I consider making a house build dependent on children to be unproductive. As you already write, it’s about the garden, possibly more space, and maybe also more peace. If small children are involved, that’s great, but it’s also not easy to manage everything. Not to mention the financial side. If the kids are already older, however, building has its charm. They can better understand the scale of this project, can also have a say. After all, the two of them will eventually inherit it. And very importantly... at this age, they can also help out and contribute. Regarding the additional rooms that might be empty later on... it’s best to separate the rooms now with a lightweight partition wall and open them up again later. Great hobby room, or spacious guest room. :-)