Do I Need Planning Permission.co.uk
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I would like to build a single story extension on the back of my property approx 4.5mtrs long by 3.5 deep, the main drainage for my property and my neibours runs approx 12 to14" out side the building line at a depth of 24". Would I be aloud to build so close to the drain

Subject to access being made available to the drain, there is no reason why you cannot do this.

It will be dependant on whether or not the drain that runs through is private or public. If the drain is public you may need to apply for a 'Build Over Agreement' with your local water board. If you contact them and ask for developer services they will be able to tell you whether or not the drain running so close is a public or private drain.

Its not always if you would be allowed to build so close it is more how are you going to build so close. For example the drain run sounds quite close and so your foundations will need to be minimum as low as the invert level of the drainage or lower so as not to impact (weight bearing) on the drain which may cause the pipework to crack causing bigger problems in the future.

I would speak with your local water board first and then employ a designer/architect/architectural technician to provide you with plans that will be approved by both building control and the local water board should it be required.

I hope this information helps you.

James