This is linked to the Question "Can I adapt code I wrote for work and release it on open source"
I used to work as a 3D Artist for a company where I sometimes wrote scripts to ease our workflow. Of course that code belongs to my employer. However, if I now were to rewrite the code from scratch at home or maybe at a different company, would I still violate that law?
You could also say, does my ex-boss own just the code, or also the idea?
First, check your contract. If it's Germany, it's probably written down.
Assuming there is no agreement whatsoever (and I doubt that) then you would be free to redo the work you already did with the knowledge you now have. Redo means you have to do it again with the tools you have available now. No copying of source code files just because they forgot to revoke your access, not using their tools.
A simple example: if you program an e-commerce platform in C#, then switch jobs, you are free to program another e-commerce platform. In C#. Just imagine if you weren't allowed to do that. I programmed a linked list for my very first employer, can I really be banned from writing such data structures in the future? Obviously not.
Exceptions are (and I cannot stress this enough) your contract and any laws that apply, like patent laws. If that e-commerce site was such a miracle that they had it patented, then you are not allowed to copy it, whether you previously worked there or not.
If your boss though it would be great to sell shirts next to socks in that web shop? There is nothing prohibiting you from opening your own business and doing exactly that. Except for maybe your contract.
It depends.
How specific is the "idea" to the organization?
Example:
If the idea is to improve the efficiency of a specific working model by introducing a new logical path using some specific tooling - well, that idea / concept might be owned by your organization. The organization might have spent some time and resource finding out the problem exists and then again used their resource to produce a solution. You happened to be the part of the team which worked on this - so this does not make the idea yours.
If the idea is to automate some download from internet and using that as a source for random noise - well, that's a common idea and any code you (re) write would be OK.
To sum it up: If the idea is available or derivable without a dependency on the work you have done in your previous organization, then any code you write based on that idea would be OK. Any conclusion or observation (however small it may be) which (can potentially) lead back to the work done in your previous organization - STOP and abandon the idea.
Disclaimer: IANAL
