Normally, summer houses don’t require planning, but this is only the case so long as it’s for ‘incidental’ use – that’s, for use as a hobby room, workshop, or for relaxing in. When you install a toilet, this may be considered ‘ancillary’ use, which applies to things you’d normally do in the house, like sleeping, cooking, and bathing. As such, you’d have to apply for permission to add a loo, and your building would also have to comply with building regs. If in doubt, check with your local planning office.