Can a photovoltaic system be included in financing retroactively?

That is completely unproblematic, simply divide the loan correctly by percentage, then it will be fine.

Yes, you can break it down. But your own loan would of course be easier.

That is completely unproblematic, simply allocate the loan correctly in percentages, then it works.

The problem here could be that depending on the equity contributed, the loan portion for the photovoltaic system decreases.
Example
without BV
Construction costs 400K€
Equity 100K€
Financing 300K€ (share 75%)

with Photovoltaic
Construction costs 412K€
Equity 100K€
Financing 312K€ (share 75.73%)

Photovoltaic share
Construction costs 12K€
Financing 9.1K€ (share 75.73%)

With separate loans, in the case of 100% financing, the interest for the entire 12K€ could be deducted.

The problem here could be that depending on the equity contributed, the loan portion for the photovoltaic system decreases. Example: without BV Construction costs 400K€ Equity 100K€ Financing 300K€ (share 75%) with Photovoltaic Construction costs 412K€ Equity 100K€ Financing 312K€ (share 75.73%) Photovoltaic share Construction costs 12K€ Financing 9.1K€ (share 75.73%) With separate loans, in the case of 100% financing, the interest for the full 12K€ could be deducted.

The tax office does not want to know how high the equity contributed was; that is none of their business. Photovoltaic is fully financed. Period.

The tax office does not want to know how much equity capital was contributed, that is none of their business. Photovoltaics are fully financed. Period.

With "period" that doesn’t mean anything. How are you supposed to provide proof of full financing without separate loans? In tax law, proportional calculation is often used or it may not be allowed at all. You can read up on the effort required, for example, with partial use and partial rental of a two- or multi-family house to be able to claim the interest. You can consider yourself lucky if they allow proportional distribution at all. In the worst case, they completely disregard any interest claims. You will never get full financing of the rented portion approved without separate loans. The photovoltaic system should be seen analogously.

With a single loan, that's nothing.
How are you going to prove full financing without separate loans? In tax law, calculations are often done on a pro-rata basis or may not be allowed at all. You can have a look at the effort required, for example, for partial use and partial rental in a two- or multi-family house to be able to claim the interest. You can count yourself lucky if they allow the pro-rata distribution. In the worst case, they completely disregard the interest deduction. You will never get full financing of the rented portion accepted without separate loans. The photovoltaic system is to be viewed analogously here.

Musketier, I don't need to look that up, that's my job, so I've done it many times already...

One loan, claim the interest for the €12,000, and that's it. The tax office doesn't ask about that...