The usual caveat applies - it all depends on the local geology/hydrology, landfill, nature of the extension and existing development, foundation design etc etc.
When I worked in West London 15 ish years ago we routinely attached planning conditions to extensions, requiring either spike testing to confirm there wasn't a problem, or requested the applicant install 'reasonable' gas protection measures to the extension by way of mitigation; typically airbricks and a sealed/lapped etc membrane. We'd usually accept the standard 1000 or 1200 gauge dpm, so long as it was carried across cavities and sealed around service penetrations etc.
Things have obviously moved on since then (not least regarding spike testing) - and I think Clive has a point that if the gas risk is considered so great that a new extension MUST be protected from gas ingress, but the rest of the building is unprotected, then what further consideration is being given to assessing/protecting the existing building ? Conversely if the assessed risk is not considered that significant, it is difficult to argue that the extension would be unsuitable for use without gas protection !
I would think in the scenario you describe it could be difficult to justify to the applicant why they should incur additional expense to protect the extension, while the remainder of the house is unprotected. That said, it could be appropriate advice in some circumstances, depending on the site specifics, and given the relatively small marginal cost of fitting the dpm membrane as a gas barrier.
As for the potential for gas to migrate more than 50m from an older landfill, I have seen several times, though it's clearly fairly unusual. Highly permeable geology, service pathways, capped landfills, local hydrology and dissolved gasses no doubt all played a part.
> Steve Moreby
> Contaminated Land Officer
>
> Environmental Health and Regulatory Services
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> Gloucester City Council T 01452 396 312
> Herbert Warehouse F 01452 396 340
> The Docks
> Gloucester
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-----Original Message-----
From: Contaminated Land Management Discussion List
[mailto:[log in to unmask]]On Behalf Of Clive
Williams
Sent: 06 September 2010 13:34
To: [log in to unmask]
Subject: Re: Gas Protection In Extensions
If the planning authority insist that an extension requires gas protection measures to protect against gas intrusion and the original house has none, can you infer that there is a significant pollutant linkage to the house and therefore does the house need to be determined?
I wonder if anyone knows of any gassing landfills where gas has migrated through the soil to distances over 50m and if so was it methane or just carbon dioxide, the methane oxidising on the way. Similarly with VOCs - anyone got any actual examples? I ask as I find it hard to conceive of a driving mechanism that would get gas through 250m of soil.
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