Renovation of an old building from energy class H to efficiency house KfW 55 EE

Just a brief better description of the upper floor apartment because you write attic apartment without balcony. The upper floor apartment would later be on two floors, upper floor and attic, connected by a staircase. On the upper floor is the balcony, in the attic is finished. In the attic, larger window elements would be installed later for good lighting.

So for the small apartment on the ground floor with 55 m², possible target group couples with up to one child,

No, that is not a target group. The size is for singles.

The 90m² apartment with balcony would then also be rather for small families with up to 2 children,

That is not the target group either.

So for the small apartment on the ground floor with 55 sqm, the possible target group is couples with up to one child, singles, or spry older tenants.

I see an 8 sqm kitchen and a 15 sqm living room. This serves as a walk-through room to the bedroom. Then there is a 9 sqm room, which you call a children's room.

Most likely target group is rather couples.

Exactly. No family, unless you have a housing shortage like in the big city.

Thus probably higher turnover.

I actually see it as a transitional apartment. Maybe an older person would like the garden, a hardworking young couple climbing the career ladder would probably feel less comfortable there.
The living room with 15 sqm is no longer contemporary. The walk-through door restricts furnishing options and also disturbs privacy.
Now ask yourself, who as a single person would pay over €1000 cold rent?

The upper floor apartment has a bathroom, kitchen, living room, parents' bedroom, small children's bedroom, and on the attic floor there would be another large room of about 30-35 sqm.

Look, you are already downgrading the square meters somewhat.
The upper floor is already in the roof. There is also the living space calculation according to DIN. What remains of the attic over 150 cm height? 3, 4 meters in width?
How do you enter the attic? Where would the stairs be? Above the other stairs? Or additionally in the hallway?
For the room sizes, I will make a screenshot of the upper floor (with slope)
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And you come with a family with 2 children...
The attic in our area (Hamburg commuter belt) would be nothing more than a bonus on top to make marketing more attractive. That is then called a maisonette apartment.
And yes, 80 sqm (or 90 sqm) sounds nice, but I see rather a single or couples passing through here.

Do you think the target groups I mentioned are rather not potential tenants?

Think it over. I was once single 35 years ago too, but I had a 24 sqm living room. With 12 sqm and 5 sqm kitchen, you might want to calculate differently.

Assumption of rent price for fully renovated apartments at the end of 2026: €14/m².

ok, if I google rental apartments in your/your community, there are fully renovated apartments with about €10-11 cold rent. The €14 is charged for a new building with status appearance. I cannot say anything about the location.
I think you are miscalculating!
Or do you want to live in one of these apartments with your wife?

That is exactly the point. I also thought at first that the room behind the building partition wall should be considered heated. However, the energy consultant said that it is not that simple and that he would have to calculate quite a few thermal bridges here, which might make it difficult for KfW 55.

No, the room behind the building partition wall must be assumed to be unheated. Because the other half is bought by someone who overextended themselves with the renovation costs and whose wife then left them because of it. Then the half stands unoccupied for years until the foreclosure and subsequent renovation is completed.

Therefore, according to DIN EN 12831, only one's own residential unit is considered heated. This makes it even more difficult with 2 residential units in your half. For the self-used and self-paid property, it wouldn't matter much, but KfW looks more closely for their loans.

Look, you are already rowing down the square meters a bit.

Yes, that's true, I am actually rowing down because I don't know exactly how far I can come inward with the interior insulation of the roof. I also haven't measured the heights exactly to calculate it precisely. But yes, it might be that I am somewhat "beautifying" this attic. Access is currently via a floor hatch in the stairwell. I would have closed that and made a staircase to the top in the living room on the upper floor. I measured that and it should work. Unfortunately, there is too little space in the hallway.

I see an 8 sqm kitchen and a 15 sqm living room. This serves as a walk-through room for the bedroom. Then there is a 9 sqm room that you call a children's room.

I would change the room layout. Swap kitchen and bathroom. Remove the partition wall to the living room so that the kitchen is open to the living room. The living room is currently a walk-through room, yes. I haven't come up with a clever solution to change that. Moving the hallway makes the "children’s room" even smaller.

No, we actually do not want to live in one of the apartments. It is really supposed to be only a kind of retirement provision.

The property has been on the market for quite a while now, and probably no one really dares to tackle it because of the costs and the then more difficult-to-find target group.

No, the room behind the building partition wall must be assumed to be unheated. Because the other half is bought by someone who overwhelmed themselves with the renovation costs, and then the wife ran away from them. Then the half stands empty for years until the forced auction and subsequent renovation are completed.

Therefore, according to DIN EN 12831, only your own residential unit is considered heated. That makes it even more difficult with 2 residential units in your half. With the self-used and -paid property it would be relatively irrelevant, but the KfW looks more closely for their loans.

Great, thanks to you too for the info.

Thanks to you all for the valuable input! I probably have to think about it even more or negotiate the purchase price down to half :p

The property has been listed for quite some time now, and probably no one really dares to take it on because of the costs and the rather hard-to-find target group.

Costs: You want to convert the attic (building permit required = architect fees), change walls and floor plans. Since you also have two separate apartments with all the trimmings, you should really think about the costs. No idea what you want to contribute to the equity, but the more you add, the more expensive it gets.