Unfortunately, it has to be: The house in case of inheritance, spousal protection

Waivers of the compulsory portion can already be arranged now and one does not have to or can wait for the event of inheritance. If the event of inheritance occurs, the parents could renounce the inheritance, yes. But one can also talk to everyone alive now, go to the notary, and draft the corresponding documents. I would then clarify with the notary what exactly is possible there.

Of course, this only works if the relationship is good now and the parents would actually sign something like that.

One can also arrange renunciations of compulsory portions now and does not have to or cannot wait for the inheritance case. If the inheritance case occurs, the parents could renounce the inheritance, yes. But one can also talk to everyone alive now, go to the notary, and draw up the corresponding documents. Exactly what is possible there, I would clarify with the notary.

Of course, this only works if the relationship is good now and the parents would really sign something like that.

they wanted to do that with us children

I say no

Well, the compulsory portion is simply the compulsory portion, the legislator had a reason for that.

If for some reason the heirs are so ill-disposed towards the remaining partner that, for example, they accept a forced partition sale, then you're just out of luck. Whether parents or children doesn't matter.

At what point the compulsory portion is waived initially doesn't play a role. The question sounds as if there are already some problems, otherwise one wouldn't even think about it.

Therefore, I don't think that an early waiver declaration will be possible.

The only way to secure it: set aside the expected compulsory portion now (if not possible otherwise, little by little).

If you have financing on the property, a right (whether usufruct or right of residence) will be subordinate only, thus it will not change the value of the property in the foreclosure sale, as it will be deleted for the purchaser at the auction.

Just for understanding: The usufruct right can only be deleted if the bank, which registered its rights first, i.e. the mortgage here, actually has the house auctioned off?

If I see it correctly, in a Berlin will the inheritance share that would go to the parents is no longer the statutory share (= share of the house, registration in the land register as a new co-owner and therefore the right to demand a partition sale). Instead, only the compulsory portion. But the compulsory portion does not establish co-ownership, it is purely a monetary claim, so a partition sale would be off the table.

But even in a partition sale, the usufruct could not be deleted if it was registered BEFORE the inheritance event, thus having higher priority than the heir's rights?

If for some reason the heirs are so ill-disposed towards the surviving partner that they, for example, accept a partition auction, then one is simply out of luck. It does not matter whether it is parents or children.

That is why the Berlin will exists. It excludes the statutory inheritance share, which would be an actual co-ownership of the house, and limits itself to the compulsory portion, which is a pure monetary claim. Thus, no entry of the heirs in the land register occurs, and a partition auction is excluded.

Then it is only about devaluing the compulsory portion as much as possible. It is a percentage value (in the normal case of statutory matrimonial property regime, it would even be that the parents together would only have a quarter as the statutory inheritance share and as compulsory portion again half, thus an eighth, together (each individual parent 6.25%).

Now 6.25% of nothing is nothing.
So if, by entering a mutual usufruct during the spouses’ lifetime, the value of the house is virtually destroyed for all other persons, then not much remains of the compulsory portion.

Ultimately, it is about the surviving spouse not suddenly facing exorbitant monetary claims.

Summary:
Calculation with statutory inheritance share and community property: parents-in-law would receive 25% inheritance share (ownership of the house): house value for example 200,000, of which 100,000 inheritance (because half share is inherited): of that 25% := 25,000 euros or 12.5%/an eighth co-ownership share in the land register including right to partition auction
Additional: if both parents no longer live, brothers-in-law/sisters-in-law can also inherit, as they move up in the statutory order of succession

Calculation with disinheritance of the parents (Berlin will) and community property: parents-in-law would only receive the compulsory portion (monetary claim): no share in the house, no entry in the land register, no right to partition auction. The compulsory portion is half of the statutory inheritance share, i.e. 12.5% for both parents together or 6.25% if only one parent is still living. That is 12,500 euros or 6,250 euros monetary claim.
Additional: if both parents are no longer alive, no brothers-in-law/sisters-in-law move up, as they have no claim to the compulsory portion.

Calculation with usufruct entry: house value is reduced by usufruct right, which depends on the age of the beneficiary (remaining useful life according to table) as well as the annual house value. For a 60-year-old beneficiary a roughly 15-year remaining usage is assumed, and with a 200,000 house value and 5% annual value one would get 10,000 (annual value) * 15 years = 150,000 euros value for the usufruct. The residual value of the house would thus only be 50,000 euros and the compulsory portion will then be calculated from THAT, whereby I am not sure whether the residual house value is halved again, since only half is inherited (with half co-ownership share). In the worst case, it remains at 50,000, then 12.5% of that would be 6,250 euros for both parents together, or 3,125 euros for one parent (corresponding to half if only half of the residual value is taken).
That is still a lot, but should not ruin the surviving spouse, since if necessary a loan could also be taken out on the house again (the land charge would at that time be a multiple of the residual debt).
Additional: to be offset against are the costs for registering the usufruct right in the land register by the notary (which are probably several hundred euros).

I exclude the waiver of the compulsory portion here. On the one hand, because it is rarely obtained without compensation (I would also not sign such a thing without payment), on the other hand because the legal certainty is questionable if the social welfare office backs the respective parent, which would certainly like to collect its expenses for the nursing home/basic security as well. After all, “artificial impoverishment” is excluded for recipients of social benefits.

Since perhaps others also have the problem of a "not so good" relationship with parents-in-law or parents in basic security/nursing care, all this may also be of interest to others.

If I am mistaken anywhere, please correct me. Thanks to you!

The registration of a usufruct is equivalent to a gift. This should not be relevant for tax purposes between spouses, but it is for the compulsory portion.

For gifts made less than 10 years before the inheritance event, 10% of the gift is considered part of the estate for each year accordingly.
The problem: as long as you retain ownership of the property, the 10-year period does not start to run.

You cannot easily avoid the compulsory portion this way.