Expanding a two-family house into a three-family house - ideas wanted

For the construction year 1951 - yes. The post-war period indeed required and produced the greatest improvisations in building materials and methods. Even with renovations to normal standards, there are surprises that will leave you stunned. In case of doubt, you should go through the structure with a good energy consultant or preferably an expert to see how problematic a renovation to [Gebäudeenergiegesetz] (Kfw 70) or at most KFW 55 standard would be.

Would you generally always prefer a new build to a renovation for a passive house?

With such an old building, definitely. You have to completely gut the house – meaning you have to relocate the current occupants elsewhere during that time – and essentially rebuild it entirely within the existing structure. And in the end, you only have a house with 1950s specifications: ceiling heights, floor plans, etc. I know what I'm talking about, since I am upgrading an old building to KfW 55.

After all, you are participating significantly more in your own discussion this time than in the two threads on the same topic from four years ago and one year ago, but since then apparently nothing has progressed; which could well be related to the fact that so far neither here nor there nor in https://www.hausbau-forum.de/threads/neubau-energetisches-efh-mit-einliegerwohnung.37931/ you have once shown the house or the property in question. That won't work :-(

My parents own an almost 900 sqm plot near Regensburg, currently there is a house from the 1950s (only a wood stove, no heating system, in line with the times). My parents live on the ground floor, and my grandmother on the first floor (attic partially converted, but without water pipe). Together with an architect, we have already created a plan for the complete expansion of the attic,

Would you like to provide a site plan, photos, etc. for your question? I think something great can be created if more is known about it. Many of the forum members here often have brilliant ideas. Maybe a compromise would be to leave the existing house as it is and design "your" extension separately to passive house standards, or only create a connection to the other house in the attic, if at all. Or something like a semi-detached house.

Would you generally always prefer a new build over a renovation to a passive house?

For the year of construction, yes... I live in a house from 1930 and I would never tear it down. We have high ceilings, wide doors (>110cm), large window areas, massive thick walls, etc. All of that was understandably not built with the highest priority after the war.

An alternative would be to build a new house with a granny flat (for grandma) in the garden and demolish the old building when great-grandma is unfortunately no longer there someday. But then the current building plot could not be used, and many trees would have to be removed …