Avoid mistakes in the second house: Home construction company or architect?

A house of about 140 sqm (let's say, one allows oneself 160 sqm) is needed, for which a plot of the same size is sufficient.

But that doesn't really match

Today, instead of two children's rooms, there are supposed to be four, plus two workrooms plus a guest room.

plus parents' bedroom (that makes already 8 sleeping/guest/work rooms) plus large living area

True, got me. Home offices are basically additional "children".

For comparison, large houses >200sqm in the existing stock cost around 700k here in the region. That also includes a large plot of land. In our case, I see another new build as not feasible, because it is significantly more expensive.

If you check all the boxes on the wishlist, you also need to bring an appropriate budget. With 3 children and a fourth planned, there is probably little to be done about the children's rooms, and with 6 people, two bathrooms are certainly not excessive. With 4 children, however, I don't see two parents working 40 hours a week from home, so perhaps one office is sufficient. And if planned cleverly, such a study can also accommodate a sofa bed for guests. They usually come on weekends anyway, when you’re not working. And just like that, you’ve saved two rooms, which corresponds to about 30 sqm including walking paths.

With existing properties, you always have to calculate precisely. Of course, older houses are initially cheaper than new builds, but there is quite a lot to be done in the coming years. Energy-efficient renovation, switching to a heat pump or district heating. Bathrooms and kitchens have a limited lifespan and must be replaced after 40 years. And then there are the other things that are a matter of taste... smaller windows, compartmentalized floor plans instead of open-plan, ...
Realistically calculated, you usually come out cheaper with a new build and can set your priorities. That seems to be important to you.

@ Wilder Süden: If, for example, the windows in the existing building needed to be renewed anyway, I would have less hesitation to replace them with a large panoramic window front. I would simply not do that in our 4-year-old house.

Since our working hours overlap and especially because my husband needs a separate room, we already need two workrooms. They also don't have to be too big (each 9-10 sqm). The guest room (now 12 sqm, which is good) is regularly used by grandma as well, and when she spreads out her things, I have to put away my work stuff every time. Therefore, I moved into our bedroom with my home office. It feels like existing properties more often have large living areas with many rooms. Probably because building today is significantly more expensive than in the past. That might still be a solution for us with the many rooms.