Floor plan for a single-family house with 4-5 children's rooms

Then you would have a much too large bedroom later on.

How about combining your bedroom and a future children's room. It can initially have two doors, two connections for controlled residential ventilation, and later a drywall partition can simply separate a children's room. It’s a matter of two days... and in the meantime you would have a bedroom large enough. The two youngest could share a room for the time being.

Then include them in the room planning!

When converting the attic, please don’t forget the escape route and build a proper staircase upstairs right away.

And proper lighting, shading, and thermal insulation in the insulation. I see the whole project rather in the area of over 220 sqm. And thus not within budget. A friend is currently building something of a similar size (3 children's rooms, two offices) almost 200 sqm, 50 km from Hamburg, and the latest cost estimate with double garage and terrace was about 750,000 euros in solid construction with clinker facade. Without land.

I am not involved in the topic "solid house" vs "prefabricated house" at all.

That is not decisive at first. You can build good houses with both variants. Certainly bad ones as well. In terms of energy efficiency, prefabricated houses have slight advantages, but nothing decisive. In terms of price, my experience was that the prefab builders want to pass on a lot to the client, so you should factor in quite a bit of leeway there.

I don't know if a sensible extension – if possible – would be more reasonable. The perfect floor plan that remains unchanged until the children move out will be far too large for the budget. I'm really not a fan of basements, but without one? The laundry, the food supplies (a freezer alone is nothing), the children's sports equipment, or the wet snow clothes for six people. Six bicycles plus scooters, skates and the like fill a garage on their own without hoarding masses. I might plan like this – as small a floor area as possible, no projections or recesses. The basement will probably fall victim to the budget, although I already consider it almost mandatory here. Ground floor: living (later separable), dining, cooking, guest WC, wardrobe (quite large, e.g. what is the house technology in the standard plans), one storage room for drinks, food, freezers, cleaning supplies. Upper floor: bathroom, 2 children's rooms and 1 large family room, utility room + storage room. Attic (is it even possible to convert it into living space?) later expansion for parents’ bedroom with bathroom, office corner and house technology. In the garden a large shed for garden tools, bicycles and the like.

We now have offers from two providers (Viebrockhaus and Hanse Haus), which still need to be revised, but initially leave enough room in the budget.

That sounds suspicious at first: what is still missing – is the apparent discrepancy more than just the "builder-provided" items, are possibly additional incidental construction costs missing from the calculation?

I am also not familiar with the topic "solid house" vs. "prefab house".

Despite all the hate preachers who only proclaim one of the two as the only true one: in terms of quality, it is basically a matter of taste, as I already wrote in "Lightweight walls in solid houses?"

And I first have to take a close look at the building and service description.

If you want less "apples vs. pears" in comparison, I would contrast Viebrockhaus not with Hanse, but with Gussek Haus.

Oh, at Viebrockhaus, quite a bit more will be added. My sister was shocked two years ago at how quickly it got expensive, too expensive.

Hanse has this one variant that would fit our room planning so well. Gussek Haus has nothing that could fit. Then so much tinkering has to be done again that it actually can't be worth it. Yes, I will wait until everything is fully compiled. The construction and performance descriptions are too different.