Construction financing as a young couple including own contribution

What you say is true. Basically, I call it a "dream" because until now I had excluded new construction due to the extremely rising construction costs. However, for the past 6 years there have been no good/affordable existing properties. Accordingly, one is now really dealing with the topic of new construction. Moreover, the new development area has been developed and affordable plots are available.

They can of course calculate what the concrete and masonry work would cost me approximately. But the total costs are what really matter. The company mainly works in hall and stable construction, concrete work, but also has experience with houses. The company is based here but primarily works in other districts, so the other trades are only conditionally an option. Locally there are also enough companies from which you can get quotes (much shorter travel distance). In the countryside, you know the local companies and know which ones you can work well with/which ones to avoid.

I still wonder: Here it is mainly written that you have to calculate with at least 3000€/m2 and that that would already be tight. At least, that's what I take away from many topics here. Friends of mine managed with under 2500€/m2 including garage and outdoor facilities, excluding land and additional costs. Calculated for 160 sqm, that is a difference of 80,000€. Can prices really vary so much regionally? I am honestly asking out of pure interest since I have only been dealing with it for a few weeks now. Or can one say: 2600€/m2 KFW55 -> 3000€/m2 KFW40? There is probably never a real flat rate; if I compare the hourly wages from NDS with Hamburg, there will naturally also be significant differences in construction costs.

I still wonder: Here it is mainly written that you have to calculate with at least 3000€/m2

+/- 3,000€/sqm These are the numbers I see in practice and that are also posted here by those who build. Catalogs always say "from". Additionally, there are always incidental costs, garage, outdoor facilities, etc. Sometimes one provider leaves out the base plate, sometimes the photovoltaic system (partly mandatory), mostly floors and walls, etc.

Can prices be so different regionally?

Different, yes, extreme, no. Material costs are the same everywhere. But these are all move-in ready prices. With EL it looks (completely) different. Saying "But I have ..." is correct, but does not change the standard. It can also be cheaper if you take a stock house and have no wishes at all.

Friends of mine recently managed with under €2500/m2 including garage and landscaping

Experience shows that such figures rarely withstand close scrutiny. When it comes to building, people fib just as much as when fishing, hunting, or having sex. Not that anyone necessarily wants to harm you, but many people don’t even know their actual numbers. Sit down with the experts in your family first and discuss this with them, clarify explicitly!!! exactly what they can/want to do for you and ask for reliable figures. Anything else is suicide.

There are also plenty of companies on site from which you can get quotes. (much shorter travel) In rural areas, you know the local companies and know who you can work well with/which ones you should avoid altogether.

Anyone can get quotes anywhere; what’s your advantage in that? A shorter trip often doesn’t reflect in the price. Nobody is going to give you anything for free. I built my first house young as self-managed, partly because I knew most people. As is well known, self-management is usually considerably more expensive and you alone are the manager on duty. Use the prices mentioned here as a guideline; if you end up cheaper, you’ll have money left over (you’d be one of the first). Better to calculate this way or only calculate this way.

Can prices vary so much regionally?

Yes!

Are all regions considered the same here in Germany? Let’s say Bavaria compared to Lower Saxony?

Here in the forum and elsewhere, people calculate with plus or minus 3000€/sqm. Of course, you cannot put 55 in the 40s cost drawer, the 16sqm window house with the houses that have 26sqm windows, plaster with clinker, etc.
And: There is a north-south gradient!
However, it is only about calculation, not about actual price determination. The construction company should figure that out.
Unfortunately, the district is rarely mentioned, many are afraid (no idea of what) and do not even name the federal state where they want to build.

Yesterday I received a new catalog from a prefabricated house builder from Schleswig-Holstein, which offers a turnkey single-family home country house 140sqm for 220k€.

Catalog prices are starting prices. There is a lot written about that here in the forum.
(Ours was offered at 149,500, we paid 205,000. Built 2013)
But there are cheap ones in the north. The service depends on the building service description, and thus also the additional costs. It can only be that some cannot keep their fixed price even after signing the contract.

Friends of mine managed with less than 2500€/m2 including garage and outdoor facilities,

I don’t know anyone who tells the truth when it comes to money. Either they conceal how much good salary they get for little work or how little salary they get despite hard work. Both are embarrassing or at least unpleasant. The same applies to house building.
By telling numbers, one achieves quite a lot with the recipient.
Most don’t even count the costs of outdoor facilities or everything together anymore. The garden gets expensive over the years, and you forget the trouble over the additional costs during house construction.
Just as most here brag that they are absolutely right and get by with their (low) calculation, it gets quiet around them later. Some report back here after several years and say that the plans were actually much more expensive.
Others say that they roughly managed with the 3000 (or 3200).
Many also lie to themselves or deceive themselves about their own work contributions.
Some don’t even count them as part of the house costs.

Look here in the forum to see who has built or signed a contract in the last six months. Everyone who posts here is quite honest (because anonymous) and has dealt with the subject. Then you read the +/- €3,000 or information about EL.

That's a lot of information at once, thank you!