Noah does not recommend placing a hot tub or conventional sauna inside a standard garden room because of moisture. Discuss a separate hot tub area or a suitable combination design with us instead.
Our current guidance allows consideration of a self-contained infrared sauna where ventilation is properly managed. Share the exact model and its installation requirements so suitability can be checked for your project.
The room around the equipment matters too. Include space for changing and a seat, and consider the route between the house, the dry room and any outdoor spa area.