Planning of a single-family house (basement + ground floor + upper floor) on a 480m² plot

I don't want to harp only on the budget, but your 900 are shaky too. For example, my rough estimate is missing the architect (around 50k) Also, there is no buffer included yet. And since you are just starting to plan... add about 5-10% price increase until you get that far. And just saying... integrating a sauna into a bathroom... unless we are talking about a DIY infrared cabin from a hardware store... you are also looking at an additional 10-15k upwards... You already said the carport can come later, etc... and you can do some things yourself. But don't overestimate that. Materials still cost... especially for the outdoor facilities (do you need retaining walls?). Regarding the budget, also take a look here

yes, I remember how in 2009 I thought it was nonsense to use electricity just to collect some virtual play money.
I should have, the computer was always on anyway, only instead of loading the donkey I should have mined BCs.

It really can't be that you can't get a single-family house for under a million... I still have to recalculate everything and see what might possibly be cut out.

I would N.E.V.E.R inflate a house for the purpose of presenting a staircase. No man can be that pretty (sang Gitte) :)

You already said it can also be the carport later, etc.... and do some things yourself. But don't overestimate that. Material still costs... and

Calculate with 140 sqm, without sauna, without basement (but larger garage or additional annex for technical room etc.) You'll come well under 1 million.