Construction financing for new builds + sale of existing property

The 500 was an optimistic estimate, but in the end we don't want and can't go all in at a million.

That's why the question is what you need or what is nice to have.

So I would roughly calculate as follows:

Land 200k
Additional purchase costs land 20k
Basement 75k
Garage 20k
160 sqm * €2,750 (still a very optimistic estimate in Bavaria) 440k
Additional construction costs 40k
Buffer 20k

Total amount 815k

Maybe you can still contribute some own work -20k
- Equity capital 350k

Financing amount 445k € - I consider that realistic.

To what extent this is feasible with your income situation others can better assess :)
If in doubt, I would simply cut the garage and the pool or postpone them - and then implement if there is still money left.
Possibly also reduce to 145 sqm, that should work quite well for five people.

Just put this into the repayment calculator, assuming 4% interest, the rate over 30 years is around €2,150. That's about 40% of your household income (excluding child benefits) – I personally consider that ambitious with three kids.

The question also arises: what can we still afford at our age to be able to live "nicely" on the side?

There are almost 30 years left until retirement or pension.

If all else fails, my wife can go full-time (2 days on-site, 3 days home office).

I think it will have to come down to that if you want to "live nicely on the side."

Or what speaks against 150 m2 without pool/basement/garage. Then you can sleep more peacefully and enjoy your life more and also have enough space for other things if you then add the attic/garden house. That should be quite possible for 500k (3000€ per m2 x 150 m2 = 450,000€ plus 50k ancillary building costs).

Attic in the cube house? The roof shape is extremely impractical for that. For storage space, a gable roof with a proper attic would certainly be more favorable.