Building documents, boundary construction, and outbuildings as living space

Hello dear forum,
basically, I am already quite sure that something is wrong with my potential house, but maybe you can assess it again.

Main house with outbuilding, whereby the outbuilding stands on the boundary. I only saw this on the cadastral map, that the extension is marked in light grey as an outbuilding (so the same color as the garage). Length of the building on the boundary is 14 meters, only one story.
There is no building file at the municipality and the district. An architect drew the house 10 years ago and marked everything as living space.
In the outbuilding are kitchen, office, and heating room.
It is 3.5 m to the neighboring house.

Can something like this even be legal? Could the cadastral office have accidentally designated a residential building there as an outbuilding? Fireplaces are not simply allowed as boundary construction, are they?

I am 95% sure I will not buy the house, but maybe there is a chance for approval? Or should I rather expect serious trouble, meaning demolition or prohibition of use?

Do you generally have to assume illegal construction if building documents or permits cannot be found?

I’m attaching a somewhat modified house from the cadastral map (H for heating room, K for kitchen and B for office).
The federal state is Lower Saxony.

Could I just ask the building authority?

Thank you all

Do you generally have to assume an illegal construction if building documents or permits cannot be found?

If the building authority also cannot find anything, then possibly yes. However, what was bombed in the war has not yet been digitized.

If the building authority also finds nothing, then possibly yes. However, what was bombed during the war has not yet been digitized.

Thank you. As far as I know, there were no major damages here in the rural area. My colleague himself received various documents for his half-timbered house.

I will call the building authority tomorrow; unfortunately, I couldn't get through today.

I will 95% not buy the house

What exactly are you looking for, where and how "quickly"?

What exactly are you looking for, where and how "fast"?

Do you mean which documents? Or what kind of real estate? I assume that about 10 years ago, a breakthrough was made to the outbuilding to install heating, kitchen, and office there. Additionally, the attic was converted. Previously, it was a 100 sqm little house that belonged to an almost 90-year-old lady. The house was really well renovated, beams renewed, and the floor plan changed. I have been working on it for 4 weeks now and am waiting for documents. But then I realized last week, there is nothing that indicates any kind of permit. An architect has now drawn up the floor plans. My colleague has a much older half-timbered house and also a new drawing for it, but his has a stamp from the building authority. I am a complete layman in this field but somehow I think something is fishy.

Often, a gut feeling does not deceive.