Approximate costs of an "extension"

Good evening,

we (my wife and I) are currently thinking about taking over my parents' house where we have been living as "tenants" for 5 years. The house was built around 1940 and has been constantly renovated and maintained in recent years. Unfortunately, from today's perspective, it offers "relatively little" space for 2 or almost 3 generations. For this reason, we would like to build two floors with a flat roof on top of a detached brick double garage (made of relatively large but also old gray aerated concrete blocks approximately 240 mm wide?!). However, we will probably have to completely rebuild the garage, which would be checked in advance. We don’t know if one can simply build on top of it.

Can someone estimate how much such a construction project might approximately cost?

Ground area approximately 7 m * 7 m
2 floors on top with 1* 2 rooms and one large room above
No basement (if everything has to be new)
With flat roof or shed roof
No water or wastewater (only living room and bedroom)
Relatively large window areas
We would do the complete interior finishing ourselves

Since we are absolute beginners, we would also appreciate information about the surrounding procedures. For example, which steps do we have to take (building authority, architect, etc.). Perhaps someone also simply has a good link or a book recommendation for us to read up on.

Thank you very much in advance.

Best regards
ChriStef

Good evening

We would do the entire interior finish ourselves
....
Since we are absolute beginners

How do both statements fit together?

Rhine greetings

Good evening


How do both statements fit together?

Rhenish greetings

I have been doing/accompanying beginners in the topics of house construction, interior finishing, and renovations since I was a child. As I said, the house has been continuously modernized for years.

Regards
ChriStef

First of all, I would clarify whether the request is even possible. The garage is detached, i.e., separate from the house? How is the access planned then (bridge?) Is the garage located on the boundary? Simply building something on top is often not possible because that would be a change of use, which is often not allowed. For example, we wouldn't even be allowed to build a roof terrace on the garage because then a 3 m boundary distance would have to be maintained. Aside from the structural issues. Do you have a site plan, because as it is, it is very difficult to imagine.

Better to go to the building authority. There’s no way around it!

Good evening,

thank you very much for your tips. I have already considered some of them.
The next boundary to the neighbor is approximately 6 m away, the next building is about 10 m away. We will definitely contact the building authority, and the structural engineer will also check the condition of the current garage.

It is simply the case that we have a budget limit, and therefore want to first know how much money we need to plan for.
If the approximate price is far beyond our expectations, we will not "make a fuss about everything and everyone". Therefore, we just wanted a rough estimate of what to expect.

Thank you and regards