Conservatory replacement

WobblyDog

My house has an ugly, disproportionately large conservatory attached, added by a previous owner, at least 10 years ago. I don't know whether it ever had planning permission. It's a poor quality thing in every respect. Most importantly, the foundations of the conservatory have moved, so it now slopes a bit. It is isolated from the house by an external-quality door, and has isolatable heating, so I don't think building regulation approval is required.

I want to completely replace it with a smaller one, including the foundations. I suspect (but am not certain) that some (previously private) small shared sewers run underneath it serving a me and a couple of neighbours. There were never any access chambers underneath it.

All the companies I had round to quote say it won't be necessary to inform the water company, because the conservatory was built before the recent shift of responsibilty for sewers. They say planning permission is not an issue because the new one will be smaller than the old one.

I might want to sell the house in a few years. Would I be wise to insist the water company is involved? Any other advice?