Is preparing for photovoltaics in new construction sensible?

Please briefly describe your roof (roof shape, available roof area, orientation, and any obstacles such as bay windows/dormers/chimneys).

For a classic photovoltaic system, you need an inverter that converts the direct current generated by the photovoltaic system into alternating current. Depending on the size of the system, the inverter must be appropriately sized.
As the power of the inverter increases, so does its size.

Briefly describe your roof (roof shape, available roof area, orientation, and any obstacles such as bay windows/dormers/chimneys).

For a classic photovoltaic system, you need an inverter that converts the direct current generated by the photovoltaic system into alternating current. Depending on the size of the system, the inverter must be appropriately sized.
As the power of the inverter increases, its size also increases.

We have a roof pitch of 22 degrees, no bay windows or dormers, only a chimney from the gas fireplace on the south side. I don't know how much roof area that is now.

South-west orientation.

What would you recommend? Rather 3-4 DN50 empty conduits or are two DN75 enough?

Sorry, what kind of empty conduits are you actually talking about? Do they all have to go from the roof to the connection room? I thought all the lines are brought together under the roof and then one cable comes down to the inverter. And then one thick empty conduit is enough or you just have the cable laid directly. Isn't it possible like that?

we have a roof pitch of 22 degrees, no bay windows or dormers, only a chimney from the gas fireplace on the south side. I don’t know how much roof area that is now. South-west orientation.

The roof pitch is positive, so it will probably be worth covering the north side as well. You need to calculate that. It’s a bit unfortunate that you have obstacles on the south side. If the house has not yet been built, I would ask if the chimney can be relocated to the north side.

Since probably all roof sides can be covered, different wiring options are possible for you:

a) very classic
2 small inverters each with two MPP trackers and one string per roof side
b) Solaredge
1 inverter with MPP tracking at module level through optimizers.
c) Solar modules with microinverters (e.g. Sunpower Maxeon 5. I don’t know if that is already available)

I would have two M60 conduit pipes laid from the attic to the utility room; this is rather generously dimensioned but then forgiving of one or two bends and you can lay cables afterwards.

We had 9 cables laid through an M40 conduit. Since we had a bend in the conduit, the floor covering in the attic at the bend had to be cut open so the cables could be laid from there. That was very annoying.

The positive aspect is the roof pitch; it will probably be worth covering the north side as well. That needs to be calculated.
It's a bit of a pity that you have obstacles on the south side.
If the house hasn't been built yet, I would ask whether the chimney can be moved to the north side.

But the north side hardly gets any sunlight, so how could it still be worthwhile? Sorry for the silly questions, but I really hadn't dealt with this topic at all before.

The chimney can no longer be moved because it is a "shaft" for a gas fireplace and it is located where the fireplace will be. On the north side, the chimney would run through the middle of the hallway on the upper floor.

Maybe I could have the roof access window moved to the east, so only the chimney would remain there. The question would be whether it really makes a difference whether it is there or not.