Is it feasible to buy a second house with a mortgage on the first house?

Regarding the minor repairs, you might be right, but consumption-independent costs such as property tax, insurance, janitor, etc., are probably not transferable without an agreement, as far as I could gather from the answers on [Frag-einen-Anwalt de].

but consumption-independent costs such as property tax, insurance, janitor, etc. are probably not allocable without agreement,

Yes, but that is theory, as in practice a wording like "Bearing of all ancillary costs according to the Operating Costs Ordinance in its respective valid version" or "Ancillary costs as a tenant would also have to bear" is included in the notary deed. That is already standard.

Who knows if your daughter will even stay near you, or if training / work / love / desire for adventure will take her somewhere else entirely.... of us 4 sisters, only one still lives in our hometown. Of my sister-in-law's 3 children, who still live in the hometown, one still lives in Germany - but far away - and 2 in New Zealand... I could continue giving examples from our closest circle of friends and family. Certainly 3/4 of the children & grandchildren, etc. no longer live in the 'homeland'.

If they are relatives after all: Are they in a hurry (because the money is needed, for example) or could you possibly postpone the house purchase to next year or something similar?

Otherwise, the pressure is somehow immediately there like "okay, now I have to go back to the old job sooner," "this HAS to work now," etc. etc. So I would just find it somehow sensible if at least the first phase of reintegration is over before you take on a big new commitment. Otherwise something goes wrong and the mess is there. I also don’t know if the bank might want to see the income for at least a few months or not (we at least had to submit a few payslips), but that might depend. But also just for yourselves...

Your question of whether this is feasible I answer with a clear yes. The only question is at what risk for which party. I would review the constellation with regard to the establishment of a management company, e.g. GmbH, for the property. If necessary, both houses can be contributed to the GmbH. The articles of association describe what happens if a shareholder dies or wants to leave – this also offers room for design, as does the distribution of shares and the valuation of the contributed capital, as well as the transfer of shares through capital increases. The assurance of "staying in the house" can be contractually secured. Property tax, gift tax, inheritance tax... this is viewed differently in this constellation. When reviewing, be sure to also explore the disadvantages. The solution primarily depends on the interests of those involved. After that, a realization path can be found.

I would review the arrangement regarding the establishment of a management company, e.g. a GmbH, for the property.

This is completely overpriced for the scale we are talking about here. There are a lot of costs incurred in such a GmbH per year, especially if you are not personally involved in the areas of annual financial statement preparation/electronic balance sheet/tax declaration/legal advice, especially since a property is owner-occupied and then counts as rental. Just the contribution of the existing property already triggers real estate transfer tax, notary costs, etc. From parents to children, the inheritance and gift tax allowances are so high that this is completely unnecessary for the original poster. It may be different with 5 large apartment buildings, but here there are only disadvantages.