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Leaky flat #2

Our house was built in the sixties and is sound, but at some point an area under the house was closed off and made into a small flat - maybe in the 80's? Cladding is cedar weather board. Looked good, but turns out build quality is terrible. Its been leaking for a while and I pulled some of the gib off today so someone who knows what they're looking at can assess it better. Sill plates are rotten and looks to have been leaking past some dodgy flashings. Not sure if any of its recoverable, might need to start over. We need expert advice so only qualified builders please.