simply already drawing the loan fully and repaying it, although the house is not yet ready to move in? Of course, we would also have to cover rent in addition, but that would probably be possible for us. The advantage would be that we could secure a good offer this year.

Many thanks in advance!