We too (like probably many others) have a non conforming fosse (‘grey’ water goes straight into fields). All our artisans (builders, gardeners etc) without exception said they would do nothing, and wait for SPANC/SIAEPA (relevant agencies) to catch up with you. That didn’t sound either sensible or the right thing to do to our ears. We are putting in an approved model of MicroStation at considerable expense (€13k).
Interestingly we have just received notification of a proposed contrôle of the existing fosse sceptique by SPANC/SIAEPA giving us a time, date and requiring sight of a plan/diagram, invoices for emptying/maintenance, and telling us to ensure removal of any soil, lawn or flower pots preventing access to the fosse for inspection. What is slightly bizarre is that the contrôle letter is actually signed by the very person we’ve involved from SPANC/SIAEPA to approve our plans for the replacement MicroStation! She has agreed to cancel the contrôle on the basis that the fosse won’t actually exist in a few weeks, so a sensible conclusion so far.