Is the construction project feasible? Is our budget sufficient for it?

Yes, me too, I don't think it will work out.

So, how is it working for you?

It works wonderfully. When requesting an offer, of course, the planned execution period is mentioned. The awarding then involves concluding a construction contract in which a time frame is agreed upon. Done. At minimum, a written acceptance of the offer with acknowledgment by the contractor should take place.

I can roughly tell you the larger items off the top of my head. Overall, I would have to look at the list tonight.

Ancillary building costs (notary, insurance, connections, ...) approx. 20,000
Shell construction 174,000
Roof including visible roof truss in the garage and sheet metal roof on the bay window 34,000
Heating (air heat pump with 1200l buffer and underfloor heating throughout the whole house) 43,000
Central ventilation system 8,500
Kitchen 20,000
Floors are calculated at €50/m2 (will be laid by ourselves, and we will probably choose one that is a few € cheaper)
Screed €5,750
Windows 20,000
Venetian blinds 10,000
Garage door 3,100
Front door 6,000
Paving 5,000 (self-installed)
Bathroom fixtures 17,000
Fireplace 13,000

That works wonderfully. Of course, the planned execution period is mentioned already when requesting offers. The awarding then includes the conclusion of a construction contract in which a timeframe is agreed upon. Done. At the very least, a written acceptance of the offer with acknowledgment by the contractor should take place.

The larger trades are even happy because there is planning reliability and everything is agreed in writing. As mentioned, smaller trades like screed,... are then agreed again 3-4 months before installation but the cost framework is already defined and a small buffer for price increases is included.

It's realistic, only the screed seems very cheap to me. Ventilation systems are extremely cheap, is something missing? It costs 15,000. Windows are also cheap if [Holz Aluminium]. Plastering is completely missing, that's another 40,000. Painting is also missing, about 8,000.

It's quite realistic, only the screed seems very cheap to me. Ventilation systems are extremely cheap, is something missing? It costs 15,000. Windows are also cheap if wood-aluminum.

Plastering is completely missing, that's another 40,000. Painting is also missing, about 8,000.

Windows are aluminum-plastic. High quality there means well processed. Aluminum-wood would have cost an additional 3,000.
The ventilation system is complete. But we really got very, very lucky to get a very good offer.
Plastering definitely doesn't cost 40,000. It will be somewhere around 30,000. At least not here in the countryside.
We paint ourselves. Paint is budgeted at 2,000€.