Developer contract with a UG!

One basically buys a finished piece of house with land from a developer and has a purchase contract as a basis. The buyer is not the builder.

If building on one's own land with [GÜ] or construction companies, then the owner of the land is the builder who has claims directly against the building companies.

Calculation example.
UG = 1€
GmbH = 25,000€
AG = 50,000€

If you are the only creditor, to what extent do helpers advance you 25,000€ for the house construction if you have a shell and the company is bankrupt?

Apart from that, there is practically no UG with only €1 equity.

But when you buy from a developer, you are basically buying a finished piece of house with land and have a purchase contract as a basis.

Perhaps to clarify what type of contract this is. It is not a simple purchase contract! Even if the developer contract is referred to by you as a "purchase contract," it is actually a separate type of contract. It is a mixed contract, to which case law has long applied purchase law with regard to the property or property share but construction law - and here especially with regard to warranties - construction law in the sense of contract for work and services law. In the case of construction defects, for example, the long warranty periods under contract for work and services law apply. According to § 634... Building Code, these are usually five years from acceptance of the building.

It is a mixed contract, to which case law has long applied the purchase law regarding the property share for the real estate, but regarding the construction - and here especially the warranty - construction law in the sense of contract for work applies.

We do not know and can only advise the OP to look closely (or have someone legally knowledgeable look closely) at what kind of structure it actually is in their case. Developer contracts are a broad field, often somewhat disguised and often classified differently regarding financing and warranty law. The only general advice that can be given to a prospective homeowner is not to be satisfied with any term explanations given by the contracting party themselves, but to have an independent expert assess what is really on the table. One who is liable, of course – and you will not find such a person in a forum, here only rough initial help can be provided.

Is there such a thing as a share deal for single-family homes? I have not seen that in 20 years and absolutely cannot imagine it.

The OP wrote,

the managing director establishes a new UG for each new construction project.

and I interpreted this here as referring to that single house. Otherwise, I only know this for entire projects, i.e. a whole development area or, if for a single building, then with a dozen residential units.

But back to the core of the initial question: valuable guarantees are, for example, completion bonds; for warranties, I do not know anything like that; a legal form or a former share capital alone means nothing reliable. What can always be said about a UG, on the other hand, is that it is relatively weak: as soon as it closes a financial year with its share capital grown to that of a GmbH, it is quasi automatically transformed. This "saving up" is carried out legally automatically (through profit retention), i.e. a successful UG naturally soon becomes an adult GmbH and as long as it is not yet that, it is usually not a preferred client by insurers for warranty risks. My comment

and you later take over this company together with the house it owns

means "in simple terms": "make sure you don’t end up having claims only against yourself (as the legal successor of your counterparty)."