Granny flat / Semi-detached house advantages

Don’t talk it down. After all, the second semi-detached half no longer requires a separate plot. That should (as long as you can get along with a tenant as a neighbor – but I’d prefer a tenant over an owner as a neighbor) enable a reasonable return.

140 sqm house at €2,750/sqm (it doesn’t have to be built as for owner occupation) amounts to €385K. Plus some additional costs and garden/parking space, so maybe €450K for the house. Rent then €1,500 + X monthly – that should be the minimum in any reasonably normal area = €18K p.a.
Resulting in a gross yield of 4%. That is completely okay. There are no management costs.

So a shared entrance to the house and then each turning left and right into their respective "apartment" would be a first step.

The problem with this is, of course, the marketability of the rented unit. If you want to sell the "apartment" later, it will be worth less than if it were a separate single-family house.

Rent then €1,500 + X per month - that should be the minimum in any halfway decent area = €18k p.a.
That leaves a gross yield of 4%. That is completely okay. There are no operating costs.

A granny flat cannot be sold separately because there is no separate plot of land for it. A subdivision declaration must be made beforehand.

In addition, I can deduct the construction interest for the rented part from taxes.

Apparently, there are some creative options to build a two-family house even with 2 separate entrances.

But you have to get the 1500 first. That really depends a lot on the location.

If you pay that much rent, do you now want to live in a semi-detached house without a garden?

Especially not if I bring the trouble with the tenant onto my property.