How do I determine the house costs before buying land?

You can initially set the value of 2000€/m² for the house and find further indications on various topics here about what the surroundings (garage, outdoor facilities, additional costs...) cost. And you can then multiply that by 1.05 for each year you wait to account for the price increase. So from (example) 350k€ now, it will be 425k€ in four years. The fact is that you can hardly manage with the budget today unless you really build modestly. In three or four years... no chance. With some luck, your income will also increase accordingly by then.

Prices are currently rising faster than you can blink.
Where do you live, or where do you want to build?

And you can then multiply that by 1.05 for each year you wait, to take the price increase into account. So from (example) 350k€ now, it will be 425k€ in four years.

With 5% price increases, that would immediately mean additional costs of 17,500€ in the first year. Personally, I find that a bit exaggerated doom-mongering, but it could happen. The biggest risk I see is in x years due to new political regulations on energy saving (Energy Saving Ordinance, etc.).

If we pessimistically stick to 5%, then the amount of 17,500€ corresponds to a monthly savings rate of 1,500€.

In our region, the price increase at the last turn of the year was closer to 10% than 5% (I know this because I "was allowed" to sign a house construction contract in both years, so this is not hearsay)

We have already looked at many prefabricated house catalogs and have a rough idea of what the house might cost. But so far, all prices are ones we have researched ourselves.
We are calculating house costs in the amount of €300,000-350,000 for a 150 sqm city villa with a garage. Additionally, we prefer more modern technology (heat pump, electric shutters, underfloor heating, etc.). The building site is in NRW.

Nowadays
€300,000 house kfw70/Energy Saving Ordinance 2016
€30,000 ancillary construction costs
€25,000 double garage
€10,000 floor coverings
€20,000 outdoor facilities
€20,000 furniture and lighting

In 4 years: stricter building regulations due to climate protection etc., one salary lost due to parental leave, roughly 10% price increase.

In our place of residence, building plots will be sold in the next few months. We would very much like to acquire a plot there. We would start house planning in 2022 and begin construction in 2023.

So, why do you want to wait? You will probably also have a clause to have built by year x.

face26, so far our budget planning has looked like this: we said we don’t want to spend more than €450,000 on a house. Simply because, in my opinion, that is already far too much money. With this amount we would be debt-free after 25 years. Therefore, the buffer upwards is not particularly large anymore; I do not want to be deeply in debt.

Will this still be your opinion in 4 years?

My actual question, why I wrote down the amount here, was more about how to proceed. Buy a plot and let yourself be surprised in 3 years about what the money will suffice for?

Reserve the plot when the building wish is there. Have a building ground survey done, calculate the financing limit, look for a general contractor or a construction manager you trust to build the dream house.

Good that you are already getting informed. A house without inheritance or similar with two normal earners is usually no longer feasible nowadays. An apartment will have to suffice.

We built three years ago, 170m2 with basement and double garage. High-end fittings. Without land, with a lot of own work, 500 thousand without outdoor facilities. Minus kitchen, basement and garage, it is about 2300 euros per m2 (including ancillary building costs).