Construction costs are currently skyrocketing

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I once knew someone who was forced to undergo an audit by the tax office.
...

A few years ago, I had an audit. Two men from the tax office showed up with really large briefcases (plural!). That made me nervous... A few hours later, they were genuinely amazed that I had reported the rather unusual circumstance (only that was audited) 100% exactly, and they left. They are (mostly) only human too. But I also know opposite stories (forced to file a tax for a type of company that was not taxable at all. I believe more than 10 years later, it was judged and hundreds of thousands of euros were returned to those wrongly convicted. The company, of course, did not survive ... :-(

But who is supposed to check that...
If Aman simply states 600 watt inverter and has 800 watt?

Many metering point operators require the unit certificate of the inverter directly upon registration.

And indeed 80% one to one, because hardly anyone is at home during the sunny hours of the day.

Yes and no. You should try to orient one photovoltaic module to the east and one to the west. You have a lower annual yield, but it is better for covering the base load. At my parents' place, I installed the modules a few months ago on the shed with an east/west roof.

How do you actually use such a balcony power plant? Purely as an off-grid system via the inverter? Or is it connected with the apartment? Without storage, I imagine the use is difficult, maybe for charging a handheld device..

Purely as an island system via the inverter? Or is it connected with the apartment?

The unit is connected to your power supply. If your yield is higher than your consumption, this electricity is fed into the grid. You don't get anything for it, but depending on the base consumption, it's usually not much.

Because it's harder to troll real numbers, here is my calculation of the profitability of our photovoltaic system from 2020, just quickly adjusted to electricity prices that make more sense for 2022. You can ignore the yellow box, that was some other calculation for the electricity tariff. :)
The price of the photovoltaic system is still the original from 2020. I don't know where systems stand nowadays, but we actually already paid too much for ours at 1770 net / kWp back then.
The self-consumption rate of about 40% is more aimed at the average employee, and it is achievable if you put in minimal effort. My husband and I both work from home and in the first 2 years with the system, we were more around 60-70%.
At 65% self-consumption, the break-even would already be in year 8.

So I don't think you can say that such a system does not pay off. Rather, it pays for itself after 8-10 years and the rest is profit.
The argument about the missing electricity from the roof when the heat pump needs it most in winter is so old, it has already been in the "Bartwickelmaschine" in Pirmasens three times. Besides winter, which of course offers the least solar yield, there is also autumn and spring. Both are heating periods here in Germany as well and we had excellent synergies between photovoltaics and heat pump.

It says:
incl:
Single-family house with gable roof and 145 m2 net floor area

[*]Architect services
[*]Fine plaster, exterior
[*]All interior walls
[*]Aluminum roller shutters
[*]Fourfold insurance coverage
[*]Outdoor space upgrade

99,999 instead of 123,999

I ordered the info package. I sacrifice myself for the forum community.

So I actually got the representative to send me the scope of work description.
Here’s the gist.
Exterior walls yes, but only up to the stud frame. Insulation of the panels (according to plan with mineral fiber), vapor barrier, and interior cladding all included as owner’s scope (EL).
Interior walls also only wood stud frame, the rest must be done by yourself.
Ceilings are wood beam structure with wood-based panels on top (I suspect OSB). The rest must be done by yourself.
Gable roof structure without insulation.
At least concrete roof tiles and USB included.
Gutters and downpipes (titanium zinc) included.
Plastic windows according to plan with Ug 0.6 included (no security or sound insulation features).
Aluminum roller shutters with crank
Plastic front door
1 planning meeting and otherwise whatever documents and evidence are necessary for the building permit.

Everything else are chargeable services, which can be additionally ordered but are not included in the basic service package.
I have to say the scope of work description here pretty well explains everything else that comes up (and then is owner’s scope). Because of the requirements they set, you almost have instructions on how to do it.
For the base waterproofing, they provide many examples of how you could do it, with sketches and descriptions.
I honestly think that’s not bad at all.

So if I had to estimate how the entire house construction might look in the end:

Shell house Massa Haus	€99,999
Geotechnical report	€1,000
Surveyor	€6,000
Earthworks, simple plot	€10,000
Foundation slab	€15,000
Perimeter insulation	€10,000
Owner’s scope walls + ceilings	€30,000
Owner’s scope floors top layer	€10,000
HVAC (subcontractor)	€60,000
Electrical (subcontractor)	€15,000
Utility connections	€10,000
Owner’s scope outdoor facilities (minimal package)	€15,000
Contingencies	€15,000
	€296,999

So for a 140 m² standard floor plan plus a lot of owner’s scope work. What do you think? Did I totally miss the mark on any of the cost estimates?