We live in semi-detached house with an inspection chamber for the rainwater sewer at the end of the garden with two inputs and one output.

The two inputs are fed by underground pipes from the rear of our house; the first is fed by the downpipe from the garage roof and the second fed by the downpipe from the main house roof.

The guttering from the adjoining semi connects to ours and thus also feeds into this second downpipe so, as far as ownership and responsibility are concerned, does this mean that the underground pipe to the inspection chamber is considered a public sewer by virtue of it serving more than one property? There are no underground connections from elsewhere.

The context of the question is that we are currently planning an extension to the rear of the house which will be within 3m of this underground pipe and therefore, if public, will require approval from the water authority and associated fees, inspections, etc.

Mathew