Building permit from 1931 not adhered to

In April 2014, my mother transferred to me by notarial deed and entry in the land register a small two-story single-family house (built in 1931) with a basement and a living area of about 90 sqm in a village in Schleswig-Holstein, approximately 10 km from the Baltic Sea. The plot is located in the middle of the village by a meadow and a village pond and has a size of about 600 sqm.

Description: The foundation walls consist of 40 cm thick rubble stone masonry. The exterior walls are double brick, the interior walls single brick. The basement has meanwhile been fitted with a screed floor. The roof structure was completely renewed in 1985 and two rooms were built in the attic in timber construction. A new staircase was installed in 1990. Plastic windows, bathroom, and toilet were installed in 1980.

I completely cleared out the house last year and removed the old thatched roofs as well as chipped off the old (loose) plaster from the walls. Now I want to partially install new electrical wiring, have the walls replastered, and have painting work carried out. So that the house becomes habitable again; because I would like to rent out the house.

My problem: When I requested the old building documents from the Schleswig State Archive, I unfortunately had to find out that an approximately 3 m annex on the right side of the house was not present. However, this annex was built at the same time by the masonry company back then, as is clearly visible on old photos.

The subsequent conversions, such as the bathroom and toilet, were also actually not approved.

My questions:

1. If I want to rent out the house or sell it later, do I necessarily have to have a building permit (structural approval)?
2. The house has stood in its current form for 87 years. Thanks to the fieldstone foundation, there is no moisture and no cracks in or on the house.
3. What happens if I receive an order from the building authority to dismantle? Or should I rather not "wake sleeping dogs"?

Please no lectures. I need genuine constructive advice and would be very grateful for it.

Thank you very much!

that an approximately 3 m extension on the right side of the house was not present.

It is new to me that demolitions have to be approved by the building authority.

Also the subsequent modifications...

... inside the building, the authority has nothing to do with it.

I don’t know how it is with rentals.
If you think the structural integrity is questionable, you are acting negligently when renting.

Please no lectures.

That applies to you too!

1. If I want to rent out the house or sell it later, do I necessarily have to have a building permit (structural engineering?)?

No. But if the potential buyer asks and you can't present anything, he might not buy or try to significantly lower the price.

2. The house has stood in its current form for 87 years. Thanks to the fieldstone foundation, there is no moisture or cracks in or on the house.

That's not a question.

3. What happens if I receive an order to dismantle from the building authority? Or should I rather not "wake sleeping dogs"?

If it gives you sleepless nights, commission an architect to take a look at the existing building from a planning and building law perspective. Provided there are at least no planning law objections, a dismantling order is practically excluded. From a building law perspective, the architect can then prepare a list of the measures necessary for subsequent legalization. Whether you implement these immediately or wait until you are officially requested to do so (which, IMHO, will not happen) depends on the severity of the "defects" (fire protection and rental can be a problem) and the disturbance of your peaceful sleep.

It is new to me that demolitions have to be approved by the building authority.

Oops. Hopefully no one from your building authority is reading this or hopefully you haven't demolished anything yet.

... inside the interior, the authority has nothing to do with it.

That depends on what was remodeled. Structural interventions, even inside the house, do require approval.

I recently had a phone call with our building authority regarding a modern building permit. They told me that, for example, doors and windows are not important. That means if they are located somewhere else than originally approved, it does not matter. And you can even see that from the outside.
In that respect... I would first inquire to what extent internal alterations require a permit at all (you can say you are interested in a property and might want to make some changes, what are the rules?). As long as the structural integrity is not altered by the modifications, it should actually not be too critical.
I also cannot imagine for the life of me that you would have to tear down the house because of the missing extension. Here in our city, someone had to tear down a building that was fundamentally not approvable. But here? At worst, you just have to obtain the permit retroactively.
I wouldn’t worry too much.

My problem: When I requested the old building documents from the Landesarchiv-Schleswig, I unfortunately had to find out that an approximately 3 m extension on the right side of the house was not present. However, this extension was built at the same time by the masonry company back then, as is clearly visible in old photos.

A friend and her husband have the same problem.

They had an appointment with an architect last week, who compared the current situation with the documents from back then.

Result: According to the development plan, the extension is allowed to be there since it is within the building plot and nothing regarding size, etc., violates any regulations.

The architect is now taking over the entire preparation of the documents and will submit a building notification to the building authority. His statement was that this will be sufficient.

Therefore, I think the approach of working with an architect is the right way to go.