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Extensions

Not sure who to ask for advice - drains under potential extension

Mr Daniel T Duke 16/10/2025 - 8.00 AM

Hi, I am not sure the best people to ask my question to, so thought I would ask here for any help pointing me in the right direction. We are in the idea phase of an extension, and have hit a few issues, some of which I can see solutions for, but there is one I am not sure how we would resolve, so would be good to get an idea on if it's a problem or not, and how it could be resolved. Basically, we are looking to do a ground floor extension over a basic patio, but there are drain covers there and an open drain too. I am guessing these would need moving somehow, or to be accessible, but not sure how that could be done, and what affect that could have on our plan. Any ideas welcome, even if just "you should hire an x as that is what they are for". Thanks for any tips.

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4 Answers

SP Builders

Rating: 4.8 out of 5

Ashford, Middlesex

hi you would need to contact the local water authorities to get permission to move them and get it in writing.

Answered13 October 2025

Edenbridge Property

Rating: 5 out of 5

Bournemouth

Drainage will need to be extended and new man holes inserted for access. Any unsupported areas will require concrete lintels and if they are owned by the local water authority a plan will need to be drawn up. Remember all extensions now require a soakaway min 5m from edge of extension to remove additional rainwater from extension roof.

Answered13 October 2025

J Baxter and Son's Building contractors

Rating: 4.8 out of 5

Hull

Hello Daniel Good news the drains are easily moved and generally don’t present an issue. You might want to speak with a local ground worker as this is right up their street and business as usual activity.

Answered13 October 2025

Anonymous user

Hi Daniel, That’s a great question — you’ve done the right thing asking early. Drains are a common consideration when planning a ground floor extension, and the good news is they can nearly always be dealt with properly. The first step is to determine whether the drains are private or shared (adopted by the local water authority). If they’re shared, you’ll need a Build Over Agreement from your local water company (e.g., Thames Water, Anglian, etc.). This is standard procedure for extensions and simply ensures access and protection of the drain. If the drains are private, your architect or structural engineer can design the extension so that the drains are either re-routed around the new foundations or kept accessible through inspection chambers built into the new floor. It’s very common to move or adjust drain runs to suit the new layout.

Answered16 October 2025