DIY work before house handover (general contractor) - what to watch out for?

Hello everyone,

first of all, sorry if I placed this topic in the wrong section – please feel free to move it elsewhere :)
As builders, we are working with a general contractor.
Among other things, we have a concrete staircase: the parquet stair coverings as well as steel railings were initially included in the contract, but we want to have them done by an external provider (i.e., as a builder’s own contribution).
If we have it done "normally" after the house acceptance, it means a delay of 10-12 weeks for us: the railing builder needs the already installed stair coverings for planning due to heights/angles and then another 8-10 weeks for production and installation (custom solution, wrought iron). In addition, the stair stringers still need to be filled – this can also best be done only once the stair coverings are installed (this should still be done by the construction company).
Therefore, we want to have the stair coverings installed before acceptance (at the time as if it were still to be done by the construction company).
The site manager just says that we can do whatever we want, but then we lose the warranty in case of any issues (e.g., scratches, etc.). I understand that as well.
How can this be properly coordinated: e.g., partial acceptance of the stairwell before the stair builder comes, somehow arranging a withdrawal declaration, etc.
Are there any tips and tricks on what to look out for?

Thank you very much for your help!

Best regards
Nils

Are there any defects that you already know of and would record during the final inspection? If not, there is hardly any reason against it, as really serious issues will hardly arise by then?

The site manager just says that we can do everything we want, but then in case of any issues we lose the warranty (e.g. for scratches, etc.). That’s clear to me as well.

How can this be properly coordinated: e.g. partial acceptance of the staircase before the stair builder arrives, somehow arrange a withdrawal declaration, etc. Are there any tips and tricks on what to pay attention to?

He means that if a scratch is complained about on the windows, there is no warranty for that because a craftsman from an external company, in this case the stair builder, might have caused it. The object of the defect does not necessarily have to have anything to do with the staircase. But if he handles iron and rods, then a dent in the plaster, door or screed (theoretically anywhere in the house) can happen.