Is it possible to build a new building for 900€ per m²?

As nordanney already writes, when deviating from the standard, you usually only get credited a fraction of the money that the production actually costs. Furthermore, many turnkey providers are also reluctant to coordinate with external trades (own contributions "in the middle" are therefore often not welcomed or sometimes not permitted at all – to avoid disputes, for example, over who is responsible for a mistake...). I don’t know exactly how it is with Heinz von Heiden.

Is there also the option to decommission?

For example, with the cheapest bungalow with over 100m2, the incorrigible B760, a small photovoltaic system makes no sense financially,

My uncle works as a carpenter and has already offered to install the windows for me at material cost and a small fee. I don’t really need something like roller shutters either.

Now I am sure that a new solid house built by an architect is not an option for me.

And what if you don’t provide any own contributions but simply leave things out altogether? For example, the small photovoltaic system (which doesn’t bring anything anyway at that size) and the roller shutters.

Often yes, but never and with no provider profitable for you.

Go into any café and ask what the price is without the mini cookie. The photovoltaic system is calculated here as a promotional gift, with which the last wavering customer is tipped into signing. It is meant to show how important your impression of the provider’s price-worthiness is to them. Mind you, only the impression.

A parquet-layer uncle would be a bit more practical. The windows don’t come so late in the construction schedule here that fitting an external subcontracting position wouldn’t be disruptive. Here, the logistics disruption penalty costs you more than a credit could ever amount to. Especially since the profit share is never credited (actually, the whole margin is not).

Then you have at least not read or understood my post #22. Go to an independent construction consultant — among them, the architect is not the worst choice — and have a preliminary draft created and get advice on the fittings. Then, either you or the consultant look for which house models from which providers are suitable. If necessary, it will be a shell construction or a shell house stage.

And what if you don't provide any own contributions but simply leave things out altogether? For example, the small photovoltaic system (which doesn't bring anything anyway at that size) and the roller shutters.