§ Section 30 BauGB regulates the permissibility of projects within the scope of a development plan.

As part of its planning sovereignty, the municipality controls urban development by drawing up qualified development plans. This municipal planning sovereignty is enshrined in Article 28 (self-administration of the municipalities) of the German Basic Law and serves to ensure that the municipalities regulate their own affairs under their own responsibility. For the structural development of the municipal area, the municipalities make particular use of urban land-use planning, which is regulated in the Federal Building Code (Baugesetzbuch GB). The usability of a plot of land for building purposes is regulated in so-called binding urban land-use planning via development plans.

In areas where a legally binding, qualified development plan exists, a project is permitted if it does not contradict the provisions of the development plan and development is ensured (Section 30 BauGB).

However, exceptions and exemptions from the provisions of a development plan are possible; Section 31 regulates the relevant framework and assessment parameters for this. Exemptions can be granted if the basic principles of the planning are not affected, reasons of public welfare require the exemption, the deviation is justifiable in terms of urban development or the implementation of the development plan would lead to an obviously unintended hardship. Exceptions, on the other hand, are only possible for circumstances that are either explicitly stipulated in the text of the development plan or contained in the building areas of §§ 2-10 BauNVO.

Irrespective of this, the provisions of the building regulations (regulated in the Hessian Building Code (HBO)) and other public law regulations (in particular the design statutes) apply in principle. These regulations generally result in additional requirements or restrictions that are decisive for the general permissibility of projects.

§ Section 34 BauGB regulates the permissibility of projects within built-up areas.

An unplanned inner area exists if there is no development plan, but a built-up district exists. In Wiesbaden, as in many other cities and municipalities, there are areas that are largely built up due to historical urban development, for example, but for which a development plan has never been drawn up.
There is also often no development plan, particularly for plots on the outskirts of towns and plots on the edge of large open spaces within a built-up urban area. Such areas are referred to as "unplanned inner areas" (Section 34 BauGB).

Such an unplanned inner area is formed from the built-up districts. In principle, construction projects are only permitted there if they fit in with the character of the immediate surroundings and build on these within the framework of the existing character. The existing, visually perceptible buildings, which form a building context and reveal an organic settlement structure, are decisive in this respect. In the case of gaps between buildings and open spaces, the decisive factor is whether and to what extent the successive or surrounding buildings nevertheless give the impression of coherence and whether this sufficiently characterizes the property in question.

In order to be permitted, the building project must fit into the character of the immediate surroundings in terms of the type and extent of building use, the construction method and the area of land to be built on, and the development must be secured. In addition, the requirements for healthy living and working conditions must be met and the townscape must not be impaired.

The decisive factor for "blending in with the character of the immediate surroundings" is the framework of the surroundings that is used to assess blending in. The selection of this "setting" is therefore particularly important. Under certain circumstances, this framework can be selected for the respective insertion characteristics - for example, a different framework can be selected for the assessment of the type of use than for the extent of use. Blending in is less about formal uniformity than about fundamental harmony within the urban fabric. A project blends in with the character of the immediate surroundings if it corresponds to the existing urban development situation and does not lead to any tensions in terms of land law.

§ Section 35 BauGB regulates the permissibility of projects in undeveloped outdoor areas. The outdoor area includes all properties that lie outside the spatial scope of a development plan and outside the built-up areas.

The demarcation of the inner area from the outer area essentially depends on the extent to which the development context extends and gives the impression of a consecutive closed development. The built-up area usually ends with the last existing building. The actual property boundaries are not taken into account in this consideration. All adjoining areas belong to the outdoor area.

In principle, the outdoor area is to be kept free of development and is subject to protection against building development. Only favored and privileged projects (e.g. agricultural and forestry uses), which are only permitted under certain conditions (§35 BauGB), may be built.

The extent to which an existing building can be put to a new, possibly no longer privileged use can only be legally assessed on a case-by-case basis. It is therefore best to contact the building inspectorate directly for advice.