My kids would love that, as for PP the national rules for England give you the two tests you're asking about. Without an application, a garden building can be up to 4 metres high with a dual-pitched roof or 3 metres with any other roof, with eaves no higher than 2.5 metres, and if any part of it is within 2 metres of the boundary the whole thing is limited to 2.5 metres. It also has to be single storey and for a purpose incidental to living in the house, and the government's guidance says a bedroom, a kitchen, a bathroom or a separate place to live is not incidental. So something taller than that, or set up for sleeping and eating in, would normally need an application, though only the council can say whether a particular building is lawful.
https://planforecast.co.uk/guides/do-i-need-planning-permission-for-an-outbuilding?utm_source=forums
(I help build that site.)
