Extension construction on the boundary

Hello everyone,
maybe someone can help me with the following issue.
On my house, there is a small flat roof extension that is located on the property boundary to the neighbor. The neighbor's property initially consists of a small garden followed by a small rental apartment of the neighbor.
In order to create more living space, I would like to build an additional room on my flat roof extension, in other words, to add a story.
Do I need to obtain the neighbor's approval for this, or do I not need it since my building already stands on the boundary and I am "only" building upwards?
Thank you very much

Your development plan or the contracts with your neighbor or the building authority of your municipality can help you with this. Without knowing the legal and local conditions, it is impossible to say anything. In principle: Yes, the neighbor must agree, or also register building encumbrances on their property. If they have already done this (to that extent), it should be possible. However, the extension must be applied for (building application). Therefore, you need an architect (or draftsman). They can also assist you with this matter.

A boundary construction is only allowed up to a certain height, usually the height of a standard garage (250/300 or so). A call to the building authority can quickly clarify for you whether you even have a chance with a building application or if you should generally give up.

Hello,

There is a small flat roof extension on my house that stands on the property boundary with the neighbor. The neighbor’s property initially consists of a small garden followed by a small rental apartment of the neighbor.
In order to create more living space, I would like to build an additional room on my flat roof extension, in other words, to build upwards.
Do I need to obtain the neighbor’s consent for this, or do I not need it since my building is already on the boundary and I am "only" building upwards?

What is housed in this "flat roof extension" and is there a building permit for it?

Basically, it can be explained that in Germany we are hardly allowed to build anything without involvement of the authorities. Therefore, I would say that you need the neighbor’s consent in any case; whether the building planning office approves this measure is a different matter.

A conversation with the responsible caseworkers will help you further

Rhenish regards