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

It may also be possible that with 20% commercial use, 20% of the construction costs can be covered through the business, which would already be a nice thing.

or build privately and rent to the company. The "profit" then comes via the special depreciation over the next 4 years (I believe this only works if completed by the end of 2021)

I can’t imagine that half a million is enough today to build a house.

It depends on the plot and the house wishes. Plot: the sum is already quite sufficient. €150K plot 120sqm house at €2,300 = €276K (decent quality!) Additional construction costs €50K Total: 476 – there’s still money left for a carport and landscaping. If the plot only costs €100K, for example, also includes a basement or a larger house, or... If you don’t have luxury wishes, you can perhaps also build a decent house for €2,100 per sqm. €500K should be a pretty good average. However, your budget will be significantly influenced by your income. Side income might not be considered – how sustainable is the income? Then in the worst case there might be no house for you if you prefer to splurge rather than save.

or build privately and rent to a company. The "profit" then comes via the special depreciation over the next 4 years (I think this only works if completed by the end of 2021)

Until 2021 is impossible. It will probably be discussed more intensively next year.

However, your budget is largely influenced by your income. In that case, the secondary income might not be taken into account – how sustainable is that income?
In the worst case, there will be no house for you if you prefer to splurge rather than economize.

I am calculating with a minimum profit of €20,000.
But it could also be €80,000 like this year.
This year, more was possible due to Corona and other factors.

I just want to consider the worst-case scenario in the planning.

The topic will come up in the next few years. I'm not forcing anything, but I know that rent is more of a concern for her because she thinks she would then be dependent on me and couldn’t afford a house anyway. I think, however, that it can be arranged so that in the event of a separation, she would benefit more than if we had been renting.

I totally understand you because I had the exact same long-running discussions. My girlfriend (now wife) was also scared for a long time by the high numbers. Only when her best friend moved from an apartment into a nice newly built house and calmly explained to her over a cup of tea that during her life, she would either pay off the landlord’s property or her own, the knot finally broke, and she started to research on her own as well. If someone’s mind isn’t there yet, you as the braver party can talk yourself hoarse. And even if one side gives in out of love (or annoyance), at the first major disagreement during the construction, you get the "You wanted a house, I didn’t" argument. Then it ends with: house finished, relationship too. It happens more often than you think. It’s nice that you’re not forcing her and hopefully not pushing, because it’s pointless. Give her time, even if it absolutely makes no financial sense to wait. Only then do the advice for you or you both make sense.

And even if one side gives in out of love (or annoyance), at the first major disagreement during construction, you get the "You wanted a house, not me" argument. Then in the end, the house is finished, and so is the relationship. It happens more often than you think.

That's exactly the problem I have right now... I persuaded her 3 years ago to move to the village because my parents had an apartment available and we were living very cheaply here at the time. I thought it was exactly the right thing to see if it would work out with us.

In hindsight, it annoys me because now I always have to listen to things about how you’re not so flexible in the village.

And I will avoid that with the house construction. Only if she fully supports it, will it be built together. Renting is not an option for me because too many points speak against it.