Honest opinion about the bungalow floor plan - approximately 175 sqm

I would discuss the topic of costs more thoroughly with the architect. €300,000 will by far not be enough and with the mentioned €350,000 I still don’t see a garage.

Is another child planned?

I calculated the total m2 with €1800 per m2 (without garage and come to €315k). So €35k for a double garage is not realistic?

Children are planned.

The construction project will not be awarded completely. Much of it is also to be done through involvement. The circle of friends consists of various trades. (Carpentry, electrical, interior finishing)

Regards

What about incidental construction costs? Features like a new kitchen? Outdoor facilities?

Apart from whether 1,800 is enough, the items above are not included in it.

Doing it yourself also costs for the materials. Besides that, only you can assess whether your friends will work for free in their free time and if so, how much.

The budget question should actually not be the focus here, but I think that 300 - 350k just for the house including garage "out in the countryside" calculated at 1.8k per m² is well calculated. However, I might be mistaken, but I am happy to share my calculations with you.

5) Why should more insulation be added there? Isn't the insulation of the exterior walls enough?

So that your water pipes inside the wall don't freeze in winter. Have you ever looked at the wall thickness of the party toilet? At the moment, all I see are "paper-thin walls".

The budget question should actually not be the main focus here, but I think that 300 - 350k just for the house including garage "in the countryside" with the calculation of 1.8k per m² is well calculated. But I might be wrong, I’m happy to share my calculations with you..

Even in the countryside there are ancillary building costs, and not just a little. Those are missing

The €1800 is basically already rather too little today, with you the construction will probably start no earlier than 2020, so it will hardly be feasible.