Rear extension and drains

Dedekind

Hello, 

I have a question, already raised with our architect/engineer but wanted to check what people here say as well. 

We are planning to build a rear extension in a semi detached house. On one side we have the neighbours and on the other side there is a driveway. The neighbours drains go through the back of the house all the way to a manhole which is in the driveway. 

As such our extension  would be built over the drains. The neighbours are reasonably asking that we make sure drains are not damaged during construction  and so on and that there will be future access in case of blockages. 

Is this a potentially complicated issue? I believe we may need a build over agreement with the water company, is this a straightforward process? I don't want to spend lots of "preparation" money (buying kitchen etc) if we won't be able to build in the end. 

I understand the drains are quite deep by the way. Can't they just be below the extension? 

Extra piece of information: deeds mention a Drainage Agreement between the properties signed on 1927 (when the houses were built). But no details on what the agreement is 

grumbler

Personally, I'd do my best to avoid building  even over my own drain, let alone neighbour's.

Dedekind

Is it something that can be easily moved? We are OK to extend our budget a few £k if that's what it takes. But I wonder how feasible it is -- if not then our plans need to be revisited!

Doozergirl

grumbler said:

Personally, I'd do my best to avoid building  even over my own drain, let alone neighbour's.

People build over drains all the time.  The Build Over Agreement exists to ensure that they are built over correctly, specifically to allay the sort of concerns that the neighbour has.  

It's not a problem. 

Doozergirl

Dedekind said:

Is it something that can be easily moved? We are OK to extend our budget a few £k if that's what it takes. But I wonder how feasible it is -- if not then our plans need to be revisited!

There is technical guidance for build ofer agreements that show how water companies might expect you to build.  As long as it isn't  over a certain diameter (usually sat in the main roadway), build over agreements are commonplace.  

If there isn't a manhole or other access plonked in the middle of the proposed extension then it's a case of getting the build over agreement which involves a fee, some CCTV work for the water company before and after, and building over the drains to ensure there's no downward pressure on them.  

If there is a manhole, it would need to be moved, but you haven't suggested that there is.  It shouldn't cost vast amounts as you've already got people digging on site and materials aren't expensive.  

Your architect will know all of this.  

Dedekind

Thanks everyone. Our architect confirmed that all such below drains will be taken into consideration in the drawings (and he will inspect the manhole soon as well to confirm the depth and so on).

Dedekind

I have one more question, was just reading some of the requirements for a "self-approval" from Thames Water:

"Any sewers where the bottom of the pipe is more than 1.1 metres below ground level must run at least 500mm away from the foundations. If it is more than 2.0 metres below finished ground level, any proposed foundations must be at least 1.0 metre from the sewer."

What I am not getting from this is whether the foundations need to be above or below the pipe -- example if the pipe is 2.5 metres below, and we have 1 metre deep foundations, then surely they are at least 1 metre apart. But maybe they mean that the foundations need to be at least 3.5 metres in that case? (That would be crazy for a simple single storey extension..)

DevilDamo

The bottom of foundations have to be taken down to ‘at least’ the bottom/invert level of any adjacent sewers. So if your sewer is 2.5m deep, your foundations would have to be at least the same depth. The clearance dimension referred to is measured horizontally and requires new walls to be linteled over existing sewers.