Hello Finch,
I first join the others and also have a strong tendency to say: "Buy it." The main reason is that when selling within the family, you could assume that you will not only get an attractive / fair price, but above all, you won't be misled regarding the condition, technical / legal / neighborhood issues, etc. However, as long as you don’t provide us with photos and plans, we can only comment on all this based on rough (and apparently consistently positive) assumptions.
One thing upfront: In a comprehensive renovation of a semi-detached house, the character of both halves must remain similar afterward. This criterion is interpreted very differently by building authorities. Sometimes not even an insulation above the rafters (let alone dormers) is allowed, while sometimes one half ends up looking completely different from the other.
Regarding the living area: It has already been mentioned that you might gain some space with dormers, possibly also with a small extension. But in my opinion, you can live well with 4 people in 100 sqm if you adapt the floor plan accordingly. You have to say goodbye to the trendy "open living" concept, which, in my perception and post-Corona, is on the decline anyway. What is important is that each family member has a retreat space. To put it simply: 2 children’s rooms of 10 sqm each are much more valuable than only one with 20 sqm. This also applies to other rooms (home office, etc.). Here in the forum, you will surely get valuable tips on how to make the best use of the space once you upload a first draft of your intended floor plan.
I will summarize what construction problems I have encountered with houses from the 30s and organize them according to "fixability":
EASILY fixable within the scope of a core renovation:
1) inappropriate floor plan
2) pipes (electricity, water) do not run through installation corridors
3) asbestos was hardly used in the 30s but often found in later renovations.
4) damp basement (often built directly against the earth without waterproofing)
5) roof in poor condition
6) poor energy efficiency values
7) roots of old trees growing into masonry or sewage pipes
8) house poorly documented, especially (but not only) due to subsequent modifications
DIFFICULT / NOT fixable:
a) weak sound insulation to the neighbor, probably a 24 cm partition wall (possibly even thinner). Furring strips help a bit (especially if the neighbor cooperates and installs them on their side as well), but against continuous structural elements such as beams, there is little you can do.
b) wooden beam ceilings with their typical problems (poor protection against noise from the upper floor to the ground floor, vibrations, partially creaking)
And finally something positive: Such a house has a soul. You can never achieve this with a new building.