Is it possible to renovate a residential house from 1954 into an energy-efficient house? How?

Yes, all documents from 1954 are available.

Well, then scan them, so we can think along here on a concrete visual basis.

As said, it is double-layered with an 8cm air gap.

Are both layers solid, or how should one imagine that? – I can’t reconcile the description with any stone format.

We wanted to open the gable wall and extend the rooms behind it by 2 meters with the annex.

I thought so – that sounds conceptually clear but is tricky. The gable wall is a load-bearing wall, meaning it would have to be replaced by beams (which is no small matter and, in my opinion, results in an awkward spatial situation if those 2-meter narrow strips emerge). One does not simply extend a ridge beam and then support the roof structure “limping” on one end on old substance and on the other on new substance.

Adding these linear “extensions” to the floor plans seems to me, in terms of effort, to be a solution that is, to say the least, significantly above economy level.

Is there a concrete accidental idea of the new floor layouts behind this concept, or is the “classic” solution (adding only part of this side length on the eaves side, inserting a knee wall in the attic) too boring for you?

So I tried to scan the papers, but the quality is just so-so.

Both shells solid, or how should one imagine that? - I cannot reconcile the description with any stone format.

yes, outside about 2cm plaster then 11.5cm stone 8-10cm air 11.5cm stone and inside plaster again.

Regards Basti

yes, outside about 2cm plaster then 11.5cm brick 8-10cm air gap 11.5cm brick and inside plaster again.

You probably copied that from the linked document? - by the way, external links are not desired here.

From your drawings one can actually recognize something, namely that the brick size is not yet the new format, but still the pre-war Reich format. You can see this from the wall thicknesses (12/25/38 cm). Accordingly, a solid exterior wall could well be present here.

That would even be "better" in the sense that with the described wall structure, the "inner shell" by itself would be "non-load-bearing," and heavier ceilings than those present could become problematic.

How certain are you that the described double-shell structure is present here? - do you infer that only from the similarity to the total thickness described on the linked website?

A Prussian building code might well no longer have applied in 1954 (I do not know exactly when it was replaced after the war). I know of a building project of my family from around 1940 where 25 cm single-shell exterior walls were permitted. 38 cm was only used for the basement there. In rougher areas they might also have used that for the above-ground floors.

For 1954 I would find double-shell unusual: houses built then probably were no longer planned before the Second World War, and in the post-war period construction was done as simply as possible. Conversely, that could be the very reason why, to save bricks, a double-shell was built here after all.

According to the plans, however, I consider the 38 cm as the raw construction measure - with 12 cm bricks, however, that would leave a 14 cm cavity.

The quality is just so-so, though.

The practiced eye still recognizes quite a bit, besides the fact that it is obviously a pretty little house even to laymen (one imagines our grandma’s little house, the one that gets drunk in the carnival song):

Image 1)
Wall thicknesses 12, 25, and 38 cm = indication of old Reich format. The window reveals show rebates, which in this form rather argues against the suspected double-shell construction. Two stoves would have been placed on the chimney flue on the left side of the plan (front parlor) and in the middle, and the stove on the right (but conducted through the rear parlor in an arc). There could be fastenings for the staircase stringer in the partition wall to the pantry, otherwise a [Rabitzwand] might have sufficed. The straight-running partition wall is load-bearing; ceiling beams probably run parallel to the staircase. There is no sign of a toilet—was it under the stairs, or maybe even in an outbuilding?

Image 2)
A mansard chamber also names 12 cm as its wall thickness. However, these walls do not run in a way that suggests they are masonry. Rather, these are stud walls; the chamber appears to stand within one bay of the roof structure.

Image 3)
The chimney pierces the ridge: there is no ridge purlin here as is common today.

Image 4)
Here you see a roof structure with double purlins. Besides the wind braces, the mansard chamber probably also contributes to the bracing of the roof structure. The planks under the ground floor’s plank flooring probably have their own strip foundations.

Post a current drawing of the property here sometime, then one can more closely explore where what might be possible. This is a gem and can become one again, but it will take quite a bit of work to transform it to “today’s” standards. Worthwhile, but not easy.

I was not aware of the link. Sorry.

I know about the masonry because my father-in-law made a breakthrough for an extension in the early 1980s where the window is placed in the kitchen on the drawing, in which the sought-after toilet (now the bathroom) is located. Thank you for your very detailed description of the building. We will see what the architect says. We want to bring the little house somewhat into the modern age but, as mentioned, we are still in the idea-finding phase.
Best regards, Basti