Single-family house 172 sqm with garage and sauna

you have about 20sqm less than we do - and the garage is about the same size. 20sqm x 3m = 60m³ we have 1250m³ - so you about 1150m³* 350 =400k€ No idea where your 300k€ come from. best regards

According to our architect's calculation, we have 797 m³. House size = 172 sqm

So the sauna is only accessible from the garden or the garage, or is that the tiny workshop?

The sauna is on the left, only accessible from the garden. The tiny workshop is on the right as a passage.

A small selection on the topic of budget:

https://www.hausbau-forum.de/threads/liste-der-anfallenden-Baunebenkosten-bauseits-teurer.9737/

https://www.hausbau-forum.de/threads/warum-ein-Hausbau-fast-immer-teurer-kommt-als-kalkuliert.16237/

https://www.hausbau-forum.de/threads/kostenanschlag-ueberschreitung-möglich.32463/

https://www.hausbau-forum.de/threads/baukostenentwicklung-noch-normal.28743/

https://www.hausbau-forum.de/threads/Bauvorhaben-Neubau-Budget-gesprengt-Erfahrungen.28751/

Regarding the floor plan:
Draw in all the correct furniture to scale as you want or have it.
Take walking paths into account.

Do you really want to squeeze a sauna into the narrow storage room next to the bicycle, lawnmower, and so on? That feels like a warehouse atmosphere, but nothing like wellness.
A bathroom as a passage room doesn’t work. Uncle Franz is really sitting on the toilet and the kids come in.

Overall, I would start over with the floor plan. After you have planned the budget.

Demolition: Ask several demolition companies. And if it’s not necessary, don’t give a timeframe. We searched for over a year for a company that does gutting and demolition. The offers ranged from 45,000 to over 100,000 including disposal. (It was more than for you and in Bavaria disposal is quite expensive.)
Have the basement carefully backfilled and compacted. Definitely demand a compaction certificate with load plates if you want to build on top of it.

Double staircase: space-saving staircase to the attic. The staircase is not properly installed yet, I mentioned in the introductory thread that many things are not yet in the right place, but here we are seeing drafts where interior walls, furniture, doors, and windows are not yet optimally positioned.

Backpack in the bathroom: The bathtub is not placed correctly, I think. They originally wanted to put it in the bay window (that's why there is a wall above the stairs). But it doesn't fit; it is planned to be in the middle of the bathroom under the sloping ceiling with a skylight. The bay window will have two large windows and the stairs will then be free-standing, I believe we don't have a so-called air space...

In my pencil drawing, I had already taken into account all the furniture that we have. The interior dimensions of the rooms were then passed on to the architect. So the measurements are correct, but the corresponding furniture is not yet in the drawing.