Sequence in house construction in difficult starting position

Hello everyone,
I will try to clearly explain my intentions and look forward to some helpful answers.

A single plot of land with 600 sqm is in family ownership. It is a rectangular corner plot. Opposite are 2 houses that need to be attached to.
On the family plot, there is an old extension house from the 1950s at the corner with a shed and garage. (Not to scale)

Party 1 only wants to demolish the right part and build the green house. The left part should either be preserved, meaning only the front part will be demolished, or Party 2 builds a mirrored house on the left, in which case everything will of course be demolished.
Party 1 and 2 do not agree; Party 1 now wants to start on the right as quickly as possible.

Problem: How can this be implemented in terms of timing despite difficult dependencies?
The existing property can only be demolished from 01/22, as it is inhabited. I could also conduct a soil survey with the existing property to save time, according to the surveyor. I can also submit a building application as a (still) non-owner. The current owner can give me the permission.
At the moment, however, it is a developed overall plot that is (still) not parceled. Can I even submit a building application without surveying the right plot part? The entire plot is surely surveyed, since otherwise construction would not have been permitted. (Existing property and shed, garage are precisely drawn in the construction plan.)
The surveying or parceling can only take place after demolition, right?
The most desirable would be as follows:
- Soil survey
- Planning and building application for the front part (because that takes the longest and then you know what and how you can build and therefore how expensive it will be)
- Demolition, surveying, and parceling
- Development (there is an old development on the right, but it should be newly done in the middle)
- Parallel land register entry
- Financing (the above steps can all be paid from equity), construction

Can the building application be submitted without surveying or is that always a must?
Can a survey also be done with an existing property that does not match the building to be constructed?

A single plot of land measuring 600 sqm is in family ownership. It is a rectangular corner plot. Opposite are 2 houses built to which an extension must be attached.
On the family plot is an old extension house from the 1950s with a shed and garage at the corner. (Not to scale)

Unfortunately, the saying "a picture is worth a thousand words" also apparently applies "exceptions prove the rule" :-(

Helpful criticism would have been better

Opposite, two houses have been built to which extensions must be attached.

Opposite usually means on the other side of the street. And build extensions there?
It’s best if you post some photos as well as a Google image/Bing image, so that one has more insight for constructive criticism and tips.

Should the 600sqm be divided into 300sqm each and then built as a semi-detached house for the respective existing neighbors? Or can a new semi-detached house be built there as well? What is the disagreement between the parties, which later mentally plays no visible role?

Attached is the original development plan, which is intended to clarify our concern. I would like to start building as soon as possible on the right part of the property with approximately 300 sqm marked by the "yellow arrow". The entire property is about 600 sqm and has not yet been subdivided and is currently temporarily occupied by a rented old building, see picture.

There are 2 interested builders and there is disagreement about who builds where and how. If I build on the yellow part, the common middle section (marked with the colored circle) is to be built together. Since construction is directly on the property boundary here, both parties must of course agree, logically. From a building law perspective, this is easily possible; I have had this clarified. If we cannot come to an agreement, I will simply mirror the existing house at the boundary and thereby maintain the 3m distance to the neighbor (red arrow), so he then cannot complain. Although this would be a waste of space, it would be better than an endless tug of war about how we build the common middle and potentially regulate the shared access over the middle of the property.

To avoid losing time, I would like to start the time-consuming planning and building application process now, even though the old building has not yet been demolished and the partial property has not yet been surveyed. The soil survey can be carried out without problems despite the old building.

Is it possible to do a building plan and building application even if only the entire property is (probably) surveyed and the existing building is still there? Or conversely, can the entire property be surveyed and subdivided if it is still built on?

In the case that only one party builds, the left part (red part) is to remain and only the right part is to be demolished. Even if I then build directly up to the neighbor (red arrow) up to the property boundary, the exact middle is still needed, or can this be taken from the already subdivided opposite property? As visible in the picture, the properties are the same length. The plans from the neighbor to whom it must be attached are available. A definitive statement (in case the neighbor has made any subsequent changes) can be obtained at a fee from the building authority.