Internal manhole on private drain?

[grandschemeofthings]

We're about to start work on an extension to our Victorian terrace. Our existing kitchen has an internal manhole at the back of the original extension. The tiles are laid into a cover so you can access it if required. We've not required access in the 10+ years we've been here.

It's a private drain (just serves our house) and only takes water from the gutters at the back of the house, the soil pipe is much further down the run. The pipe is less than 150mm wide.

Am I right in thinking we don't need a build over agreement for this because a) it's in the existing footprint not the new extension b) it's a private drain not a public one and c) it's less than 150mm diameter?

In theory we could move the manhole outside, but that would mean laying another 2m of drain that would never have anything running through it which seems a bit mad, and a manhole would be more obtrusive in our small garden. The biggest issue we can see with leaving it where it is is there would be a cold patch where we couldn't lay underfloor heating...

Are you working with an architect? If so, they will know (we have just been through similar, though slightly different as the water company didn't know anything about the drain which is shared by a few houses and both the drain and building we are replacing predates any of the companies)

Yep, architect hasn't a clue (have yet to meet one that does tbh!) and all the builders who've quoted have advised us differently.

A friend had this point raised and your understanding is the same as mine. It is a private drain if no others feed into it, and therefore manhole covers can be inside the property.

If any others feed into it then the manhole cover would need to be moved since it’s a public/lateral drain and manhole covers cannot be inside the property.

The architect (and planners) never noticed it was a lateral drain manhole on my friend’s extension and the manhole (and drains to meet the manhole) had to be moved outside the extension once building had started… (and build over agreement required).

@Bumblebee47 thank you, that's good to know. We had a drains camera survey done a few years back so we know what connects where and it's definitely all ours, no lateral drains involved.