Buying an apartment despite planning to buy a house in 3-4 years?

Well, you should also include maintenance costs etc. that come with a condo, especially when renting it out. For the apartment to be suitable as an investment property, it must be properly maintained, and that costs money.

Contribution to the maintenance reserve of EUR 100 per month (in our case) already taken into account; see above.
But that's true. In the end, everyone has to calculate and decide for themselves.

So initially, "only" the 7,000€ remain.

Plus appreciation... in our case about 30,000 EUR to date. And that in less than 3 years...

The increase in household surplus can also be achieved through a cheaper rental apartment.

How so? If I move out of the rental apartment, I only save the rent paid so far. In contrast, with the condo, future rental income stands against the previously paid loan installment plus ancillary costs. The liquidity advantage can therefore only arise with ownership and subsequent renting out.

Whether the rental apartment really pays off in the end is definitely not certain. With a single property, it can quickly tip due to missed rent payments, tenants who damage the property, politics, etc.
Of course, it can be worthwhile in your individual case. But I think it is wrong to generalize.

That is why I wrote "in my personal opinion [...]"

That is absolutely correct. You should choose the right property for that.

Thank you very much for the numerous responses. We have now decided to buy the apartment, but will use slightly less equity and have reduced the installment to 500. This way, maintenance and reserves can almost be fully financed from the net rent. Looking at the purchase purely from a financial perspective, there are certainly better investments. However, since we will initially live in the property ourselves and want to make a few small changes, we at least gain a certain amount of quality of life.

For the second project, we are looking for an existing property, or currently the discussion is emerging with a couple of friends to build a semi-detached house. With this, we would certainly manage with 420k, including ancillary costs we would quickly be at 450k. If we now find the dream property that significantly exceeds the budget, we can still sell the apartment; the prepayment penalty is very low. Since we are staying in the region, we also remain in the price development. In other words, if property prices rise more strongly, the apartment will also become more valuable, at least that is our consideration.

We made this decision 2 years ago and bought the apartment we were renting at the time. In hindsight, I was somewhat annoyed that I hadn't negotiated better, since, as mentioned, a rented apartment is worth less. However, since we had no notice period protection and there had been 2 viewings for personal use, we weren’t tough enough. But maybe just as a reminder regarding purchase price negotiations. Currently, we are looking at houses – not an inexpensive matter in the Berlin area. Therefore, we are currently rethinking our former "strategy" of renting out the apartment. After all, that means further risks. We had the apartment financed with a 90% loan, which was the best compromise in terms of interest rates, monthly payments with 2% repayment, and remaining equity for a future house. I will only soon have the first talks about financing. What is not entirely clear to me yet: Do the 10% equity in the apartment and the installments already paid help me get a better interest rate for the new project? (Assuming renting instead of selling) As long as it’s not the same bank (keyword: repaid land charge) probably not, right? Another bank could at most register a second-ranking land charge, but I don’t know if that is common and if the associated costs even bring any added value? This question should come up within the scope of this thread at some point anyway, so I’m just joining in here.

Do the 10% equity in the condominium as well as the installments already paid help me get a better interest rate for the new project? (Assuming rental instead of sale) As long as it is not the same bank (keyword: repaid mortgage), probably not, right? Another bank could at most register a second-ranking mortgage, but I don’t know if that is common or if it brings any added value considering the associated costs?

My brother just had this situation. He bought a condominium a few years ago, a significant part is already paid off and the apartment has also increased in value, so that "actually" there is already substantial ownership. In addition, they have saved a very considerable amount.
Now they looked at a house. They went to the bank that also financed the apartment. They would support the financing. Then they went to Interhyp and the project was completely rejected with the note that the ancillary purchase costs for the house would exceed the liquid equity, so that it would amount to over 100% financing. The condominium played no role in the consultation, with the note that the first rank in the land register was otherwise occupied. (Whether sale or rental was also irrelevant).
The project did not come about due to other discrepancies.

Then they also went to Interhyp and the project was completely rejected there with the note that the incidental purchase costs for the house would exceed the liquid equity, so that it would result in a financing of over 100%.

What kind of idiot did your brother get? It is totally normal to have to pre-finance the sale of your apartment – daily business.

The condominium played no role in the consultation, with the note that the first ranking lien in the land register was otherwise occupied.

That does not matter either, since no land charge is registered for the bridging finance of the sales proceeds anyway.

The fact that the financing (as you described it) was rejected is not due to the constellation, but simply because of the idiotic advisor.

It is totally normal to have to pre-finance the sale of your apartment - everyday business.

Maybe it was because they were not yet sure whether they actually wanted to sell the condominium or rather rent it out. I was at the same advisory office at the time and felt very well cared for there. (Of course, there are several advisors, maybe not the same one.)