Floor Area Ratio in Land Parcel Division

Sorry for the inaccuracies.
Therefore, I am reposting, did not want to confuse with details and measurements.
The house is not planned but already stands. The plot is larger than the required area of the standing house.
1. According to the floor area ratio, the necessary area of the plot results from the footprint of the house.
This corresponds to the red line.
2. I am not a garden enthusiast and would accept the division directly at my terrace.
That would be the green line.

This would result in the ceded plot share being larger.
If he built on it, he would have a larger buildable area, since the buildable area depends on the plot size according to the floor area ratio.
He would have a larger plot, but my remaining part would no longer correspond to the permissible floor area ratio for the standing house!
All clear?...?

All clear?....?

Yes, but what you intend to do is not acceptable at all. Besides, the calculation is done exactly the other way around, so your approach is incomprehensible. You have the actual, you have exhausted xy, etc. Doesn't matter. Calculate for yourselves how much can still be built on. The second house may then have that much ground area. But I don't believe it will be larger than a shed size. And the building authority must not object to a second house being built. Keep in mind that a driveway and parking spaces will also be added.

It probably varies in our countries,

Actually not, since the floor space ratio is regulated in the Land Use Ordinance, which is federal law. However, Berlin is "so fuckin' special" because parts of Berlin did not belong to the Federal Republic before 1990.
But if we stay in modern times, meaning the applicable law after reunification, Berlin does not differ from the other federal states.
Through the subdivision of the property, no building law violations may arise. The exceeding of the floor space ratio could be remedied by a unification building encumbrance. However, a lot of information is missing to assess the meaningfulness.

The exceedance of the floor area ratio could, however, be remedied by a unification burden.

Haha, the OP actually intends exactly the opposite: curing the compliance with the floor area ratio by severing a partial area of the property in advance.

However, a lot of information is missing to assess the meaningfulness.

I cannot imagine any information that could still bring meaning here. Presumably, the OP is not understood precisely because his intention is such nonsense without any substance, so I will try once more to summarize it in my own words:
1. There is a property. For inheritance tax reasons, the OP wants to transfer a partial area "slice by slice" in advance to a future heir during his lifetime.
2. For this nonsense command, the OP has selected the partial area "beyond the terrace," which he considers dispensable for his own enjoyment.
3. The future heir does not want to develop the property part to be marked off; thus, a remaining share of the floor area ratio attributable to it would be irrelevant.
4. The said property share represents exclusively an asset that is to be transferred now ahead of the inheritance due to tax reasons.
5. The OP is troubled exclusively by the following two questions:
a) whether the desired operation can be denied because the existing building would then exceed the floor area ratio of the remaining property (in my view, most likely yes), and
b) whether gift tax is incurred for the same purpose if he gives the heir an ideal share of the remaining entire property instead of a real share of the then divided current entire property (to my knowledge, yes: gifting is fully equated with inheritance).

The conclusion from my point of view is therefore clear:
I. The coupon idea is a crackpot idea, whether real or ideal – “the perfect murder of inheritance tax” will not succeed by either means;
II. At the “red line,” the crackpot idea can succeed – if by then the terrace’s setback obligation is also fulfilled.

Haha, exactly the opposite is what the OP intends: remedying the compliance with the floor area ratio by splitting off a part of the property.

That is wrong. At the moment there is nothing to remedy. And the building easement would only be public law, while both parts of the property would be independent under private law.

I cannot imagine any information that could bring sense into this.

For example, the floor area that the undivided property still allows to be built on. If it is the shed assumed by Yvonne, further considerations are unnecessary.

The later heir does not want to build on the part of the property intended to be demarcated,

Where does it say that?

Is this now a tax avoidance forum for privileged people?

Even if the matter here is apparently debatable, one should not support the intention behind it. The OP should go to a tax advisor.