Attic conversion with a 25-degree hip roof - a sensible project?

How old are the children?

Here is a wardrobe bed 180 * 200 with a sofa.

Does a utility room with washing machine & co. need to be heated?

At least it makes little sense to have an unheated room within the thermal envelope besides the pantry. In utility rooms that are also HAR, there is usually waste heat from the equipment.

How old are the children?

Here is a wall bed 180 * 200 with sofa.

Nice curtains. They match the zeitgeist of the wall bed.

In the utility room, which is also a multi-purpose room, there is usually always waste heat from the equipment.

That is probably sufficient if the heating is also in the room, but not with a single washing machine that is only occasionally on. I mean, what equipment would be in there anyway?
It's also a question of whether the dedicated housewife wants to hang laundry out at -20 degrees.

How old are the children? Here is a wall bed 180 * 200 with sofa.

The kids are still small (4 & 2) and the children's rooms are very spaciously designed.

That is probably sufficient if the heating is in the room, but not with a single washing machine that is only on occasionally. What I mean is, what kind of technology would be included? It is also a question whether the inclined housewife wants to hang the laundry out at -20 degrees?

If you build a wall in the very large garage, that is outside the thermal envelope. The heating room is directly in front of it, but separated from the garage by a heavy (insulated) door. In the new utility room, the washing machine and dryer should find their place and maybe a few wall cabinets to store detergent/cleaning supplies & co. It should be warm there since in winter you would otherwise have to do the laundry wearing a jacket… although at my parents’ place the machines are in the basement and it was never really warm there either