Costs for an extension to an existing semi-detached house

Hello

I have already read quite a bit here, but I would still like to ask the community for advice.

About my problem.
We have been offered a fully developed plot of land (270 sqm) for 125k.
The building will be an extension to an already existing semi-detached house.
One full floor on the ground floor, a second full floor in the attic.
Living space approx. 120 sqm.
I will find out more next week.

We have a total budget of 400k.
30k as a reserve.

I will also get an offer from tremmel Haus next week.
I have already spoken on the phone with Tremmelhaus.
On the phone, I was also told that we can manage with the budget.
The equipment would include a heat pump with underfloor heating.
But more details when we meet.

The question:
Is the project realistic with the budget?
I am well aware that compromises have to be made on the equipment.

Unfortunately, this whole thing is not giving me peace.
I have also already asked acquaintances who have built themselves.
Opinions vary widely, as always.
I would like to hear your opinion!
Thanks in advance.
Best regards

Budget is roughly rounded to 250k after deducting the land and ancillary costs. Divided by 120m², this results in approximately €2100 per m² of available budget. Reserves not included. So far so good. On the other hand, many details are missing, for example regarding the [Keller], [Garage] or special requests.

A basement would be nice, but it is not planned.
So it will be without a basement.
There would be space for a garage.
But it will only be realized when you see that the budget is sufficient.
That also comes off the extra 30k.
The kitchen too.
Extras from my point of view would be.
Walk-in shower, underfloor heating, and possibly electric roller shutters.
I would do the outdoor area myself (with my father-in-law).
I also have electricians and so among my friends.
But I don’t want to rely on anyone for now.