Costs for partial extension and flat roof with roof terrace, any experiences?

No, I have no experience with construction, the prices are the result of Google searches, where I have already oriented myself at the upper end of the suggested average prices.

I could afford to take 4 months of parental leave financially, I have a vocational training in craftsmanship and various craftsmen in the family.
I think that both the wall and roof construction could mostly be done by myself.
Costs for crane, construction debris, applications, fees, architects, etc. are not included.

Since the KfW loan [Jung-kauft-Alt] with €150k is to flow into the project, at least the standards for the KfW70EE house must be met.
The roof itself can be very simply (with roofing felt?) covered, since the terrace will be above it.
I have no detailed plans regarding the exact details of windows and further execution.
The electrical system in the house itself is up to date, there should not be much to do in my opinion.
The house should be as low-tech as possible, I am considering a masonry stove (as an oven of course)
and a fiberglass ceiling heating, which at the same time serves as reinforcement for the clay boards.
The house is a backyard house and has a long side attached to the neighboring building
(here the approval is still pending, on which the project stands or falls)
However, there are no large trees that directly shade the house, but it only receives direct sunlight relatively late
since the east side is built up.

The huge window on the south facade alone is probably easily five figures, plus shading.

The house is already mostly like that, it’s only about the left side, which has no upper floor.
And the roof terrace, admittedly, the window is already large at the moment but would probably be replaced with a larger one because of the otherwise poor lighting.
However, I would also be open to used building materials.
I have to stay under €200k for the renovation measures; otherwise, it will be difficult to reach a purchase price of €320k.

The house is a backyard house and is attached with a long side to the neighboring building (here the approval is still pending, on which the project depends)

Does the house where the extension is to be built have no windows?

Have you submitted a building application or a preliminary building enquiry? Is there a development plan or a statute of the municipality that regulates the extensions/annexes?

A roof terrace is more expensive than a roof.

Show us the plan of the existing building.

Here the right way up. The ground floor already has very large windows approximately 2+ meters wide.

Doesn’t the house, where the extension is to be built, have any windows?

Have you submitted a building application or a preliminary building inquiry? Is there a development plan or a municipal statute regulating the expansions/attachments?

A roof terrace is more expensive than a roof.

Show us the plan of the existing building.

Yes, it has large windows, which were only recently replaced, plastic, insulated glass. The preliminary building inquiry is currently being prepared, an architect is currently making plans for the project. However, it all depends on the neighbors’ approval; I have already spoken with the city, which gave me positive feedback. The neighbors are also generally in favor.

Do you happen to have experience with building a roof terrace? After all, it would also lead to a significant increase in value. (Average rental price about 16€/sqm+ for new rentals)

If this is going to be a single-family house, life usually takes place in the garden or on the terrace in our regions. [Dachterrasse] is more typical for condominiums. Or is this supposed to be a multi-family house? The [Dachterrasse] will probably be rentable for 8 or 4 euros.

Is the sketch from the architect?