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

Similar situation as with you (grandparents' house): We are currently in the process of (core) renovating an end-of-terrace house from the 1960s and have looked for an architect from the very beginning.

At first, we had considered contacting craftsmen directly and requesting quotes for various modernizations (of course, electrical work, windows, plumbing, heating, floors were also planned). However, we quickly abandoned this idea because coordinating such a project as "laymen" seemed too demanding. We were also concerned that the different trades might not be well coordinated in the end.

Therefore, from my point of view, I can only recommend "buying" experience. An architect may seem like an "expensive" pleasure at first glance, but so far we are very satisfied – you still have to think a lot for yourself and sometimes take the initiative, but you have a lot of influence in return, and the architect (hopefully) has a good overview of the market and costs. Our current design feels like more than just the sum of the renovation measures – somehow coherent.

You should definitely also try to clarify whether the renovation "pays off" financially. For us, discussions about amounts soon arose that others might pay for their developer house in a new housing estate. It probably also depends very much on how much you want to intervene in the floor plans of the old building (larger rooms?). Architects often have good ideas that unfortunately cost money – but once you have seen them, in the end you still want to implement them.

The house has a 38cm wall thickness with about an 8cm air layer in between. Windows, roof, and heating (gas) were renewed in 1993 right after the reunification, but certainly no longer state of the art, façade only re-plastered in 1995 but all without insulation.

We are considering an extension in timber frame construction to enlarge the living space, about 8x2 on the southern gable.

- Re-insulate and re-roof the roof, - add 2 dormers

That means from the original construction year double-shelled - that is unusual, and I can't make sense of the total thickness, how this exterior wall might be constructed. Is anything more known about this?

I also can't really imagine what kind of rooms would be in this strip between the old and new exterior walls.

The "classic" when modernizing such a building is actually more of an attic conversion (e.g., installing knee walls). The gain in space could roughly compensate for an annex.