Renting, ownership of a condominium, or just building after all?

I find the house too expensive for how much still needs to be done. The same is available at Danwood for 150k euros including laminate and paint, if this standard is sufficient. I wouldn't build smaller.

If I look at it correctly, you want to put all your equity into the house? Or is there still a buffer that has not been considered so far? If not: what will you pay for the kitchen, new furniture, lamps, and all the small stuff you suddenly need when you have to fill twice as much space?

Total: €370,000

At least the garden (fence)/ terrace, exterior areas like the entrance platform, paving to the house are missing.

The backfilling of 90cm is in no way included in your ancillary construction costs or the buffer.

There is no room left for upgrades either.

With the 15,000 for interior finishing, you won’t get far if plastering still needs to be done.
25,000 euros for personal contribution is too much.

Nothing has been budgeted yet for the outdoor area. At least 15,000 euros should be planned for paving, terraces, etc.
Otherwise, you will be entering the house over pallets or through the dirt for years.

However, you still don’t have curtains, a mailbox, new furniture, possibly a kitchen, washing machine, garden tools, etc.

For me, the income at that age would definitely be too tight.

I wouldn't do it either. How are the extras like kitchen, garden, etc. supposed to be paid for?

Also, the financing is a huge nightmare, please find another financial advisor. He might get a lot of commission with his building savings contracts, but for you, it's a very expensive option.

And KFW doesn't make any sense anymore, the interest rates are very bad and they have abolished the special repayment privilege.

Overall, I wouldn't approach it that way if I were you. The plot is also rather expensive at 90,000.

A city villa in the late 40s.... If it is supposed to be something for old age, then I would build on one level. A bungalow with about 100m2 would be completely sufficient for 2 people. And I don't think much of wild financing constructions either, preferably an annuity loan with, for example, an additional [KFW] module, is kept clear and better comparable between providers.