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Waterproofing issue in downstairs bathroom

So in our downstairs bathroom, which was renovated about a year ago, the doorframe to the bathroom and skirting close are getting quite mouldy and soft to the touch. We have had a plumber through who is confident it’s not a water leak from a pipe. There is also access to under the house just next door which might make investigating a lot easier, I’m of the opinion that water is somehow coming from outside, but no idea how. Would love an expert to come through and give us an opinion on what work needs doing, and if possible, perform that work. Cheers!