Yes Permitted Development for 56 days is an option but 56 days is only the summer holidays really.Under the 28, now 56 day rule, you can use your permitted development, for most places.
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What about dry compost loos, which are basically a shed, which you separate the solids from the urine, and add shavings. Make it in to an eco feature.
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As for the compost loo technically it would require planning as it is a structure. In reality it depends how permanent the structure is.