In summary: For the purchase, we already have to indicate the ownership relationships in the land register, possibly attach a declaration of division. But without a building plan, we cannot foresee this. And without land, we cannot create a building plan....
There seems to be a misunderstanding. A "declaration of division" is not a statement about who contributed which parts to the purchase price and therefore wants residential rights or shares in the building. The ownership relationships on the land cannot be entered as divided as long as the land
is a single land register object. So
one entire thing cannot have owners of individual parts. At the time of purchase, it will therefore initially be
one property and
one land register object. However, this can have several owners, e.g. the spouses Rubiwan own 7/10 and the brother 3/10. Or the entire property belongs to you, and the brother has a land charge registered corresponding to his share of the purchase price. A residential right probably cannot be registered yet, since the building does not yet exist.
If you actually want to choose the route of making the brother’s residential unit a share of ownership during the development,
then the declaration of division is made first. After this declaration of division, the subunits arise as separate land register objects, e.g. 1. your apartment (completely owned by you), 2. the brother's apartment (completely owned by him), and 3. the surrounding land, paths, parking spaces (e.g. 7/10 owned by you and 3/10 by the brother). Upon handing over his residential unit, he then has his land charge registration (or his co-ownership share, depending on the chosen route) deleted from the land register sheet of the current whole property.
Before planning, you do not yet have to explain (or want to or can explain) to anyone how the later ownership should be divided in rem and legally. Rather, you spouses and your brother together are
one purchasing party towards the seller of the currently whole property, which legally has no parts yet. Who pays which part of the purchase price does not matter – there is
one purchase price. What the brother contributes is secured by one of two ways (theoretically also combined): a) You become joint new owners with shares registered as described above, or b) you and your wife alone go into the land register, and the brother has the money he contributed registered as your land charge in his favor on the current whole property.
I would probably – since the values of the residential units are not yet clearly determinable – register the brother’s share so that the value of his residential unit preferably exceeds this share in the current whole property in case of doubt – hence the advice to possibly combine both ways –
but a business consultant is not a lawyer, so one should also consult the latter about this.
Your presumably suspected chicken-and-egg problem is therefore a misunderstanding; the declaration of division is only needed at a later step – and only if you then really want to divide according to the Condominium Act (WEG). For the purchase itself, you do not yet have to explain (or be able to explain) who should ultimately own how much of the "final product house with several residential units," and – apart from the issue of the Money Laundering Act – you do not have to account for who puts which cash on the table. The question is therefore initially only between you and your brother whether he wants/should be a co-owner or a land charge holder in the current whole property’s land register sheet. Apartment land register sheets logically cannot exist yet.
Assuming you actually want to divide according to the Condominium Act later and the brother’s apartment share of the total property would be, for example, 28%, then the question to the expert would be: should the brother now be entered in the land register with 1/4 = 25% (and pay you the 3% exceeding his deleted land charge upon buying the apartment) or should he be entered with 3/10 = 30% in the land register and later receive 2% from you. In your place, I would ask an expert who is both a lawyer and a tax advisor – then
one expert has
both professional opinions.