A garden guest room is the use that changes a garden building's legal position more than any other. Permitted development for outbuildings covers buildings incidental to the enjoyment of the house, and sleeping accommodation is not incidental. The building regulations exemption for small detached buildings also falls away once somebody sleeps in it. Those two things happen together, and neither is a formality.
Planning: incidental use does not include a bedroom
A room for a home office, a gym or storage is incidental. A room provided as sleeping accommodation is treated as an extension of the dwelling itself, and that normally needs planning permission rather than falling under permitted development. Building it as a study and using it as a bedroom is the arrangement people describe on forums and it is not a legal position, it is an unresolved one.
Building regulations: the exemption depends on nobody sleeping there
The exemption that lets small detached outbuildings be built without approval is conditional on there being no sleeping accommodation. Take that away and the building is ordinary building work, which means thermal elements, structure, ventilation and, most importantly, fire safety all have to be dealt with properly and signed off.
Fire safety is the requirement that bites
A detached building someone sleeps in needs a means of escape and a means of warning that works when they are asleep. In practice that means a door or window that can be escaped through, interlinked smoke detection, and attention to the fire performance of what the walls are made of. This is the part that a garden building sold as an office has usually never been designed for.
Do it properly and it is worth more
A garden guest room built with permission and a completion certificate is a documented, lawful piece of accommodation and it reads that way to a buyer and a lender. One built without either is a question at the point of sale, and the answer is either retrospective consent or a discount.
Questions people ask about garden guest room
Can guests stay occasionally in a garden office?
Occasional informal use is not the same as providing sleeping accommodation, and nobody is inspecting a spare bed. What matters is what the building is provided and used for: if it is where visitors regularly sleep, it is a bedroom whatever it is called.
Is an annexe the same thing?
No, and the difference is self containment. A room with its own kitchen and bathroom used as a separate household is an annexe, which brings a further set of planning questions about whether it is a separate dwelling. A guest bedroom without cooking facilities is a simpler proposition.
What paperwork should I end up with?
Planning permission or a lawful development certificate, a building regulations completion certificate, and an electrical installation certificate. Those three documents are what a solicitor asks for, and assembling them at the time costs a fraction of reconstructing them years later.