How do you start? When do you assign whom with what?

Land value reference is the current one, right? In 2019 there were new reference values, if the plot has been "lying around" for a while maybe the old land value reference applies. But even then: a plot of 600 sqm (and that's roughly what you need for a single-family house) can easily cost 70,000 euros. If the market value is above the reference value (usually it is), then it gets more expensive. The 100,000€ for the plot is a good benchmark.
Then you have paid for the plot - notary fees, land registry etc. - all that comes on top.

But the land value reference is not a value that should concern you - that interests notaries and the tax office. You need the market value and depending on the region it can differ significantly from the reference value.

For the house, take a rough estimate of 2,000 - 2,700€ per built sqm - with your requirements rather towards the upper end.
If the house should be enough for 4 people, 150 sqm is rather on the lower side - that means if you assume 2,500€ per sqm and only build a smaller house with 150 sqm you are already at 375,000€. And that’s without anything else, no plot, no shovel of earth moved (you want a basement, don’t forget that), no notary, no land registry entry, no architect - not to mention the garden. And the basement comes on top with 60,000 - 100,000 (depending on equipment and soil conditions).

The houses from the catalogs and on the internet: nice photos, quite decent floor plans and the minimal equipment. So pitiful tiles and floor coverings (or even no flooring), cheap doors, cheap fittings, double glazing, max. 2 sockets per room (especially you as an IT specialist should know that will be tight) etc. In short: nobody gets by with that and they usually charge well for the upgrades.

So all in all:

100,000€ plot
375,000€ house
about 80,000€ basement

That already makes 555,000€ - and I would say that is still without [BUS].
On top: additional building costs, garden, garage/carport, furnishings (usually you end up buying a new kitchen etc.)

You’re a strange IT specialist...actually they are good with numbers..

no, Steffen, the IT specialist only needs 0 and 1

My assumptions for the costs are based on various websites where the costs for such a house are indicated between 150,000 and 250,000 (without land).

Your research is correct. Unfortunately, these prices do not include all the costs you have to bear as already writes. Building a new house is different from buying an existing property.

I simply included additional costs for the heat pump based on a gut feeling in the estimate, since I also do not know which model I would need here, and prices on the internet are difficult to find for this.

The additional costs for house equipment consist of material and labor. This is opaque especially since labor costs can vary regionally and from company to company.

I have now found a plot of land via gdi-sh that I could imagine fits. How do I know whether it is for sale or who owns it? The info I have is a parcel number, the standard land value (115€)

The standard land value and purchase price have no direct connection as aptly describes. Still, it is worth investigating. Sometimes you get a plot before it hits the market or because you become the welcome friendly neighbor or... To refrain from trying means to exclude luck.

And never forget, having is better than needing. And beauty fades, the hectare remains. This also applies to sqm. In 25 years the money will be enough for mahogany built-in cabinets, it doesn’t run away. But building an additional missing room is more difficult.

In our extremely privileged society, we have forgotten to distinguish what we want and what we need. Important are: health, being there for each other, food, shelter, and meaning. I am very grateful that these five are with us. I also enjoy the nice extras daily — but needing? I don’t think so. Having without real functional or aesthetic added value is unattractive to me.

You are a strange computer scientist... actually they can handle numbers well..

They can do that too. In addition, they are good at abstracting, constantly dealing with false assumptions, correcting them and learning connections intensively. Asking questions helps with this and that is exactly what is doing here.

- clarify your budget

- Look around here in the forum
or should roughly be within your budget
then look around here
https://www.hausbau-forum.de/forums/Baukosten-Förderungen.180/

- Get in touch with your desired municipality and ask about available plots of land. The Sparkasse often has some as well
Depending on what the plot costs and how high your budget is, it will become clear whether you can even build.
Unfortunately, 300,000 is not a lot and you can forget about a basement, double garage, KNX.

Forget about the prices circulating on the internet. They are so much sugarcoated and sometimes shameless that it’s still published.
Behind the prices there is a construction service description. I once had one in my hand that looked nice, including complete electrical installation. That included 10 sockets. Even in 1950, more was installed.
Then all the notes about construction-side work or costs that are quietly omitted
Sometimes the prices are turnkey. Sounds good. Turnkey does not mean the moving van arrives and you move in. Often turnkey means a lockable door is installed. No tiles, no flooring, no paint on the walls.

When you know which plot is an option and what it costs, talk to the next house builder around the corner. Then it’s clear
- Slope location
- Orientation
- Development plan
etc

Yes, that’s roughly my price range. However, if the [Grundstück] is excluded and things like the kitchen and/or the occasional extra wish are also not included. Otherwise, with everything we’re at around 350k. Should the 300k here be just for the house or including the [Grundstück]? If the latter, then you will really have to say goodbye to all your wishes like the basement, garage, various technology, etc. If it’s just the house, you will still have to give up some of your wishes (especially the basement) and then manage well, bringing in a lot of [EL]...

I just spoke with my banker. So 400,000€ would be the recommended upper limit. 500,000€ would theoretically still be doable, but it wouldn’t be fun anymore. That’s what the house + land may cost. However, these were only estimates and not calculated down to the exact euro.

Then clarify what the plots cost. You can forget about your special requests. They are not possible. Most of us have made compromises. Wishes and budget are usually a square peg in a round hole. Why build now and not save more?