Honestly, better leave it alone. I don’t want to get into the topic of "building with/next to friends," but you are still very young. A house certainly has advantages, I wouldn’t want to miss mine either, but it also has disadvantages. It binds you to a place (do you already know if you want to stay there forever?) and it binds you to each other. Now people will come again and say, well, you have the house as an asset and can sell it, but in the first 10 years (with zero equity) both of you still end up with considerable debt. I have a couple of friends (with 2 kids) who currently really can’t separate because they financially can’t. That’s not a pleasant life! Right now you may have the most wonderful relationship in the world, but just the house building alone will put your relationship to a tough test, believe me.
There is also the financial aspect. In my view, the equity is not the decisive factor. The more, the better, of course. But what matters is how high the amount borrowed is, as it determines the monthly burden. Even if you don’t get the best interest rate, nowadays anything under 2.5% or rather 3% repayment is actually irresponsible. Even if you can pay off for a long time because of your age, very high remaining debts remain over a long period. Let’s take the example with 2.5% interest and 2% repayment. You pay an installment of €1125! Of that, €625 go to interest. That’s almost the amount you mentioned here as rent. To me that doesn’t make sense at first, because you don’t have any counterpart for it (apart from loaning the money), whereas you can already live in the apartment. Additional costs are also higher. Sure, older buildings have higher heating costs, but when you go from a 70m² rental apartment to a 130m² house, the cost advantage is not that big anymore. Water, electricity, property tax, insurance, waste disposal—all of that becomes more expensive. Plus, there are all repair and maintenance costs. These can come quickly even with a new building in the worst case.
I also always hear the myth that when you build a house the repayment isn’t higher than the previous rent. Personally, I don’t know anyone where that is the case or they are not interested in 100% repayment (that also exists). In my view, that can only fit under two conditions:
1.) I move out of a big city like Hamburg, Munich, Frankfurt, etc. to the countryside. That can fit, but then you also travel 50 km or more to the city every day. Here, you’re rather comparing apples to oranges.
2.) I have a lot of equity (50%). Then it’s naturally easier, because I only have to finance "half" the amount and can therefore immediately halve the repayment. But that calculation is wrong, too. If I invest the other 50%, I get interest income that I don’t have afterward. I would have to deduct that from the rent to be paid.
Those who belong to group 2 can of course think about homeownership, since interest income is low and interest rates are also low, as you get great rates due to the high equity share. Purely economically speaking, however, it must then also be a property that appreciates in value over the long term to make sense from a purely economic perspective.