House construction planned for the future, how best to proceed in the Marburg area

Good morning everyone, I need some food for thought from you. I would like to build a house. Preferably in Marburg (but that is probably not feasible). At least somewhere relatively close in the surrounding area, for example Ebsdorfergrund. My girlfriend is a bit concerned and currently does not want to think about it. However, we are still living in my parents' house at the moment (€400 warm) so very cheap. But that shouldn't stay like this in the long term. We both work in Marburg and for me personally, renting an apartment is not really an option since we are planning to have a family. Regarding the financial situation... my partner earns €2000 net with an 80% position + €2000 special payment. My financial situation is a bit more difficult because I can hardly plan. €2500 net + €3000 special payment. In addition, there is a secondary income that at least this year exceeds the main job. However, this may and does not have to remain the case. That means guaranteed income is €4500 net + €5000 special payment. In addition, there is €20,000 - €50,000 net from the secondary income. €100k would be available as equity but would rather be important as cash flow for the secondary income. I haven't been to the bank yet; I wanted to first gather some ideas so that I can deal with the topic a bit more concretely. What I imagined: a house that is sufficient for a family of four. Some space will also be needed for the business. Costs should also be deductible there. Exactly what is possible, I will have to clarify with my tax advisor. That means a basement of 30-40 sqm should be planned as storage space. In addition, everything should be equipped with KNX. Lighting, blinds, heating, (possibly ventilation), etc. If you base it on the financial situation, what could be realized? How many sqm does a family of four need? It should not be bigger than absolutely necessary. In the end, my girlfriend has the final say. She wants it even more urgently than I do but currently does not want to deal with the topic. But that will come sooner or later. What costs can I expect if building not as cheaply as possible but of high quality? What would be the appropriate plot size? Thank you very much for your input

And I will avoid that when building the house. Only if she is fully behind it will we build together. For me, renting is not an option because there are simply too many points against it.

if you bring up the topic every few weeks and apparently engage with it, but the partner does not. Then something is wrong with the relationship

Very sensible comment.
I neither dug up the topic again a few weeks ago, nor can anyone allow themselves to doubt relationships where they have no clue at all...