Can this be called a semi-detached house?
I asked Master Yoda to discuss your question with Confucius. The answer is: "You can also call it Torben or Sheila." But since:
Background: A residential building insurance for a semi-detached house is cheaper in my case than if I take out separate insurance for each house.
we already have the jumping comma: if the background is a natural hazard insurance, then let them judge it themselves. What matters in case of damage is only that they knew the risk and assessed it themselves. For this, you attach the cadastral extract and the building permit to the insurance application – then they have all the facts. You apply for insurance for ONE house and make the scope of the insured object clearly identifiable. If they then issue the policy, that is what both contracting parties see in it. Is this some online shop that no longer has customer district managers (a la Günter Kaiser)?
On one there are among other things "two" solid houses. House 1 was built first. After a few years, this was extended by House 2. Both houses share a common roof, attic (converted in both houses), have separate house entrances and are only separated by a boiler room. [...] Both houses have a ground floor and an attic. Between House 1 and House 2 there is a boiler room on the ground floor, whereas the attic directly adjoins each other. [...] Both are on one land and cadastral plot.
From my point of view, it would be a maisonette granny flat in an extension, and I suspect that in the building application this is also called an extension. Here the fire protection aspect will have already played a role. I also assume that the two-family house will only have a single/common lightning protection system. Water damage risks are not examined by a building authority, but noise and thermal protection are. Conclusion: on the way to the philosophical answer, the building application for residential unit 2 will be a talkative friend to you.
But in the contractual relationship with the property damage insurer, it is more relevant that you assess the object without dispute. And here the insurer will not be bound more effectively to any viewpoint than to its own. If you had the "wrong" view in this sense and we even unanimously confirmed it to you, you would look foolish in a serious case. So delegate the blame to your opponent ;-) (the latter was the Confucian part of the answer).