Calculate additional costs!?

Hello home builders!

I am new here and my wife and I are about to buy a house.

What unsettles me the most is the calculation of the additional costs.

The house (120 sqm) with plot (280 sqm) costs €249,000. Everything except wall and floor coverings (including bathroom and guest WC) and the outdoor area is included.

What does a realistic calculation of the additional costs look like for construction incidental costs, wall and floor coverings, and minor changes such as more power outlets, etc.?

What should we plan for with a small buffer?

If it’s important, the house is being built in NRW in the district of Euskirchen.
The developer says that changes with him are manageable in terms of costs. He has been building houses for 20 years and does nothing else. The times when he made his money with surcharges are over.

Example: We want to swap the guest WC with the utility room; the surcharge would only be €400 for an additional ventilation in the guest WC.

I know, you can say a lot, and only the written word counts, but I buy it from him.

How is anyone supposed to know now what an additional socket costs.

What is already covered according to the construction service description and what do you want/need more of. With the list, you then go to your construction partner and he should write a new price below it.

Other additional costs are at least: property transfer tax, notary, land registry office, soil survey, if necessary surveyor, internal development, insurance, building permit.

Notary, office, and tax costs can be found using various calculators on Google.

Notary and land registry each independently incur costs twice depending on the situation.
Once when buying and if financing is planned, also the costs of the mortgage registration.

If you buy house and land from the same seller, property tax is due on both.

Land transfer tax, property tax is something different and is paid annually to the city / municipality. :)

Then simply go through every listed item in the [Bauleistungsbeschreibung] and inquire about every conceivable change, especially different tile sizes on walls and floors, every additional two-way switch and socket, internet and TV sockets, potential floor shower, surcharges for other front doors or shutters, interior doors, tile edges and tile skirting, etc.
He should provide you with the additional costs of each item in writing.
Also of interest are the wall coverings, to what extent will spackling be done in your case ([Dachinnenraum]), what kind of plaster is included? Do you want a different plaster quality, or do you plan to wallpaper anyway?

Additionally, there may be other material costs (tiles) or whatever.

There are also quite a few threads here on the topic of costs for flooring when doing it yourself.