PLANNING CONDITIONS - Waste water treatment

Builderfromhell

I have just completed a very large house extension to a house in Co. Clare. The water treatment unit and percolation area have just gone in and been certified by manufacturer and Site Assesor Engineer as per conditions of planning.
The planning conditions called for Water treatment unit and percolation area to be certified as correctly installed before work commences on site.
we rang planning, before work commenced, explaining that as ground levels would be changing due to excavation and spreading soil we would prefer to install WTU and get certification later.
I had written to planning a year ago seeking agreement with them for type of unit to be used but never got areply. After calling for past few weeks I finally got aletter from them saying WTU proposed was acceptable. The letter also stated in bold type that any work carried out prior to compliance with Conditions of planning would be considered unauthorised development and be dealt with accordingly ( I don't have exact wording.
My query is; Is this just a standard clause to a standard letter? The WTU and percolation area are installed and certified, so are they likely to take proceedings?
have others had similar experience, that is to say build house first and fit WTU and perco area later?
Hope someone can help. Don't want to have to apply for retention.

muffler

Once is enough to post this.

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