Reuse very good windows?

We have bought a house with an attached winter garden. The winter garden is about 3.5×4 m in size and attached to the back of the house. It is heated and has a proper foundation, so it can be used all year round. It has a solid roof, not a glass roof. Otherwise, it has floor-to-ceiling glass walls on two sides. We use the room as a dining room. Now we would like to enlarge the winter garden. We want to add exactly the same area on the side. For this, a glass wall would have to be removed.

The winter garden is in excellent condition and only 8 years old. For sustainability and cost reasons, we would now like to reuse the glass wall (consisting of three floor-to-ceiling, fixed glass elements).
Has anyone done this before?
Which company should we contact and what is the best way to proceed with the planning?
Unfortunately, we are very busy and hardly have time to do anything ourselves.

That sounds like a construction project that requires a building permit.

A conservatory without a glass roof is by definition not a conservatory.

1. Definition of Conservatory
A conservatory is an enclosed extension to a building, a standalone structure, or a construction integrated into the building with at least one wall surface and most of the roof surface made of translucent building materials. A conservatory is a lightweight structure that is rain-tight, airtight, and windproof, separated from the outdoor climate. The transparent side surfaces of the conservatory structure are classified in the technical building regulations as "curtain walls" (glass facades), the transparent roof as a "glass roof." Conservatories can be heated if they are equipped with thermally separated walls/roof/floor in accordance with the Energy Saving Ordinance or can be unheated.

The conservatory must be distinguished from the terrace roof, which can be equipped with lateral wind protection elements but are not rain-tight, airtight, or windproof.

Not included under this definition of conservatory are, for example:

[*]a generously glazed room with a completely solid roof
[*]greenhouses used for professional plant cultivation/production or
[*]a terrace roof with partial lateral or all-around wind protection made of elements that are not wind- and rain-tight (e.g., with all-glass systems).

In my opinion, you first need a licensed professional who can submit the building application for you. They can then also tell you how to proceed.

I am just imagining that the extension is not in the building plan?

Windows can be reused.

Oh, I didn't mention this in the initial thread: a building permit is not a problem. Floor area ratio, plot ratio, building envelope, etc. are not maxed out, and most of the neighbors in the settlement have also built like this. You can also see the extension in the development plan. So everything is properly registered.

Then by definition it is probably a glazed extension and not a residential conservatory. But that doesn't really matter for the actual question.
We would just like to reuse the fixed glass windows from the side where the extension is supposed to be attached.

We just don't know who to turn to? We live in a city with a million inhabitants, so there are plenty of companies that carry out extensions. But don't they want to use their own windows, etc.? Does anyone have experience with this?

Who pays commands. If you say the window is to be reused, then the contractor should also do it that way. If someone doesn't agree with that, they won't get the contract. But as already said, for an extension you need a building permit. Depending on the municipality or state, the contractor of your choice can also apply for it. Otherwise, if you don't have time, an architect can do it as well. It's all a question of cost.

Have the "Wintergarten" expanded by the same company that built it. Among the "majority of the neighbors," there should be someone who still has documents about it (and usually the manufacturer and/or window installer is also indicated somewhere on those things).

We just don't know who to turn to?

First of all, it has already been said and you have agreed to the condition, you need an architect who will guide you through the building application and approval. Then, good idea, to inquire with the former BU.