Surface water soakaway -extensions

[snb375]

Can anyone help with some advice on soakaways in the garden? We are having a really small side extension done and halfway through the build the building inspector has come round and insisted we build a soakaway.

This was never on our plans and is the first time we have heard we would have to do this. My issue with this is that the soil in our area is clay and drainage has always been a nightmare, we had a patio laid last year for the exact reason that garden was waterlogged. Even though we pointed this out and asked if there was an alternative, he wouldn't discuss this at all and insists we must dig up the patio. I have done some research since and it seems it must be 5 meters away from the property and a road and at least 2 meters away from a boundary. The hole itself must be 1m wide.
Our garden is 9 meters long and 4.7 meters wide of which we are end of terrace, so one side is against a neighbours boundary and the other side borders a pavement then the road. Surely this means that will be too close to the boundary on either side? Coupled with the fact the soil is clay surely this is a bad idea? Or can he override this guidance and just force us to dig it? Would appreciate any advice before I pick this up with the council building control.
Thanks.

Have a look at nhbc standards chapter 5 clause 5.3.11. There is information on soakaways in there but you are right that if you have clay soils it is unlikely to be appropriate. Also right that must be min 5m away from property to avoid water softening the soil around the foundations. Planning policy probably requires that you have 'sustainable urban drainage' SUD's which is why they are requesting it. And soakaways are the easiest and cheapest way to comply. It may be that you can provide some form of attenuation to limit the discharge into the sewers. Provided this is less than greenfield run off rates I don't see how they could reject but ultimately you do need them to agree to it and it can be a problem. Start reading up about SUD's in your area.

Building Regs dictate an order of priority for the selection of surface water drainage from buildings:

1 - an adequate soakaway or some other adequate filtration system, or where that is not reasonably practical,
2 - a watercourse, or where that is not reasonably practical,
3 - an appropriate sewer

A soakaway must always be the first choice but can't be used within 5m of a building or road, 2.5m of a boundary or where the water table is too high.

Page 29 onwards here:

assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/442889/BR_PDF_AD_H_2015.pdf

Given the confines of your garden plus the fact that you're on a clay soil, it seems that a soakaway is likely to be a non-starter.

So, you can discharge the rainwater into a sewer (assuming a watercourse is not nearby), although you will need to get permission from the water company and they may require evidence such as percolation tests or a letter from building control.