Is KfW40+ possible even if photovoltaic and controlled residential ventilation were already required for KfW40?

We have just done that or have had it calculated. To achieve the standard KfW40, there are several ways, but the building envelope, roof, windows, and insulated floor slab are important points that significantly influence reaching the goal. A little more here, a little less there... a proper energy consultant can tell you exactly that based on your planning. It could just as well be that your used masonry is rated so “poorly” that you would have to upgrade extremely in other areas. So it is by no means the case that you just need to have this or that and that’s it; you also have to prove that you actually installed these things (with evidence) and that it is technically flawless. If you don’t get the stamp, you won’t get any money from the KfW. Photovoltaics would even be okay with a small system of just over 3 kWp, and equally a small storage unit. Then you get the bonus after you have first reached the “40”. In the end, it remains a calculation exercise for everyone, and if you spend more than you receive in subsidies, you still have the better insulated house. I’m not building for or because of the KfW.

Does the KfW possibly appear a bit strict there?

No, just exactly as it says in there.

Otherwise, for me, practically only the controlled residential ventilation + storage would be necessary to jump directly from KfW55 to KfW40+.

No, unless you were already so close to KfW with your KfW55 house. Ask the energy consultant, preferably a good one, then it will work out. We are building the house solely with infrared heating, and it is working just fine.

Exactly, the energy consultant is on board and is currently making his calculations. The initial forecast was that we will definitely achieve KfW55, and even slightly exceed the requirements regarding primary energy demand and transmission heat losses. With the photovoltaic system, we will probably already meet the KfW40 requirements in terms of primary energy demand. For the transmission heat loss – since we apparently already slightly exceed KfW55 here – a controlled residential ventilation system might then be sufficient to also meet KfW40 requirements.

There you go. The energy consultant is the one who decides. If the controlled residential ventilation is then the crucial point and you also want it, then it fits.

So far, we are still undecided about the controlled residential ventilation because it is another five-figure item. However, if the controlled residential ventilation gives us a repayment subsidy that is €6000 higher (KfW55 -> KfW40), that could more likely move us toward a decision in favor of controlled residential ventilation. And assuming we decide in favor of it and the controlled residential ventilation ensures that we get KfW40, the question arises what it would be like if we then take one step further to KfW40+. Does the KfW make a distinction as to how the low transmission heat loss levels were achieved, i.e., only through (more) insulation or with the help of controlled residential ventilation? I don’t know if I was able to express it clearly, but is it possible that the KfW says:
“To get KfW40+, KfW40 must already have been achieved without controlled residential ventilation so that with controlled residential ventilation a significant reduction of the effective transmission heat losses can then be effected.”
The idea behind KfW40+ is to further reduce the energy demand of a house compared to KfW40.
But if I already have photovoltaic + controlled residential ventilation at KfW40, then the difference between KfW40 and KfW40+ consists only of the (slightly) increased share of self-consumed photovoltaic power through the storage.
Maybe I am also thinking too complicated and would rather be stirring up sleeping dogs.

As far as I know, the only difference between KFW40 and KFW40+ is the storage for the photovoltaic system. You can simply take it with you (as small as possible).

As far as I know, the only difference between KFW40 and KFW40+ is the storage for the photovoltaic system. You can simply take it with you (as small as possible)

Or you do it in a way that actually makes sense, meaning possibly larger photovoltaic and storage. It has to fit your own system and lifestyle. If I didn't want the controlled residential ventilation anyway, I wouldn't install it just for the subsidy. But if you plan to do it anyway AND get a subsidy, then that's good. That's how it is for us. Best talk to your energy consultant, because they ultimately have to approve it for the Kfw. We fiddled with various settings for a while and only did it when it suited us as a whole. Additionally, we got an important part of the measures almost cost-neutral, which made the decision easier. In principle, I would have built an energy saving ordinance house and would not have felt any worse living in it. I think you have to be careful not to chase subsidies and keep the actual costs in mind. Often people think they get the Kfw money as a gift; ultimately, it is only supposed to be support or an incentive for an already intended measure. If Kfw40, then I would also do Plus and build a proper photovoltaic system on the roof with the appropriate storage. Just some "toy" somewhere, just to barely meet the minimum requirement, would not appeal to me.