Gas heating new building - Is solar thermal necessary for hot water?

My opinion. I wouldn't invest more here than technically necessary.

Necessary to be able to (later on) operate a heat pump. That would be my minimum requirement. Then gas condensing boiler, controlled residential ventilation [for comfort reasons] and if possible (ask an energy consultant) better to insulate a bit more than to install something expensive on the roof.

Do you really think that in any foreseeable future someone would throw out an existing gas heating system with connection and boiler and spend tens of thousands on this pump? I don't believe that. Why would anyone do that?

Why should one do that?

Because the legislator requires it, for example. Or because electricity is much cheaper than gas and the boiler needs to be replaced anyway. If the rest (i.e., the underfloor heating) fits, then one has the choice whether the heat is generated with oil, gas, bad electricity, good electricity, or pellets, and then has the opportunity to choose the best option available.

I see it differently. They can't prohibit existing things at all. There will still be oil heating systems in 50 years... And my life experience tells me: gas prices fluctuate, electricity prices only know one direction: upstairs, Karsten

I see it differently than you, Karsten. If the underfloor heating is well planned, you simply have the option to also install a heat pump. So you are overall super flexible. If it is poorly designed, you can NEVER install a heat pump without high monthly costs. And by the way, it doesn't cost tens of thousands even with corresponding own effort. It only costs an extreme amount with the general contractor because they make a hefty profit.

I agree with Karsten.
(It may also be due to our calmness in old age)
Gas plus solar thermal system is the cheapest way to satisfy the state. You yourself have more money available for other things. If the house is around 200,000 because it was built in the north or is quite simple, it rarely has any other intention than to install an inexpensive heating option.
Anyone who likes to spend over 300,000 on their house will look more than others at how they can implement 55 or 40.

Once you live in the house, you eventually have no desire to deal with replacement or renovation (unless you have to).

Nowadays, you can no longer really speak of a potential loss in value, since we are already at a very high energy-saving level with the Energy Saving Ordinance and the differences in consumption are quite small – unlike the older houses from times when nowadays people are looking to switch away from oil because there is little insulation and the closed windows serve for air exchange.

Of course, everyone considers this somewhat individually, because for one person the savings of 120 are very much, for another just peanuts when viewed annually.
So there is no significant reason for me to update to a heat pump etc., and gas is a solid thing.

In my opinion, far too much money is planned for things that are then argued with “but you could do it later.” And I do not believe that if you don’t intend to do it right away, you will implement later what you could...

Brief regards