700,000€ house - Are we overextending ourselves?

I find your calculation a bit confused... I don't know whether you are on parental leave now or if the 4800 is from the fictional calculation for later. Also, the cold rent won't be saved next year because you will still be living there during the construction.

So currently the income is about €7000.
Fictional income (2 children):
Woman on parental leave: €6100
Woman stays at home: €4300
Woman works 50%: €5600

Our average monthly expenses over the past years:

Household €450.00
Clothing €100.00
Food at work €80.00
Leisure €140.00
Vacation €200.00
Phone/Internet/TV €44.00
Electronic purchases €70.00
Miscellaneous €20.00
Gifts €50.00
Car 1 €500.00
Car 2 €500.00
Cold rent €1,075.00
Additional costs €50.00
Gas €70.00
Electricity €60.00
Pocket money €100.00
Purchases €150.00

The costs for the cars are full cost accounting including depreciation, etc. They are also this high because we simply have a 40km and 50km commute to work.

Oh yes, the equipment should be quite decent:

central ventilation, ground-source heat pump, electric blinds, bus system KNX, good insulation, underfloor heating, glued real wood floors..

oh yes, the equipment should be decent:

central ventilation, ground heat pump, electric blinds, KNX bus system, good insulation, underfloor heating, glued real wood floors..

Central ventilation: max. 15,000 EUR.
Ground heat pump: 20,000 EUR, incl. drilling – only what the ground heat still effectively provides with today's kWh? Air heat pump costs 3,000 euros and the fan no longer really disturbs on a 1,200 m² plot either.
Good insulation can be achieved with a 50 cm brick, no wooden construction or 20 cm facade insulation additionally needed.
Real wood floors for 150 m² cost around 15,000 euros.

? What is a KNX bus for in a private house? I see no added value of a bus in a single or two-family house.
That is money thrown away.

What exact added value should KNX bring? Gira nameserver, blind control from the iPhone, wiring sockets, ....

Whoever wants to throw their money off the balcony... feel free to do so

If not you, then who?

400,000 equity and 7,000 net..... and you think that 300,000 loan is a problem? It can't get any better. If this financing doesn't work out, then you would be seriously making a mistake with your money

I am rather one of the cautious ones and find a loan of 300k high. I am also not someone who likes to be trapped in repayments for the next 30 years. But that is a matter of personality, because for you the fundamental data are fine. In my opinion, this concerns less the income than the fact that you bring in a lot of equity. Even in the worst case, you will come out of the "situation" positively.

I would generally build more cheaply, without large loans, and set aside reserves for renovations. After 30 years, the temple is only worth half the money, roughly > 300,000 euros burned. If you then want to cash in such a property because you are old and sick, you have to reinvest and first dispose of all the old stuff. ps. the whole construction financing is more like money destruction. You can have comfort for half the price - well, those who buy their food at the pharmacy and gas stations... gladly