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Surface/rainwater drainage design for extension

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Proposed Drainage.docxExisting Drainage.docx

Proposed Elevations.pdf

Hi All,


I was hoping that you might be able to point me in the right direction please!


Mid planning for a 5m rear extension to my bungalow. I have designed the extension myself and submitted planning and now looking at getting started and planning for building regs. I'm concerned about the rainwater/surface drainage and have prepared a couple of ideas in the existing/proposed drainage plans and elevations attached and was hoping for some advice before submitted to my BCO. The extension is 5m to the rear making the total roof size 14.55m long x 8.15m wide. The garage is 8.25m x 2.75m. I have calculated the total area to be drained at approx. 197m2 making for a 3m3 soakaway if it was just one (but I think this is far too big for 1 soakway?)


A couple of questions if I can please:

  • Is it normal to drain one side of the roof to the rear and the other to the front? I couldnt see anything online. 
  • Are my roof size/volume calcs correct?
  • Can I drain all to the rear? Wouldn't this cause a large influx of water to the rear of the house over the 14.55m span of guttering?
  • If all done to the rear from reading online a 3m3 soakaway is just too big for one soakaway so would need 2 one at front and one at rear?
  • Does anyone have any other ideas please, I'd really like to not have to dig up the front garden aswell if possible!


Many thanks in advance


Edited by Alex3012
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Do not assume that currently there is a soakaway serving the house. Dig up and follow the rwp and see where it gets to. Mine led to a storm sewer running shallow right under the  planned extension, but I learnt that only when digging for foundation trenches exposed it...

You need to keep specific distances from the boundaries to build a soakaway to start with (2.5m from boundaries, 5m from the nearest building as far as I remember), so you may end up with that already being a limitation. If still lucky, your percolation test may fail.

If there is a rainwater sewer around (must be) you'll end up negotiating connection with water supply company, otherwise some clever water attenuation solutions would have to be the way to go.

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Hi Olf, thanks for the response! I have previously followed the RWP and have only gotten to clay pipes about 1m down and then gave up and assumed it went to a soakway. Nothing shows up on the Wessex Water sewer map for surface water drainage here so thought this would back up the soakway theory??

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17 hours ago, Alex3012 said:

Nothing shows up on the Wessex Water sewer map for surface water drainage here so thought this would back up the soakway theory??

Nope: water companies 'adopted' what got defined as public (shared) drains in 2011. These drains can run in the gardens and Wessex Water have no idea where - the map only shows what they build, so likely only pre-2011 along the roads.


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That's really interesting, and a but worrying! Hopefully there is something there to make it easier to drain rather than soakaway but at same time hope it doesn't interfere with my foundations during excavation!

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