development plan, building line, building boundary, building window

Since was already there...

I too am puzzled by the term building line. Especially since the definition is that you must (compulsorily) build on it - unlike the building boundary.

that the solid yellow line in the front yard and in the backyard in the residential area might perhaps be the building envelope.

Certainly not, otherwise the same could not be true on both sides of such a line anywhere. Also, building boundaries are typically thick and blue, and building lines are typically thick and red, and here no reason for a special case is apparent. But you should be able to find clarification if you have the legend.

From the answer "no because massively exceeded" I would read that they would have no problem with a moderate exceeding. What does the development plan say, for example, about the dimensions / volumes of garden sheds, or about bay windows / conservatories?

I am afraid, however: nothing that will please you, something like 2 x 3 m or twelve to fifteen cubic meters, for example. Attaching an entire living room right away will, to my no small surprise, apparently be seen as softening the plan too much.

The best thing is certainly to go to the building authority and inquire on site/at the location.

Yes, and indeed before commissioning an architect. Or you ask the architect to obtain the necessary information before preparing the construction documents. Then you could probably have saved all the work.

However, for me it is important to gather enough information and knowledge yourself before entering into a discussion with the building authority.

By then it is already too late. There have already been much more than just one discussion. There was even a decision. What do you expect from the forum, to which you withhold information already available to you (e.g., the textual stipulations), if not even your architect, who hopefully is familiar with all the circumstances, creates a solid basis for negotiation with the authority.

It is right that one should not write with conjectures in the forum

I do not want to see it quite so absolutely. Often the original posters simply do not reveal the facts; then you have no choice but to express conjectures. However, I find it strange that you answer your own questions with possibilities that in the next sentence already become facts to which further incredibilities are added.

Could handle building lines ... that the building line may be exceeded by 1.5 m ... and thus not fit the streetscape.

And all this against the background that much more has already happened than just the question about plan symbols. As it turns out from the information now provided, your initial question was misleadingly posed. It is not about the meaning of the yellow lines at all, but about a rejected submission. For your property, the yellow line is completely irrelevant, as it is almost identical to the edge of the red area here. This means, whatever it means, your buildable area ends at a construction depth of 10.00 m and your project is simply not approvable.

Hello, no, there was only one conversation with the building authority requesting the submission of the development plan and text section. From the feedback, we agreed to submit a simple draft/sketch to discuss further after the review.

Now we have received a response with the quote that an exemption from the building regulations with T=5 x B=6 (m) would massively exceed the building boundaries.

The clarifying meeting is ahead of us, and I am considering whether it is a) due to the yellow line (whatever that may mean) or b) the missing justification for the draft/sketch.

In any case, there must be reasons, although on the parallel street there is already a similar extension with a T=approx. 3 x B=approx. 7.

whereas on the parallel street there is already a similar extension with a T=approx. 3 x W=approx. 7.

The illustration does not even reach this comparative object.

and I am wondering whether it is a) due to the yellow line (whatever that is supposed to mean) or b) the missing justification of the design/sketch.

The originality of the justification does not earn any points. The essence of an exception includes the scale “of the little finger, not the whole hand.” Attempting to overlay the intended extension of about 30 sqm onto a base area of about 80 sqm exceeds this scale (by so much that the question "by how many grams" is probably redundant).

Hello, no, there was only one conversation with the building authority with the request to send the development plan and text section. From the consultation, we then agreed that we would submit a simple draft/sketch to be able to discuss further after the review.

What you wrote elsewhere paints a different picture: Dear Sir or Madam, my architect submitted a building inquiry for an existing project with a draft to the city building authority. We had previously received the plans for the development plan and text section from the city building authority. According to the basic documents, there is no restriction in the development plan. On the internet, I researched that a minimum distance of 3m to the neighbor must be maintained. In the text section of the development plan, a minimum distance area of 3m to the neighbor and a maximum building height of 6.5m for 2 floors is stated. Now we have received feedback from the city building authority that we may build a maximum depth of 1.5m and a width of 5m with a building height of up to 9m. ... Why does the building authority in my case only allow a depth of 1.5m, width 5m, building height 9.5m? ... What is the next instance? Legal action or where else could I turn?

is it a) due to the yellow line

No. It is due to the red area.

or b) the missing justification of the draft/sketch

If it were known, something could be said. Would, would, bicycle chain.

In any case, there will be reasons, although on the parallel street there is already a similar extension with approx. L=3 x W=7.

Yes, and on the other side of the street to the east, there is a 13-story high-rise. Instead of putting the facts on the table from the beginning, I find myself unwillingly cast in the role of Count von Seidewitz in the staged joke "The Cow Elsa" by Didi Hallervorden, which I find less funny than exhausting.