District heating does not meet KfW55 requirements?!

-> Sorry, do you mean a central ventilation system here? At the moment we have planned a decentralized system without heat recovery?

Yes, a central system with heat recovery.
First, read a bit about decentralized systems and whether you really want that...

Our developer alternatively suggested installing a 5sqm solar system via photovoltaics for electricity generation. What do you think of that? By the way: When speaking of photovoltaics: How do you distinguish the terminology when using it for hot water generation compared to electricity generation?

Photovoltaics = electricity
Solar(thermal) = hot water

A 5sqm photovoltaic system is economically nonsense, it never pays off. But presumably it was about tackling the topic of renewable share with the lowest possible investment. Although, as I wrote above, I am not at all sure whether a photovoltaic system can provide that share and is not just positively considered in primary energy demand. As far as I know, that only works if the generated electricity is also used for heating (-> heat pump or electric heater in hot water tank. You don't have the former, the latter is extremely inefficient). For heating support, solar thermal makes more sense, except with a heat pump.

Yes, you are right. The 5sqm photovoltaic system is probably only about meeting the criteria for KfW55. We are currently at the point where we are considering whether

a) we should most likely switch to a central ventilation system. The additional costs would then have to be clarified as to who bears them and to what extent. As I said: KfW55 was assured to us according to the contract.....

b) we give up on KfW55. If it only fails on this small point, then we just have "KfW55+X". It's all just pure calculation anyway. I think the standard is only relevant for maintaining the loan. In a few years, no one will ask anymore whether it is KfW75, 55, or 40. There will be completely new requirements then, right?

a) we will most likely switch to a central ventilation system. The additional costs would then have to be clarified accordingly who has to bear them and to what extent. As I said: KfW55 was promised to us according to the contract.....

If it is contractually agreed, that looks good for you at first Your contracting partner will surely prefer solutions that are economical for him. Meet halfway. At least you get something from the controlled residential ventilation with heat recovery. Argue it as an upgrade to the decentralized ventilation, which can be offset, and if you then add 2-3k€ worst case, everyone benefits. Otherwise, your general contractor will just make back the sunk money at the next upcharge

b) we waive KfW55. If it is only missing at this small point, then we just have "KfW55+X". It's all just pure calculation anyway. I think the standard is only relevant for getting the loan anyway. In a few years no one will ask whether KfW75, 55 or 40. Then there will be entirely new requirements again, right?

The financial advantage of the KfW 55 loan through interest benefits and repayment subsidy (5k€) is around 10,000€. Naturally +/- depending on which terms you compare and what your own bank conditions look like. Not world-shaking, but quite nice.
However, you have to have the regenerative share according to the Energy Saving Ordinance. That has nothing to do with KfW. Just to make it clear again: Your problem must be solved, otherwise there is no house!
Check if the city requires KfW 55 as a minimum standard. In our area it is a modern bad habit to do this in new development areas. For the green conscience of the authorities.

You have to have the regenerative share according to the Energy Saving Ordinance. That has nothing to do with KfW

Yes, apparently before 01.04.2016 it was still sufficient as currently planned. However, I am not aware of anything that requires building KfW55 in our area. That is up to us.

Your general contractor will otherwise recover the sunk coal at the next upgrade

You are right here as well. Therefore, it is feared that possibly the cheapest controlled residential ventilation with heat recovery will be selected just to fulfill the requirement. I will get in touch with the general contractor tomorrow and have him present his alternative.

To revisit the topic: If the GU recommends a solution involving a solar system with a buffer tank, does that mean photovoltaics or solar thermal? Since we get the hot water through the [Fernwärmeanschluss], the latter doesn't make sense, right?

That will be solar thermal, which heats water when exposed to sunlight. Hence the (large) buffer tank.
The point ... well, if you assume that solar thermal does something, it will reduce your hot water costs. Whether that's such a big deal, I would at least doubt.
Also consider, a large buffer tank needs floor space, and the floor has to be able to bear the increased load (I wonder if the general contractor factored that in ...). Since it typically stands in the basement/utility room, that might not be a huge drama, but without the buffer tank you could fit an extra shelf in there.
Personally, I see the greatest benefit in the controlled residential ventilation with heat recovery. But that was probably already clear.