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Have you already used the gov portal for flooding? It will give you a good idea of the build up of rain water and best location for drainage.
We did a similar thing in Devon and realised we had a much larger risk of flooding from a southern boundary, despite the obvious risk being water coming down a lane at the front. We moved drains around accordingly, etc.
https://flood-warning-information.service.gov.uk/long-term-flood-risk
JonoMarshall-
I have but it doesn't seem to offer anywhere near that level of detail and I thought it was automatically calculated and you couldn't rely on it for detail anyway?
I wonder if it's more useful for you because we're buying a terrace in E17 and you presumably have a mansion with land being in Devon so you the macro level of detail is more useful?
Fox
'Ours' (or what will hopefully be ours as long as it doesn't go tits up) is not lovely, although we plan to make it lovely. But removing the silly little roof, taking off the render and pebbledash I know is underneath and rebuilding the bay will be lower priority than stuff inside, so it might have to look unlovely for a bit (we couldn't afford a lovely one)!
I haven't looked into insurance yet but 'our' street and Chris's have never actually flooded despite being the closest two to the waterworks/marshes, so as I understand it there shouldn't be a problem.
I am treating the possibility of flooding as more than theoretical though. We'll need to demolish the back and put a new side return extension/kitchen in, so when we do I'm planning on putting in a non-return valve into the foul drainage so if it does happen our kitchen won't fill with poo and make it as flood resistant as possible (raised electrics/boiler/appliances, flood resistant materials etc.). As you can see we'll need a new front door so planning on a flood resistant one and anti-flood airbricks.
Hopefully none of this will ever be needed and I'll have wasted a few grand, but if it is I'll be glad I did.