Residential construction on existing building - parents' property

Can you explicitly recommend sources, books, etc. on the topics "extension," "basic house construction," "basic house financing"?

My information about the property is limited to ...

The building is currently a catering business. There is enough space all around for a possible staircase, parking spaces, etc. The roof's structural design was made to allow for two more floors to be added. On the neighboring property, a total of 27 residential units, including row houses and granny flats, have been built.

... so on the one hand the property is concretely unknown to me, but on the other hand there are (only) "suspicious indications" that it is rather non-average to atypical. These are extremely unfavorable conditions for making general recommendations, even if they are only literature tips.

Can you specifically recommend sources, books, etc. on the topic of "extension" "basic house construction" "basic house construction financing"?

There is a whole series of books for builders from Stiftung Warentest: financing, reading construction descriptions, inspections. That is definitely worth reading. Consumer advice centers offer similar materials. However, the topic of extension might be quite specific; at least I have not seen anything on that topic in my books.

I'm not a fan of building the first property for strangers who will live in the property

Me neither. That's too much financial Russian roulette for me with an unhealthy dependency!

The building is currently a catering business.

So far it all sounded good, apparently structurally okay and approvable...
But now you come with the fact that this is not a residential area at all.
Even if dozens of terraced houses are being built on the other side of the street, it may be that your development plan does not apply.

Can you recommend explicit sources, books, etc. for the topic "adding floors", "house building basics", "house building financing basics"?

No. Reading material is not necessary at first. An architect has to look at it and check all the requirements and possibly submit applications.
Besides, I would consult a tax advisor or notary who can look into the family ownership structures.

Further to the other thread: what floor area ratio are you considering if suddenly 2 additional floors with 280 sqm each (4 apartments) are built?

It was reclassified as building land and according to the development plan, 2 full floors + a half floor (roof) are allowed to be built on the building, as well as 3m width in one direction, 1m width in the other direction.

It was rezoned as building land and according to the development plan, 2 full floors + half floor (roof) may be built on the building, as well as 3m width in one direction, 1m width in the other direction.

Where is the property located? Is it a commercial area? Mixed-use area?

The building is currently a catering business.

The property is located between Düsseldorf and Cologne, the catering building was constructed when it was still a mixed commercial area, but it has been reclassified. As mentioned, we have spoken with the city and 2 1/2 full floors can be built for residential use.