Unexpected sale of rented apartment. Options?

Depending on the federal state, the ancillary purchase costs range between 5.5-8.5% of the purchase price. For a 65sqm apartment, that would already be at least 10k. Probably even more, since the OP mentioned a large open space. With a broker and moving costs, you're at 5k. That's quite a difference.
And discussing "what if" is pointless. What if the homeowners' association decides to install a heat pump and facade insulation in the year of purchase?

Buying and betting on rising prices to recoup the ancillary purchase costs in 4 years is pure speculation.

I would definitely not base it solely on money. It's also about quality of life. And it seems a bit silly when people who can buy property for 400k or so are eager not to spend 10,000 euros and would accept losses in quality of life for that.
A bet on prices is essentially a bet on quality of life. You can hardly evaluate them independently of each other.

True, very good idea!

Regarding the incidental purchase costs: NRW private purchase 6.5% property transfer tax + notary + land register

New rental apartment: couch and kitchen would need to be new + moving costs (and those who know my posts know what I need in the kitchen…)

The fact is also: Our apartment is worth about €2100/m2 based on remote internet valuation. Asking price €3000/m2
If it can only be sold with us still living there, and we don’t take it, I don’t see a buyer, not even in the current market situation. I will also calmly reconsider the pricing.

No, unfortunately not. Otherwise, I wouldn’t have shown up here ;-) – of course, that would be the optimum otherwise.

It is completely irrelevant whether a property also has a risk, because you advised to invest the equity in an ETF until the house is built if he wants to stay in rent. And that is definitely much riskier than fixed deposits, which currently cannot offset inflation, that’s true, but also do not suddenly crash by 20-30%. At most, one could recommend putting a part into an ETF.

What crystal ball are you using? The Optimist Mk II?

And that is exactly why a 15-year investment horizon is recommended.

It is by no means irrelevant whether a property also has a risk, because it was about the property for the original poster. Besides, I advised ETFs as an alternative because everything about properties has already been said here.

I actually do not have a crystal ball.

I always find it astonishing that apparently there is hardly any risk with properties but there is with stocks!

It is by no means insignificant whether a property also carries a risk because the original poster was concerned about the property. In addition, I recommended ETFs as an alternative because everything about real estate had already been said here.

No.

In case you remain a tenant in the apartment but want your equity to be worth as much in 4 years as it is now, I recommend investing in a broad stock ETF that holds shares of the largest companies operating worldwide. You can't go wrong with that.

You recommended ETFs as a value preservation measure in case of not buying. I argued against that.

In my opinion, recommending ETFs as an alternative to buying real estate is fine. But then (in my opinion) rather when the property is one to be rented out.

Here the scenario is different. It is about the owner-occupied property, which is not supposed to change until the time the capital is needed.
So besides the purely financial aspect, there is also a personal component involved.
That is why I advised accordingly. I consider the risk of loss low but I also do not say that there is none. In four years, he can decide anew whether to sell the apartment and essentially shift into ETFs or put everything into the house or rent it out. But even here, there is more than just the purely financial decision level.