House extension / extension of half the house in MOL / Brandenburg

Hello everyone,

my partner has been the owner of a semi-detached house in Märkisch-Oderland / Brandenburg for 9 years, half of which she uses herself.
Now we want to convert the terrace (unsecured) into an additional room and accordingly extend the upper floor in order to also have another children's room available there.

Unfortunately, nothing exact is known about the house itself; apparently there are no plans in the archive of the district office.

I would carry out all construction-related work myself, but the preliminary process is not entirely clear to me.

What is clear, however, is that a building application must be submitted since it is intended to be usable living space.
Do I need an architect/structural engineer for the inventory survey?

Does a soil report need to be prepared?

Do I need an architect/structural engineer for the survey?

... or a surveyor, I haven't been to Brandenburg for about 25 years ; ). Could you create the survey including a processable documentation yourself? I haven't checked your state building code, but in NRW you need an authorized submitter anyway for such a project. Discuss with them whether they prefer to make their own survey (then they are also liable for errors) or accept a plan from you.

What Escroda wants to say: You need someone like him, and he would be willing.

You need someone like him

as another option

and he would be willing

No, too long ago and too far away. :)

But you can surely tell me what the whole thing should look like in terms of form. What do I have to/can I (in any case) prepare so that, ideally, it doesn't cost me a fortune?

As I said, I can build alone, I would have support from a few friends who can also procure materials, etc.

I just need the way to an approved building application.

First of all, the planning and building law situation is important. Is there a development plan or is it an unplanned inner area (§34 Building Code, roughly speaking within a locality) or even an outer area (§35 Building Code, i.e. isolated location in an agricultural or forestry area)? Are the semi-detached houses each located on their own plots? Are there similar buildings in the vicinity?
Your first step should be to obtain a current excerpt from the cadastral map ([Brandenburgviewer], scale 1:500) for building consultation with your approval authority. There you will find everything about planning law for your property. If you mark the outlines of your planned extension in red on the map, the case officer will (hopefully) also give hints regarding building regulations, e.g. separation distances, fire protection, entitlement to submit building plans.
Once the fundamental feasibility is clarified, find a qualified person entitled to submit building plans (architect, civil engineer) and clarify the details with them.
If you do not have floor plans of the existing building, a complete survey of the building is probably necessary for the building application. In particular, in the area of the extension, location and height accuracy to the centimeter level is required. Since the geometry must also be correct (existing or non-existing right angles), this is hardly feasible for laypersons, especially as almost all planners work digitally and expect, for example, a dxf file. If you can get support from a draftsman or technical draftsman, doing it yourself is conceivable. However, your planner must also cooperate.