Starting layout: 4m × 4m. This is an illustrative design prompt; check furniture, equipment and internal clearances.
Start with the layout
Start with the furniture layout, views and how the doors open onto the garden.
Get the specification right
Consider privacy, evening lighting, summer shading and a comfortable path back to the house.
Plan the details early
A leisure room and separate living accommodation are different planning proposals. Discuss sleeping or self-contained use with the local authority.
Build a complete brief
Record internal dimensions, intended use, ventilation, services and access. Share the same brief with every supplier so quotes can be compared. Check planning and building-control requirements before committing to a particular height or footprint.