New construction after triplet birth. Looking forward to your ideas.

Hello,
here is the development plan and a sketch of our house on the property. We are allowed to build 0.4, but with garage driveways... 0.6 is also possible.

Regarding the budget, we are flexible. The nicer the house becomes, the more we are willing to spend. I would initially assume ~250-300k€.

Our heating system is currently massively oversized at 26kw, so it can also heat the extension. We do not want to install additional bathrooms. The children's rooms should be 15-20 sqm.

We are allowed to build 0.4, although with garage driveways... 0.6 is also possible.

I have some problems with PDFs on my phone, but whatever.

You have to calculate what is still possible. You don’t want to tear down, after all. What is possible, what should be, how much must and can be added.
You should - if not here, since I believe this thread is too chaotic - create that for yourself. For that, YOU need the numbers from the development plan and your house...

An architect is indispensable...
I basically consider 20 sqm for a child to be excessive and, in this case, also disproportionate. It would also be difficult to implement.

The currently existing children's rooms also do not have 20 sqm.

I don’t know what the building envelope is like, but spontaneously I would extend the living space and kitchen here with an addition of the same width as the existing house and then also extend the upper floor by the same footprint. The master bedroom and bathroom remain the same; from the newly gained space and the 2 existing children’s rooms, there will be 4 children’s rooms and, depending on the size of the addition, possibly a children’s bathroom as well as the [Laube].

During the renovation, the older child has to move downstairs; the three little ones sleep in the parents’ room; the kitchen can still be used until the very end, as can the living room.

And please understand that I
a) am reluctant to open PDFs when I am on the move
b) somehow also do not feel like reading through an entire development plan and extracting the essentials (I would rather expect that from you as input)
c) although I am now sitting at the desk with a PC and PDFs do not cause any difficulties, I am still not willing to read documents upside down

I don't know what the building envelope is, but off the cuff I would extend the living space and kitchen here with an annex of the same width as the existing house and then also extend the upper floor by the same footprint.

Yes, unfortunately the PDF only contains the changes to the textual provisions, not the image section of the development plan. A gable-side extension would be significantly cheaper in terms of effort - but for that, we would first need to know what "is allowed".