House construction in Saxony, how do I plan realistically?

You should have an external construction supervisor anyway. And with Bauherrenhilfe, for example, the examination of the construction specification and the contract for work is included in the price. Among other things, it is to ensure that you are not suddenly surprised by additional costs.

Where do we get an external construction supervisor from, is he permanently on site or does he only come occasionally and inspect the construction work?
I looked at Bauherrenhilfe yesterday but couldn't find anything where I can check the construction specification, how exactly does that work?

... and I see the allocation of house numbers procedurally almost at the very end.

I agree with you, except regarding the allocation of house numbers. This morning I had actually been thinking for a while about whether I should recommend to the thread starter to apply for it "almost first," because it’s fun and, at least for me, it was easy, quick, and inexpensive, but the emotional added value for me was enormous. From my point of view, it’s simply great to get the house number as soon as you have the plot ... everything else can still take "forever," ;-) but no one can take the house number away from you anymore. :-) Admittedly, it’s an emotional matter and perhaps doesn’t make everyone as happy as it makes me. As soon as the plot was paid for, I even called the municipality to ask when I’m allowed to set up a mailbox. ;-)

Where can we get an external construction supervisor from, is he permanently on site or does he only come occasionally to inspect the construction work? I looked at Bauherrenhilfe yesterday but didn't find anything where I can check the construction performance description, how does that work exactly?

Bauherren-Schutzbund, Bauherrenhilfe, etc. Just ask Uncle Google. You should never build without an external construction supervisor, especially not with Heinz von Heiden.

From my point of view, it is simply great once you have the property to get the house number ... everything else can still take "forever", ;-) but the house number, no one can take away from you anymore.

Oh, you sister in spirit, I did something similar: I hadn’t even registered for driving school yet, when my dream car radio model was on sale, and even with installment payments. So I had the radio before the driver’s license ;-)

But if someone could name the perfect one for me, I would be very grateful.

There is no perfect one. You just have to know what else must and can be included.

We hope that if we hold back on the selections, the price will remain the same.

It’s not just about the selections. Most of it comes later when the craftsman comes in. For example, it’s about additional costs for earthworks. It’s about the missing painting work, which concerns not only walls but also roof overhangs, stairs, etc. It’s about containers and porta-potties. It’s about additional technology, better exterior plaster, possibly better interior plaster, sewage connections, which rather affect the property again and are therefore not even listed, and so on.

How can I have the construction specification checked? Do you even have a chance to renegotiate the construction specification, or is it a "take it or leave it" choice?

Association of Private Builders (Verband Privater Bauherren), for example. No, the construction specification is part of the contract for work, which was once prepared for the company and is watertight. Additional items are attached with prices upon request and desire. You can also try to negotiate. But since Heinz von Heiden is represented throughout Germany, I would be surprised if they need to.

How can I have the construction service specification reviewed? Is there even a chance to renegotiate the construction service specification, or is it a "take it or leave it" situation?

A construction service specification has two dimensions: a legal and a technical one. Lawyers or home builders’ associations can well examine whether tricky formulations or dangerous omissions are included. Technically / qualitatively / in terms of sample levels, I advise different approaches depending on the type of construction project: For individual designs I would not passively review the construction service specification, but actively develop it myself together with the independent architect; for catalog houses my Steinemantra applies accordingly: build Müller’s way with Müller and Meier’s way with Meier – no mixing, or a big rearrangement that turns the cordon bleu into a hunter’s schnitzel, and also not building according to the construction service specification of the first queried with one of the others.