A garden guest room with bathroom is two projects sharing a base. The room itself is the ordinary garden building question, made harder because somebody sleeps in it. The bathroom is a drainage, water supply and ventilation job that has to reach the existing foul system, and that second half is where the money, the digging and the approvals are.
Gravity or pump, and the levels decide
Foul water leaves a building by gravity if the invert level of the existing drain is low enough and the fall across the garden works, which is the cheaper and far more reliable arrangement. Where it is not, a pumped system lifts the waste to the connection, which means a sealed chamber, a pump, a power supply that must not fail, and a maintenance commitment nobody enjoys. Find out which one you have before you agree a price.
Hot water and the supply run
A shower needs water at a rate a long thin pipe will not deliver. That means a supply pipe sized for the job, buried below frost depth, and a decision about whether hot water is made in the building by an electric unit or brought from the house. Making it locally is simpler and adds electrical load; bringing it out means a long insulated run and a wait at the tap.
Ventilation, because a shower in an airtight room is a problem
A modern garden building holds moisture in, and a shower produces a great deal of it. Mechanical extract ducted to outside, running on an overrun timer, plus background ventilation, is what keeps the moisture out of the wall build up. Without it the water finds the coldest surface and stays there.
Do not forget the room itself is a bedroom
Everything the guest room question raises still applies: permitted development does not cover sleeping accommodation, the building regulations exemption for small detached buildings does not apply once somebody sleeps there, and fire escape and interlinked warning are requirements rather than options. The bathroom is the expensive half and the bedroom is the half that needs consent.
Questions people ask about garden guest room with bathroom
Can I use a macerator instead of a proper drain?
For a WC and a basin it is a common answer where gravity is impossible, and it comes with conditions: it needs power, it needs the right appliances connected to it, and it fails eventually. For a shower room in a building used by guests, a designed pumped station is a more robust answer than a domestic macerator.
Does the bathroom need building regulations approval?
Yes. New sanitary appliances, hot water and drainage are notifiable work with their own Approved Documents, and it is inspected and signed off. That is separate from whatever the building itself needs.
Will the water freeze in winter?
Only if the pipe is too shallow or the building is left unheated with water in it. Burial below frost depth, insulation where the pipe rises into the building, and either background heat or a way to drain down for long empty periods are the three defences.