7 Oct 2017
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United Kingdom
Hi I am planning on building a one storey extension to the rear of my garden flat which is on the ground floor of a victorian terrace house in london. There is an existing sunken patio to the rear of my flat and there is a 1.2m high retaining wall holding the raised garden. The extension I want to build will infill half of my sunken patio.

question 1: I do not know if I can build my new blockwork wall right up against the existing garden retaining wall or not, or should there be a gap left? Or should the retaining wall be taken down and rebuilt stronger to be integrated into the extension? Ideally I would like to have the new blockwork wall as close to the existing garden retaining wall as possible in order to be able to save space.

question 2: In terms of tanking I am foreseeing having a cavity-drain system on the inside of the new blockwork wall, but would I let all the damp and moisture from the garden and existing retaining wall through the outside of the new blockwork wall?

thanks in advance for your advice...


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What the the construction of the existing retaining wall? Masonry?

I don’t have answers to all your questions but I do have a few comments.

A drained cavity is an effective way of tanking a basement bit you’ll need a good good builder with experience of this kind of work. Quality needs to be good otherwise you’ll have problem.

For a grade 3 basement which is what you’d be wanting for a habitable space I’d suggest having two barriers to water, maybe a drained cavity and an external membrane.

For more info on waterproofing of basements this is a useful guide
You're in the relams of specialist design for what is effectively a basement, and structural retaining wall.

You certainly would not be anticipating movement of a retaining wall.
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