Floor plan of a semi-detached house with a guest room

I didn't understand the access to the garage; if the garage is on the left side of the house

... the property unfortunately has no street in front of it on this section anymore.

This is all just a numbers game. You can probably calculate a really great KfW value with 1m of crushed glass under the slab and 50cm of PUR insulation on the top ceiling even with 36.5 unfilled monolithic. Haven't checked it, but it's possible.

That's all just a numbers game.

... and in any case, "three times zero is zero remains zero" - because no matter how you try, KfW40 for 1.3k/sqm* is never possible.
*) I just had a brainwave: the planner might be confusing square meters and cubic meters!

Alright, I have now considered it independently of the context because I think it is an academically interesting question. But it doesn't help the OP, that's true.

House is now planned with the dimensions approx. 8.5x13m

Ok, I found it. However, I calculate about 180 sqm from that. Two years ago, that would have meant construction costs of about 360,000, namely at a low 2,000€/sqm. In some areas, more.

Number of people, current ages 3 (35,33,1),

A small family had big dreams.

I am puzzled that no incidental building costs are mentioned. It is even stated somewhere that if you build together with the other semi-detached house, 15,000 can be saved. That can only relate to the incidental building costs, so the 360,000 must include the incidental building costs!

You should ask around in your area how construction prices are. Maybe you are the bubble that we in the rest of Germany know nothing about. That is actually not meant ironically, but it may be possible, although I don’t believe it.
Plan B:

We have to build to KFW40 standard and if photovoltaics, then 40% of the roof area as a photovoltaic system (city requirement)

I would question that with the municipality or file an objection. For us, there was also a requirement for KFW55. That was the reason the new development area was not built on, because it found no approval among the people. And so this obligation was removed by the municipality.

…third children’s room (he added it because the ground floor is so large), pantry, open kitchen, although reluctantly ‍♀ we have already rationalized down from our “dream house”

Unfortunately, the “extra” room also has to be paid for. You are basically building sqm you don’t need. And I mention the attic, which you also have and could theoretically convert for further lodgers, children, or similar.

And here we come to the sore point:

You absolutely cannot do without: guest room/office and shower toilet on the ground floor

Grandparents will stay with us about 4 months (spread out) per year (older, grandpa not good on his feet anymore)

What is that supposed to mean? What does “4 months” mean? These are your grandparents and your grandpa, or your parents, who basically take turns living with you?! I mean: visits are nice! … if they leave after three hours. Or after three days and don’t come back for 3 months!
But it already reads like a second residence. Every weekend. Or one week in one month… It reads like this has already been agreed upon. As if you were a vacation home.
Or is it because they want to see the child? Maybe there is the alternative to find a small apartment for grandpa or for you to visit him? In the past, one gave up their own bed or set up a camp at the children’s place.

You simply don’t have the money to build for others, even if they are dear family members.
You should at least discuss this guest room with them. Maybe they will chip in something for the planned “right of residence” for the house construction?!

Edit: the OP has limited his profile as a newcomer. So I assume we won’t hear from him anymore :oops:

I have now also seen the incidental construction costs. 80,000 is already very well planned for a flat plot. Could it be that the calculations are done completely differently there? A lot is mixed up? How does the architect calculate? Like a freelance architect or like a general contractor?

and yes, the architect seriously calculated 223 sqm.

Maybe the 1300€/sqm are shell construction costs? :oops: