Is a tax advisor useful? House construction, renting, depreciation, photovoltaics, etc.

I had "back then" explicitly checked.

We did too. Two small businesses without sales tax + photovoltaics including sales tax. Ran for three years (sales tax, trade tax, income tax...).

We too. Two small businesses without VAT + photovoltaics including VAT. It ran for three years (VAT, trade tax, income tax...).

I would claim from my gut that guckuck is right.
You only file one VAT return for all businesses of a natural person.
Does the photovoltaics perhaps run as a GbR?

Alternatively, do you possibly have tax-exempt revenues, e.g., from brokering loans or insurance?

Is the photovoltaic system possibly operated as a GbR?

The GbR would be an option, although it must explicitly not be used for tax planning purposes. But well, that is probably rather the rule.

If the other small businesses are operated with KUR and have tax-free sales, this can also be one of the exceptions that currently exist.
There is also a de minimis limit so that no tax is levied ... but then you do not use KUR but rather this exemption limit.

Otherwise, I strongly advise caution, as the determination of whether VAT is owed or not lies with the taxpayer. That means you are not explicitly prompted.
If you have mistakenly applied the KUR, this is at your own expense. That means paying VAT retroactively on all invoices. Ouch.

To come back to the topic once again. There are some pitfalls regarding the allocation of costs, especially for mixed-use properties. I recently read a current article about this, where the tax authorities and courts denied the allocation of costs if they are not paid through separate bank accounts. For the re-rental of the existing condominium, you will also need a basis for depreciation (Afa), etc. Get advice on this. If necessary, you could also consider having the tax return prepared for one year and then doing it yourself the following year if it becomes too expensive for you.