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Drainage issue - flooding of sleepout

Hi, we have a double garage that has been converted to a self contained sleepout by the previous owners. We are having problems with flooding when we get very heavy downpours. The drainage can’t keep up and the water rises up behind the sleepout and comes up over the slab. I suspect the drainage once did an adequate job but has become clogged and less effective over the years. I’m not sure if the drainage needs to be redone or just unblocked. We are looking to be guided by your expertise. Ultimately we just need piece of mind that we won’t get flooded even in the heaviest rain.