Separation Distance Between Drainage Field And Buildings

[EViS]

Building Regs (Part H) states that a drainage field serving a STP (or septic tank) needs to be located at least 15m from any building. Does anyone know whether this refers to the distance from the ground floor external walls / foundations only, or does this guidance include the distance from any part of the building, specifically upper level cantilevers and protrusions?

[Temp]

I couldn't find a reason why it's 15m written down anywhere.

[EViS]

8 hours ago, Temp said:

I couldn't find a reason why it's 15m written down anywhere.

At least I'm not the only one in that case 🙃. All I can think of is that the minimum specified separation distance is recommended to reduce the risk of subsidence from creating any overly dry areas next to foundations?

Edited July 5, 2023 by EViS

[Temp]

I asked on Quora and someone replied that the 15m distance came from BS 6297 which I've not seen. Apparently its to do with the risk of a flooded drainage field getting into the building causing a health hazard. So on that basis I would suggest you don't need to consider overhanging parts of the building.

 

The responder also reminded me that the Approved Documents aren't the law but represent one way of demonstrating compliance with the Building Regulations Act. So it's upto the BCO to determine if an alternative proposal also meets the Act. I would try explaining to the BCO  that your drainage field will be 15m from any part of the building that could be affected by the drainage area flooding which you understand to be the intent of this part of Approved Document H and BS 6297.

 

Ideally a drainage field would also be down slope from the building?

Edited July 5, 2023 by Temp

[EViS]

That's a very thorough response, thank you. Interestingly, BS 6297 states "position as far away as is practical, consider prevailing wind direction. Recommended minimum: 7 m" from "habitable buildings". It does mention "health hazard or nuisance" but not directly in regards to the position of the field in relation to buildings.

Edited July 5, 2023 by EViS