Can anyone help/clarify?

We have our own fosse to install on our own property in the Creuse, which we are capable of doing, but our neighbours (here in the UK) would like for us to help and carry out the installation of their fosse in the Dordogne area. They have been told that we would need to register before carrying out this work, as we could be reported to the police for operating without the relevant paperwork.

If anyone can through some light on this we would be most grateful, the last thing we want is to be locked up by the Police even before we get to live in France.

Look forward to hearing something from someone,
Thank you